Nuclear magnetic resonance spectrometry - Analytical Chemistry

Nuclear Magnetic Resonance Spectrometry Jerry

P.

A.

Heeschen, Chemical Physics Research laboratory, The Dow Chemical Company, Midland, Michigan 48640

INTRODUCTION AND EXPERIMENTAL METHODS

T

HIS REVIEW COVERS the period July, 1967 to July, 1969. It is a n attempt to summarize or refer to developments pertinent to the broad understanding and practice of S M R spectroscopy. Attention is given especially to theory, techniques, and new or unusual observations. Although a balanced presentation is attempted, the choice of material is necessarily subjective. Let us start by reviewing the reviews. The preceding article in this series covered July, 1965 to July, 1967, in a similar fashion (-4181) Other articles cover general developments in 19661967 (A118) and 1967 (A199). High resolution X M R is summarized thoroughly for 1966-1967 (AdOl), 1967 (ill32), and 1968 (A133). A general review of high resolution K M R covers significant recent developments (A135). Another high resolution review stresses resolution improvement (A 143). Two journals devoted entirely to magnetic resonance began publication in 1969. One covers “theory, techniques, methods of spectral analysis, and results of nuclear and electron magnetic resonance,” and extends ‘‘to such related areas as quadrupole resonance, cyclotron resonance, Xossbauer effect, and magnetic properties of the solid state” ( A 4 7 ) . The other “is devoted to the rapid publication of original work dealing with the application of NAIR, KQR, and E P R , to the structural problems of organic chemistry” (A269). Another series of annual reviews publications appeared (d268). Three new general references emphasize high resolution methods and their application to structure problems (A22, A51, -4189). Second editions of two older books have been issued (A190a, A327a). Two large tabulations of high resolution parameters taken from the literature are very useful for organic structure elucidation (A60, A62). A “workbook” is designed to teach the interpretation of NAIR spectra (A%). NMR in the Earth’s magnetic field is the subject of another book (A@). A collection of introductory lectures, discussions and short communications on magnetic resonance and relaxation is offered (A39). A series of survey articles on nuclear and electron magnetic resonance covers fundamental concepts and interactions which affect lineshape (A160, A162, A284, A285). I

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An NQR review covers fundamentals and structural applications (.460), and tabulations of NQR frequencies (A35) and coupling constants (AZSg) have been published. Review articles and other larger publications dealing with more specific topics are given as the subject is discussed. INSTRUMENTATION

Lists of manufacturers of spectrometers and accessories available in the United States are given (148, A310). Major changes in commercial instruments are the introduction of small high resolution spectrometers with variable temperature capability and detection of other nuclei than protons ( A l 9 8 , A282, .4346); large-gap magnets for high resolution at 90-100 NHz ( A H , A198, ,4346); and a computer package for use with and control of on-line spectrometers (AS&). A very informative review article discusses “limits of measurement in magnetic resonance, by considering sources of noise and modes of enhancement” (Al85). The question, “HOW does a crossed-coil spectrometer work?” was discussed in the literature (A104 and references therein). Many approximately equivalent descriptions are possible, including a view of the process as spontaneous coherent emission into the detector circuit. Theoretical description is given for a two-coil liquid flow maser, using proton spins (AdOl). High Frequency NMR Using Superconducting Solenoids. The construction of a “simple” spectrometer is described ( A l 7 l ) . Most components were commercially available, the probe design is novel, and sample temperature can be varied from -100 to +lo0 “C. Improvement has been made on a spectrometer reported previously (A113). The operation of a spectrometer a t 220 illHz for protons is described ( A Z l ) . Field sweep of a system is made linear by maintaining constant voltage across a controlling coil (A339). The bulk of published work a t high frequencies has dealt with high resolution spectra of macromolecules and complex molecules of biological interest. It is suggested, as well, that broad-line spectra of solids a t such high fields can be analyzed for chemical shift information; criteria for establishing the

ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970

feasibility for polycrystalline materials are esamined (A338). High Resolution Spectrometers. Two spectrometers designed for multiple resonance (A43, A331) utilize frequency synthesis and a separate channel for stabilization, and have highly linear frequency sweep. Methods for measurement and control of magnetic field homogeneity are described and applied dramatically (AlW3). Here the second moment of a line, and related criteria, were considered in constructing a complete set of noninteracting shim coils for automatic control by digital computer. A time-division scheme is described for the purpose of avoiding beat signals during honionuclear double irradiation (A182). ;1 brief description is given of apparatus used for Ivhite noise decoupling of the entire proton spectrum during observation (A306). Heteronuclear double resonance with a synthesizer was accomplished by phaselocking the observing frequency to a harmonic of the synthesizer’s reference oscillator (A79). Circuits are given for double-tuning a probe to observe both ‘H and simultaneously (A120). Other double-tuning schemes are noted for heteronuclear double irradiation (A80, A229). Remote digital control of a commercial frequency synthesizer was simple in principle but not straightforward in practice (-4196). A large frequency sweep was achieved on a commercial spectrometer by use of a voltage-controlled oscillator with ramp derived from the spectrometer sweep potentiometer (-4158). Circuit modifications are given to equip an A-60 spectrometer for measuring relaxation times of chemically shifted nuclei ( A 9 ) . Accompanying an argument for recording the derivative of high resolution lines, in order to detect line shape changes better, is the schematic diagram for modification of an A-60 recorder for this purpose (AZO7). Weak signals may be distinguished better in the presence of strong signals by recording the inverse of the spectrum (A344). Very high resolution -W1,2 = 0.1 Hz- has been reported (A196). Synchronization of a frequency sweep with the memory scan of a time-averaging computer has been accomplished by use of an optical incremental digitizer (Ad). Pulse Spectrometers. An automatic swept-frequency spin-echo spec-

trometer is described, which was built for observing ferromagnetic materials (A96), and two ot'her spectrometers built for the same purpose (AZO4, ASS?). A high-power untuned transmitter (A2S3) and fast recovery singlecoil probe and receiver (A2S2) are described. 12 new pulsed spectrometer was interfaced to a digital computer, yielding TI measurements to 1% accuracy (A336). A simple voltage-to-frequency converter with maximum counting rate of 2 X H z was built for pulse work (A 272) . Broad-Line and General Methods of Detection. Coaxial hybrid tees covering t h e radiofrequericy range have been recommended for the assembly of spectrometers a t almost any radiofrequency (A209). An improved version of a design proposed above provides improved stability and sensitivity while eliminating the R F amplifier (A69). d broadband dispersion detector is described; it can be swept over a wide frequency range, and is especially well suited for frequency-swept searching of narrow easily saturable lines (-442). Dispersion signal enhancement was used to observe 13C in some paramagnetic solids (d84). X simple detection method involves counting the superregenerative quenching frequency (A329). A stable and sensitive linear autodyne circuit is described (A81). An autornatically stabilized bridge circuit imparts long-term stability, has the capacity to compensate for small frequency drifts, and offers simultaneous presentation of both absorption and dispersion signals ( d f 47'). 1 helium cooled R F preamplifier utilizing field effect transistors improves sensitivity by a factor of three to ten (A%%). Unsaturated magnetic resonance signals in a single coil circuit may be simulated by a resonant calibrator, which should find general use in testing and comparing systems ( A @ ) . A frequency marker delivers pulses every 1 and 10 kHz for frequency-sn-ept spectrometers (A167), and a simple frequencysiveep circuit is described (AW19). An improved super-regenerative XQR spectrometer, with frequency marking, is discussed in detail (AS19). A low frequency spectrometer n a s built bo accommodate sample volumes of 0.2 to 1.25 liters, being intended for observation of whole bodies such as eggs and small animals (Af9f). Two spectrometers were built to measure relaxation times greater than 0.05 seconds (A188, A325). A unquie oscillating detect,or has been found in voltage-biased superconducting weak point contacts (A316). Two probes, designed for calibrating magnetic fields in superconducting solenoids, utilize Al(27) resonance (-4183, A S 1 f ) . A magnetometer is described

( A S S @ , as are a n X M R magnetic field discriminator for automation of a magnetometer (AZf4) and a simple proton resonance automatic field calibrator for ESR (A65). Constant field modulation amplitude is maintained over large field sweep with a stabilizer (A7), and a bidirectional square wave generator may be used for modulation of R F or field (A265). The need for manual adjustment of the magnet regulator voltage is eliminated by incorporation of a preregulating loop (A281). h triangular sawtooth sweep generator which may be stopped and held a t any point is offered for manual or time-averaging operation in N N R applications (AS6.4). The operation of a computer of average transients t o study rapidly changing spectra is described (Af 39). One device for crystal alignment in the probe w s constructed for an ENDOR spectrometer (Ago), and another permits temperatures between 90 and 273 "K to be held stable over long periods of time (A352). h probe insert of solid sulfur avoids spurious signals from this source (A129); another boronfree insert was built for a commercial spectrometer (ilS63). Variable Temperature and Pressure. X single-coil spectrometer for operation a t 4 . 2 t o 30 OK utilizes a coaxial line and stabilizes temperature to 0.05 OK (A334). Temperature control to 0.1 "C is accomplished over the range - 185 to +350 "C by blending three independent nitrogen gas streams (high, room, and low temperatures) (A294). Long term stability of 0.1 "C is reported also for a cryostat in rt hich the sample coil is immersed in a temperature-regulated nonhydrogeneous liquid such as CCI4 (A357). Design of a spin-echo probe with temperature control from 20 to 350 "C is discussed well (A278). Temperatures up to 1300 "C are achieved in a simple furnace which has been used for both steady-state and transient experiments (A35S). An inexpensive heating system consists of placing a n electric heater around the probe insert (A158). A safety device protects variable temperature probes against overheating (A180). Calibration of methanol and ethylene glycol as thermometers in high resolution NMR was achieved with a static thermistor probe; deviations from published calibration charts are found a t both high and low temperature ends of their useful range (A341). Subsequent description of a spinning thermistor probe F a s published (AS42). -4 fluorine N M R thermometer is found in a mixture of CFC13 and CFz:CC12 as solvent (il269). A cylindrical polytetrafluoroethylene weight which slides over the sample tube and rests on the

turbine rotor serves to prevent the sample tube from lifting by nitrogen flow during variable temperature runs (A34O). Variable temperature (23 to 190 "C) and pressure (0.07 t o 1.9 kbar) were applied with a new device (A293) in a study of proton spin-lattice relaxation in n-heptane. A variable pressure liquid sample container, to 3000 Kg/ cm2, was designed also for relaxation measurements (-458). A vapor cell was designed for use in the range 200 t o 1200 mni Hg (-491). EXPERIMENTAL TECHNIQUES

General techniques are treated in this section. High resolution methods are discussed in the next section, and methods for observing and interpreting certain other experiments are covered elsewhere. Subsidiary resonances a t 2w,, 3 ~ , and , 4w, are reported for solid methyl anthracene a t low fields (10-50 gauss). The intensities of the lines a t 2w, and 3w, are predicted and found to have field dependence (HLIH,)' and (HL/ H,)4, respect'ively, where HL is the local dipolar field; this contrasts with previous predictions of second power dependence for both (A92 and reference therein). Kuclear resonance may be detect'able in conducting samples via interaction of the nuclear spin with "helicon" magnetic fields generated by carriers having the same sign as the niagnetogyric ratio; the effect was predicted and demonstrated by observation of 207Pb and lz5Te in monocrystalline lead telluride a t l .2 OK (ASOO). Kuclear moment ratios have been measured in various ways. The free protonjcyclotron frequency ratio, UP/ we = y p LICje, was found to be 1.5210329 X l o p 3 i 0.6 ppm by observing in the same field water and a narrow cyclotron resonance with reduced space charge shifts (-4206). Nuclear moments in free atomic ions may be measured to good precision by charge-exchange collisions with optically pumped atoms in the presence of an inert buffer gas; this was done for R b + (-4255). High resolution multiple irradiation methods - particularly INDOR or heteronuclear stabilizationoffer the most convenient and precise measurement when applicable, but still require correction for shielding. Proton magnetic resonance has been established as the source of a modulation observed on electron resonance peaks recorded by the "hlouette 11" satellite (A163). Relaxation Time Measurement. T1 measurements of spin-1 nuclei in liquids have been measured via NRIR spectra of spin-'/? nuclei coupled t o them. I n one case the 14Srelaxation


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in 2-fluoropyridine was determined by observing spin-echo trains of fluorine; this is a new technique, and a thorough investigation was made of the requisite experimental conditions (AS). Another study utilized the 19Fband shape and 19F-15N coupling constant of fluorinated pyridines to obtain the 14N relaxation time (-488).1 4 5 relaxation in formamide was shown to be observable via the proton relaxation (A131). A number of useful modifications of established techniques have been intended to simplify or extend the applications of these methods. Adiabatic fast passage followed by a reverse scan a t the proper interval of time will generate a null signal; Tl can be calculated simply from the known value of the time interval. A device for generating such a sequence is described and applied to deuterium relaxation in several liquids (A167). d very similar method utilizes asymmetric modulation of the magnetic field, adjusting the period of the modulation until the second passage gives a null (A101). h comparison was made between symmetrical adiabatic fast passage and a n - ~ / pulse 2 sequence for measuring T I in solids; it was found that the former gave shorter TI values, but the two values can be made to coincide (A295). The use of a tone-burst generator to apply a modulating signal for specified periods of the modulation permits simple adaptation of a continuous wave spectrometer to T I measurements. I n the first instance reported, the magnetic field was modulated by a set of triangular waves up to amplitudes of 50 guass a t frequencies up to 30 cps in order to record absorptions up to 5 gauss in width (AZSO). Sequences of such bursts can be repeated a t set intervals and synchronized with a time-averaging computer to achieve significant enhancement. The authors recommend that non-adiabatic passage conditions be met in order to simplify interpretation. I n this way one records a series of absorption curves which decay to a constant height. I n a subsequent publication (A2S1) the same authors describe the use of a tone burst generator to generate RF pulses of selected length. Here one observes free induction decay, and repetition rates can be faster than for the field modulation method with ordinary probes. Interest continues to grow in the measurement of relaxation times of individual absorptions in high resolution spectra. h variety of sophisticated pulse experiments on individual lines is described, using low R F power and long pulse duration (A146). The above experiments were performed on a conventional high resolution spectrometer by pulsing the audio sideband used for detection; parameters measured were 420 R

T1, T z by spin echo, Tl, by forced transitory precession, and Trotby rotary spin echoes. The rotary echo technique in high resolution N M R is given a full treatment regarding experimental method and interpretation (AS61). A proposed extension of rotary echoes to observation at zero frequency should permit measurement of very long relaxat'ion times and spin-spin coupling (A25). The saturation recovery method for measuring T1 on high resolution spectrometers has proven to be very convenient. An initial transient signal is noted, but it \Till not affect the measurement if the observed line is inhomogeneously broadened and has no discontinuities or steep slope (-4343). The same aut,hor describes a logarithmic converter for recording saturation recovery curves as straight lines (d345). A saturation recovery method was used to measure a long T I in solid hexamethylbenzene-a long pulse of RF power saturated the absorption, followed by very short pulses to monitor the recovery (A27). The progressive saturation method was used to measure TI values of individual lines in an AB spin system; it was shown that' under certain conditions a plot of (Hl/signal height) us HI2gives a straight line with slope proportional to T I (A138). Pulse Techniques. I n a general discussion of pulsed X X R in solids, special attention is paid to information contained in the shape of the free precession signal and of echo signals (A46). It has been demonstrated theoretically and experimentally that extra "allowed" echoes from nuclei with spin 5 / 2 can be enhanced by a factor of almost 5 by phase-shifting the second pulse by 90"; the experiment was performed on lZ71 of fused K I ('4360). A niodification of the multiple solid echo experiment was performed on NaCl by sine and square wave modulation of the amplitude of the RF field; the sinewave effects were interpretable but the square wave effects were not ( A I 7 2 ) . The description of N l I R transients in solids has been extended to substances having a second but non-resonant kind of spins, and tested by spin-echo measurements a t different temperatures and field st,rengths on 1,2-diphenyl-2-picrylhydrazil ( A S I S ) . Lattice disorder effects have been evaluated by analysis of the spin echo line shapes of 87Rb, 79Br, and 8IBr in RbBr single crystals (A250). Some detailed calculations, by published methods, of K M R transients in solids have been found to differ markedly from some new experiment'al results, and the differences are explained (A31-4). New echo phenomena, similar to spin echoes, have been observed in metallic powders a t liquid helium temperature, but not explained (A@. On the other


hand, it has been predicted and shown that the apparent spin diffusion coefficient in liquid 3He, as measured by spin echo when the initial pulse is not 90°, should show a maximum value as a function of temperature (AB%$). Deuterium dilution combined with an automatic data acquisition system were used to evaluate the inter- and intra-molecular relaxation of n-dodecane (d365). Pulse studies in the earth magnetic field have included transient nuclear demagnetization in the rotating frame (-424)and examination of phase memory (-430, A31). The intensity of a noisy pulse signal has been calibrated by alt'ernately sampling the signal and a reference signal with a single boxcar integrator (A93). Fourier Transform Spectroscopy. Little has been published on this important technique since ibs introduction to high resolution S I I R in 1966 (A126). -1great deal of developmental work has been carried on during this period, however, and will bear fruit in the near future. Two significant applications are reported. I n one case the very weak signal of l*'Os in molten Os04was detected by Fourier transform of time-averaged free induction decays (A308). In a more elegant study the free induction decay following a 180'90" pulse sequence was transformed in order bo study the two different relaxation times in a water-acetone mixture. The authors suggest that this method is capable of detecting two different transitions accidentally superimposed on one another, by monitoring the time development of t,henet, intensity (A348). Multiple Resonance in Solids. The technique of pulsed double resonance has been used primarily for det,ecting weakly absorbing or lo^ abundance quadrupolar species in solids. The experiment consists of spin-locking the strongly absorbing species, irradiating the weakly absorbing species without heating its spin system, then recording the height of the free induction decay of the directly observed nuclei. The amplitude of the free induction decay will be decreased to the extent that the two spin systems are coupled to one another. The technique has a n advantage over CW detection of S Q R because the effective frequency sweep rate can be much faster, a t the expense of resolut'ion (A177). Application is noted in observation via. '9F of 39Ksurrounding a substituted S a ion in KF (-4176, includes description of the instrument), 39K and 41K via lH in K H z P O ~(AS%), and 14?J via 1H in NH4HzPOl ( r l l 7 7 ) . Sensitive detection of the NQR of low abundance species by pulsed double resonance a t zero field was given special

attention in a study of impurity centers in NaCl (AS18). A steady-state double resonance technique is reported, as an extension of the pulse method (A200). The observed nucleus is irradiated off resonance by a coherent R F pulse train and the receiver output monitored only when the transmitter is off, so that leakage and balance problems are eliminated. The second spin system is irradiated continuously, shifting the phase of the R F by 180’ regularly in order to keep that spin system hot. This continuous technique gives the same sensitivity as the normal pulse method, and offers great experimental advantage in ease of use. 40K a t natural abundance was detected in this fashion via C1 in KC103. Proton spin decoupling in solid CaHP04 was observed via changes in the second line (A264). moment of the 31P Acoustic-Nuclear Resonance. Transitions among nuclear spin levels in solids can be induced by applying lattice vibrations (phonons) a t the nuclear resonance frequency. The absorption causes saturation of the nuclear spin alignment and is detected either as a loss of signal in the normal XMR spectrum or as an attenuation of the acoustic wave. The intensity of absorption depends upon the magnitude of coupling between the lattice and the spins, generally being stronger for nuclei which have electric quadrupole moments. One thorough review of the field covers work to 1966 (A45). Other reviews cover experimental methods (AS05) and equipment (AS12). The latter reference describes a versatile spectrometer which includes ultrasonic pulsing capability. Observation of acoustic dispersion has been reported for 19F of RbMnFl (A252). Saturation occurs a t higher acoustic power. The spectrometer used was the CW type rather than marginal oscillator (-4253), and has been equipped with a scheme to lock the frequency to a standing wave acoustic resonance of the specimen (A254). A transducer is described for recording acoustic NXR spectra under pressure (AS17 ) . Two interesting double resonance experiments have been reported. I n one case 53Cr3+impurity in ruby was detected by monitoring one peak of the A1 NMR dispersion curve while sweeping acoustic or R F irradiation through the Cr resonance region; the double resonance line shapes were not the same for the two kinds of irradiation (AS47). I n another report low frequency acoustic irradiation caused transitions among nuclear magnetic energy levels in the rotating frame, as observed in a spinlocked pulse experiment (A17S). A proposal, with supporting arguments, is made for detection of dynamically oriented nuclei by acoustic N M R (A66).

Expressions have been derived for influence of the motion of dislocations on the ultrasonic saturation of the N M R line of cubic crystals ( 4 6 5 ) . Optical Nuclear Resonance. The Nobel lecture of A. Kastler (A20S) describes the principles and main features of optical pumping and related relaxation processes as observed on atomic ground and excited states. Best experimental set-ups are determined for various problems encountered in the optical detection of optically pumped vapors (A216). Magnetic resonance was induced in mercury vapor by frequency modulation of the light source (A78). Some interesting experiments have been performed in the effective magnetic field of the rotating coordinate system generated by applying HI off resonance (A114, A273, A274). Free atomic R b ions have been generated and their nuclear resonance detected optically (A255). Dynamic Nuclear Polarization (DNP). Also called the “Overhauser effect,” D N P refers to the enhancement of the polarization of one set of spins by causing a deviation from equilibrium of the polarization of another set of spins to which the observed spins are coupled. The conditions discussed here are the saturation of electron spins to cause strong enhancement, positive or negative, of the NLIR absorption of associated nuclei. Strongly related is the “solid state effect” where saturation of a “forbidden” (electron plus nuclear) transition causes nuclear signal enhancement in solids containing paramagnetic impurities. Internuclear Overhauser effects (NOE) are covered later in the high resolution section. The inverse, ENDOR, experiment observing electron resonance while irradiating nuclei is not considered in this review, beyond noting a good discussion of ENDOR techniques (A187). A general survey of nuclear-electron double resonance methods is recommended (A100). Two thorough reviews treat D K P in liquids (A119, A1 79). A unitary theory of dynamic polarization in liquids and solids has been developed (A279). The shape and width of the observed NMR line depend upon the ratio of E P R to NMR line widths and the degree of saturation of the E P R signal, effects which should be considered when evaluating experimental data ( A 2 ) . Theoretical descriptions of the solid state effect take into account asymmetry of the ESR line shape (A47, A70, A296). A low-field spectrometer, a t 30 gauss, is described (A8S). Studies of D N P in liquids usually have employed free radicals, as solute or solvent, as the electron source. One

obvious application is enhancement of otherwise weak absorptions such as 1aC (A166). Often more interesting, however, is the possibility of characterizing the nature of collision processes between the radical molecule and observed molecule. T o this end studies have been made of I9F and 1H in fluorocarbons (A292 and references therein, A1 16, A266, A270, -4366)of in phosphorus compounds ( A l l 7 and references therein), and of the stable free radical ditertiarybutyl nitroxide as solvent (A326). For both fluorine and phosphorus the results a t low field are easiest to interpret because scalar electron-nuclear coupling dominates, while dipolar effects become important a t higher resonance frequencies. I n general, trivalent phosphorus experiences positive enhancement and pentavalent negative a t low field ( A l l y ) . Organic complexes of Cr(V) and Mo(V) have shown proton polarization (AISO). D N P of nitrogen was obtained in solutions of sodium in liquid ammonia (A217 ) . D X P of 27Xlin ruby was given close attention (A16, A223, A271), with conflicting reports of temperature dependence (AWbS,A271). It is proposed that NMR quadrupolar and dipolar effects unresolved in normal spectra can be resolved by saturating the center of an inhomogeneously broadened ESR line (A222). Proton D N P in charcoal by free radicals is interpreted as a combination of “solid effect” and normal Overhauser effect (A98). The transition from an Overhauser effect to solid effect was studied in viscous isopropyl alcohol a t low temperature (A221). Optical generation of non-equilibrium electron spin systems can result in dynamic polarization of nuclei by the usual relaxation routes. Various schemes are proposed (A197), and the first successful optical pumping in solids was reported by generating polarized electrons in silicon samples (A218) and metastable triplet states in anthracene (A941). High Resolution in Solids. I n this period was reported the first successful resolution of chemical shift in a solid by a pulse technique, observing 19Fof CaF, relative to CsHs CF3 (AS55). The general problem of attaining “high” resolution requires the effective elimination of the line-broadening effects of the dipolar interactions among the nuclei, so that chemical shifts and indirect spin-spin coupling structure may be resolved. The first approach used was fast spinning of the sample about an axis canted an angle of 54’ 44’to the magnetic field. This averages out anisotropy in the chemical shifts as well. Significant narrowing has been accomplished this way. Current investigations are centered on better

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theoretical descriptions of the spectra ( A l l and references therein). A major instrumental problem is attaining the very high rotation speeds required; small gas turbines are described for reaching rates up to 10 kHz using helium as propelling gas ( A I S ) . Applications reported include observation of spin-spin multiplets in KAsFG and KSbFB (AIL$),fine structure in the proton spectra of polystyrene and poly(vinyl chloride) (ASO7), and a proposal to determine the anisotropic Knight shift in metals (ASOQ). An alternative to rotating the sample is rotating the magnetic field. A preliminary step in this direction was rotation of a 270 gauss field a t 25 kHz rate, a t an angle of 90' (A5S). Pulse techniques are emphasized in a very thorough study of coherent averaging effects; this is directed toward the specific goal of removing dipolar structure while retaining chemical shifts and scalar coupling (A169 and references therein). The rotating frame at the magic angle and modifications such as tilting the coil and pulsed analogs are covered, then a series of newer pulse sequences is proposed. I n general, pulse methods detect transients which are Fourier transformed. They reduce the measured chemical shifts by factors as high as 3, and do not remove chemical shift anisotropy. The various sequences are designed to remove or reduce broadening effects due to dipolar interactions, quadrupolar interactions and H1 inhomogeneities. Spinning the sample during a pulse experiment will remove chemical shift anisotropy; spinning need not be so fast as for sample rotation only, because the dipolar broadening already is gone. The development of similar pulse methods and their understanding is described in detail (A243 and references therein). Applications of the above pulse methods have been reported. I n a copolymer of perfluoromethylvinyl ether/tetrafluoroethylene a shift of 73 ppm was measured, with possible effects of shift anisotropy ( A l 2 l ) . The nonequivalent phosphorus nuclei of solid ZnaPz were 84 ppm apart ( A l 4 2 ) . Oriented Molecules in Liquid Phase. I n contrast to the subject of the preceding section, it is desirable at times to introduce dipolar and quadrupolar splittings into high resolution spectra. By partial molecular orientation in the magnetic field, with fast averaging over other orientations, one obtains highly structured and wellresolved spectra from which can be determined internuclear distances, chemical shift anisotropy, and absolute signs of scalar coupling constants. The most significant development over the period of this review was the successful electrical alignment of nitrobenzene and nitromethane, seen as line 422 R

splitting in the 14Nspectrum via quadrupolar interactions which were not averaged out (A184). The authors suggest that a previous attempt on nitrobenzene may have failed because the electric field was not homogeneous. The use of liquid crystals, as solvents, to orient molecules has increased, and the discovery of new systems promises to encourage still further applications. A mesophase of solvent, between solid and isotropic liquid, becomes oriented in a magnetic field. Solute molecules are restricted in their motions, and dipolar structure appears. Nost work has been done in the nematic phase, where the orientation is in the direction of the magnetic field. Several helpful reviews are noted (A29, A228, A251, ASO2, ASRI). Dependence of spectra on temperature, concentration, and spinning speed was studied; degree of orientation increased with lower temperature, and the degree of orientation of the solute is related directly to that of the solvent (AlO7). Nematic phases over the temperature range 23-130 "C can be obtained by proper choice of cholesteryl esters (A297). Limitations in experimental technique are blamed for large difference between a measured chemical shift anisotropy and theory (A67). Para-anisaldehyde azine Tl values underwent discontinuities a t the phase changes between nematic and others (A@). Poly-7-benzyl-L-glutamate as solute a t high concentrations causes the solution to become liquid crystalline upon long standing in a magnetic field, with ordering in the field direction (AS22). Further study of this interesting system has shown quadrupolar splittings in D and C1 of solvents, and that the direction of orientation is the same relative to an orienting electric field (A156 and references therein). An external electric field also was found to rotate the molecular axes of nematic p-anisalaminophenyl acetate without loss of resolution in the NMR spectrum (Arb). Lyotropic mesophases have been found, and prove to be very useful because they form a t room temperature, and can be spun in normal spectrometers because orientation is perpendicular to the magnetic field (AS7, AS8, and references therein). Potassium oleate in DzO, smeared between glass plates, is known to have its optic axis perpendicular to the plates; placing the plates in a magnetic field a t 55' to the field direction gave a sharp NMR line which broadened on changing the angle (A1OS). The analysis of oriented molecules under high resolution conditions is treated in the high resolution section. Flowing Samples. Observation of flowing samples offers the possibility of monitoring processes continuously, measuring relaxation times, and mea-


suring flow rates. A probe designed for continuous quantitative analysis allows sufficient residence time in the magnetic field for the nuclei to reach equilibrium polarization; this gives decent resolution and maximum potential signal enhancement over static methods (A249). Quantitative analysis was studied elsewhere, also, including use of relaxation time measurement to assay paramagnetic ion concentration in water (A291) and to detect the generation of free radicals (ASS). I n the former reference is described the apparatus used (A291). The influence of flow on spectral lines was studied experimentally (A168). A book on the subject has been published (A28O). A commercial NMR flowmeter has been introduced (A17). It uses the "time-of-flight" technique, for which the design of pulse detectors is presented (A2461* Line Shape Analysis and Modulation. A calculation based on relaxation theory shows t h a t when the chemical shift between lines of like amplitude is of the order of the line width, the line shape deviates from Lorentzian (Al54). Many properties of Gaussian and Lorentzian line shapes are discussed, including fast identification, moments, and peak integration (A286). A variety of functions for simulating line shapes is presented for possible use in line shape analysis (Al4Z). Line broadening by local pertiirbations in a non-linear fashion has been discussed theoretically (AS27). Phase modulation in magnetic resonance experiments was compared with energy level (usually field) modulation; complete equivalence was established between the total time rate of change of the exciting fields and the instantaneous energy level (A260). A mathematical treatment of the unsaturated dispersionmode Lorentzian line shape dependence on low frequency sinusoidal modulation shows that the apparent line shape is strongly affected by the modulation amplitude (A128). The amplitude of an imperfectly tuned wide-line signal can be corrected by a published procedure (A288). Magnetic Susceptibility Measurement. The static nuclear magnetic susceptibility of water was measured and found to agree with previous results (A26). Measurement of the volume magnetic susceptibility of any liquid by XTMR is presented (A102, A178). A wide-line spectrometer was used in an apparatus for measuring the susceptibility of MnO over the temperature range -190 to $120 "C (ASSS). HIGH RESOLUTION TECHNIQUES

General. A short review of signalto-noise enhancemenb covers the broad variety of techniques available

(Air@. Another review article deals thoroughly with this problem as it applies to examination of small samples (A236). Further methods for collecting chromatographic fractions are given (-4141, A245). A new coaxial cell design prevents volatilization of the reference material (A186). Some internal chemical shift reference compounds have been found lacking in certain respects. The chemical shift of T M S varies as much as 0.15 ppm in aromatic solvents (AIWO). Sodium 4,4-dimethyl-4-silapentane- 1-sulfonate (DSS) should be used a t low concentration to avoid likely formation of micelles (A112). Chlorine isotope effects cause small but resolvable shifts among the isotopic isomers of CCl3F (A74). Two promising proton reference compounds have been introduced; 1,1,3,3,5,5 - hexakis(trideuteriomethy1) - 1,3,5- trisilacyclohexane, in organic solvents, has its protons (CH,) more shielded than TMS and boils a t 208 “C (A289), and (CH3)3SiCD2CDnC00Nafor aqueous solutions (A290). The 19Fshift of p difluorobenzene has been measured against gaseous ethane over the temperature r a n g q 5 to 100 “C, so it can serve as a good reference-standard for other I9F absorptions (AS23). It should be noted that heteronuclear referencing is becoming common in INDOR spectra. Dissolved oxygen can be removed from water-immiscible solvents by shaking with aqueous sodium dithionite or other oxygen scavenger (A59). A deuteration technique makes it practical to simplify proton spectra (ASO4). Although low-field spectra may yield relative signs of heteronuclear coupling constants-for example JHP in organophosphorus compounds in the earth’s field (A116)-the same information can be derived a t high fields using careful computation and double irradiation (A257). Computer Techniques. The digital computer has become a standard tool in analysis of high resolution spectra. Two articles review the various methods for spectrum analysis using computers, one covers the general methods (A164) and the other gives closer attention to the use of many specific programs that are available (A268). New programs described are PROSPECT I, an iterative program for the IBM 1620 computer (-487); an adaptation to the M-20 computer (4096 words) of a program published elsewhere, incorporating symmetry for magnetically equivalent nuclei (A215); and a program for AA’BB’X3 pattern (A44). A technique is given to simplify use of the SwalenReilly method where all sets of energy levels cannot be linked (A52). A useful method of displaying the energy levels and transitions computed in the

LAOCOON program has been demonstrated (A76). R. R. Ernst has continued his pioneering applications of computer techniques to NMR spectroscopy in describing automatic correction of the phase of NMR spectra by means of a numerical Hilbert transformation, and shows that fast passage absorption and dispersion mode spectra are Hilbert transforms of each other (A124). With others he applied four computational methods of resolution enhancementband shape calculations, convolution, Fourier spectroscopy, and analogue transformation-to the detection of very small spin-spin coupling constants (All?’). A study of convolution and deconvolution of Lorentzian bands has been published (A299). One manufacturer offers computer “packages” with its systems, which include many simple spectrometer controlling features (A346). An ambitious attempt a t automated structure elucidation with a computer, using mass spectral, IR, UV, and proton N M R data has been recorded; some inconsistencies occurred in solving practice problems because of unanticipated N M R coupling effects (A301). Spectrum Analysis. A broad review of methods for spectrum analysis is recommended (A36), as well as a qualitative survey of various methods in use (A277). A series of instructional articles covers second order approach ( A l b l ) ,three spin systems (A152), and four-spin systems (A153),while another instructional article presents an elementary description of the theory of irreducible components of multispin systems (A86). Full treatment is given to the analysis of three-spin-l/Z systems in a review (A316). Methods of sub-spectral analysis are treated thoroughly in a review article (-4106). An alternate decomposition of multi-spin spectra into two or more subsystems, among which mutual coupling is weak, has been developed using perturbation theory (ASSO). A direct method for calculating high resolution spectra is presented as a function of complex shape and spectral vectors (A&). Advantages of composite particle basis functions are illustrated by a detailed discussion of the (AA’X)z system of perfluorocyclobutene (A17 4 ) . Constructions given for the partial traces of any spin system should be useful in high resolution spectrum analysis (A328). Some simple rules for assigning equivalent sets of nuclei in nonrigid molecules have proven to be useful as a teaching aid (A263). The fact that the F(19) spectrum of fluorobenBene is asymmetric shows that all H-H and H-F couplings have the same symmetry (A.276). A hand iteration on some AZBZspectra is described with criteria for choosing a best fit ( A 6 ) . A

variation of Pascal’s triangle is used to predict the splitting pattern by n nuclei with spin greater than and the width of such a pattern when the individual lines are not resolved (A283). Specific Systems. Unambiguous analysis of ABC spectra is described using spectra a t two different magnetic fields (A324). Two methods for direct analysis of AA’BB’ spectra cover situations where all lines are not detected (A73, A239), and one gives better accuracy than other published methods when enough lines are detected (-473). A tabulation of line positions and intensities for ethyl group spectra as a function of shift and line-width should prove useful (A190). The ( A X ) 4 spin system is analyzed in detail by treating each AX pair as a unit and writing the Hamiltonian in terms of interchange operators ( ~ 2 4 0 ) . The use of 18C satellites of X,AA’X‘, spectra for analysis is described, and certain non-useful circumstances noted (A136). The large A-A’ coupling in such a system has been deduced by detailed treatment of the X&’ line shape (A295). The system ABB’XX’ could be analyzed almost completely using sub-spectral analysis, while ABB’XX’R required iterative computation ( A I ) . Nomograms of the X portion of deceptively simple ABX patterns are useful for determining relative signs of the A-X and B-X coupling constants (A19). Many examples of deceptive simplicity are found in the three- and four- spin systems of 4-substituted prolines ( A l l ) . Following is a list of papers which should be of instructional value in their use of a variety of methods to analyze spectra: (ARO, A105, A137, Al76, A237, AR38). Multiple Irradiation. An exhaustive review of nucleus-nucleus double resonance in high resolution N h l R covers theory, experimental technique, and applications into 1967, drawing on the author’s own experience and 629 references (A226). Another sizable review covers chemical applications of heteronuclear double resonance (A247). Shorter general (A99) and technique (AR48) reviews are noted. The “direct method” for calculating spectra has been extended to double resonance spectra, thereby exhibiting some properties of the spectra in a straightforward way but offering no apparent computational advantages (A10). A perturbation treatment was used to extend “tickling” theory to the case of nuclei with spin greater than l/z, and corroborated by observing the effects in liquid HD at 20 “ K (A287). A careful experimental study of line splittings and splitting thresholds in tickling experiments included effects of magnetic field inhomogeneity (A227a). In a


423

R

study of triple tickling line shapes, a theoretically predicted combination line was observed (A7R). A “hole-burning” technique has been introduced as the most precise method available for determining line positions in high resolution spectra. It is a double resonance experiment where an R F field weaker than the spatial inhomogeneity of the magnetic field, H,, localizes saturation of a line in a restricted region of the sample; this saturation is transferred to lines connected progressively and regressively to the irradiated line as a sharp “hole” and use was demonstrated by measuring line separations to an accuracy of *l mHz (A146). The same authors also have demonstrated other tickling and transient population rearrangement methods for detecting very small splittings ( A I & ) . A periodically pulsed double resonance experiment, where H1 and HZ are applied alternately, gave spectra which are complicated functions of the pulse spacings and bear unusual features (A169). The related simpler experiment of observing transient nutations following saturation of one line in the spectrum was extended to heteronuclear application in determining lacchemical shifts and lac-H couplings (A368). An investigation of strong decoupling found optimum HZ value, beyond which partial recovery of coupling effects were seen, in agreement with theory (A86). A re-investigation of the double resonance of a single spin-’/2 system, using carefully purified and degassed chloroform, gave good agreement with the Bloch approximation (Ad06). Partial decoupling has proven useful in conjunction with other techniques (A134, A367’). A general method for analyzing the AA’BB’ system uses tickling information to identify pertinent sets of lines (Add@. Careful lineshape matching with and without decoupling was used to measure a small coupling (A96). White noise decoupling of all protons, or a close approximation thereto, has become important for simplification and even detection of the spectra of other nuclei, especially lac. Experimental apparatus is described in some reports of its use (A1R6, A306, AS69). “Decoupling” via relaxation of quadrupolar nuclei with lower temperature and solvent has been known for some time. A quantitative characterization of the effect is examined for deuterium in a series of deuterated organic molecules (A26.2). The effect has been studied for l0B as well (A364). INDOR. This technique monitors t h e intensity change of a line as connected lines in a spectrum are traversed. It is a spin population redistribution (nuclear Overhauser effect) via double resonance, with R F power 424R

usually less than needed for tickling. The spectra obtained are relatively clean and simple, permitting ready assignment of the connection of transitions as progressive or regressive. I n this way INDOR spectra often are preferred for determining relative signs of coupling constants, both homo- and heteronuclear (A16, A94, A810). A mnemonic for describing interconnection of transitions is suggested (AS68). This technique has proven, also, to be a very good way to observe spectra of other nuclei than the one monitored, either as a convenience or as a powerful means of enhancement. Published instances include chemical shifts of loaRh via of a ligand ( A 6 1 ) ,solvent effects on 14N shifts in amides via 1H (AdOd), ll9Sn shifts of organotin compounds via 1H (A89), l*C a t natural abundance via 1H ( A N @ , and le9Hg and 18C of mercury dimethyl via lH (A94). It is convenient to tabulate the shift of another nucleus relative to that of the monitored nucleus to great accuracy. Nuclear Overhauser Effect (NOE). This term refers to the transfer of nuclear spin polarization from one system to another by irradiation of the first. It proceedsvia relaxation mechanisms and results in changes of line intensities in the spectrum of the second system. I N D O R spectroscopy is one manifestation of this phenomenon a t low power levels. At high irradiation levels measurable enhancement of absorptions can occur when the irradiated nuclei contribute significantly to the relaxation of the observed nuclei. A dipole-dipole interaction usually is expected to dominate. Thus, strong NOE behavior is commonly taken to indicate close spatial proximity between irradiated and observed nuclei, especially if there is no scalar coupling between them. Applications and techniques of NOE in organic chemistry are reviewed in one article (A861). I n one thorough study of an all-proton system, positive enhancement of a neighboring proton and negative enhancement of the next adjacent proton were interpreted as conclusive evidence that the dipoledipole mechanism dominates the relaxation ( A B ) . I n the same paper is a report of the first example of an NOE between geminal protons. Studies of better-defined systems have been made to evaluate relaxation mechanisms. I n one study a correlated random field was found to dominate relaxation in an AB system, while an uncorrelated random field dominated in an ABX system (A349). I n another study of an AB system a method of measuring the enhancement even for split lines was used (AXd7). A detailed discussion of the relaxation effects accompanied a study of NOE in an


aromatic AB2 system (A81S). Interand intramolecular dipolar interactions were discussed for NOE in methyl methacrylate (A148). enhancement of 2.98 in formic acid, oia proton irradiation, agreed well with the 2.988 factor based on a dipole-dipole relaxation mechanism (ARIR). Complete d e coupling of protons increased the *lP signal 50% in (Me0)aP but not a t all in MeaP (A120). Saturation Transfer. Spin saturation of a set of nuclei a t one resonance frequency can be carried measurably to their absorption a t another frequency if their relaxation rate is comparable to, or less than, the rate of the exchange process t h a t causes the resonance frequencies to change. Such observation of partial saturation of one absorption during irradiation of another has proven to be a useful device for establishing like signs of H-F coupling constants in the two rotational isomers of CHBr2-CFBr2 a t low temperature (A16d) and for establishing the rearrangement mechanism for methyl group transfers in the heptamethyl-benzonium ion (A97). Even more interesting is the potential for still further transfer of the saturation, via strong spin coupling, to nuclei that were not irradiated. Specific attention has been given to this situation for selected primary alcohols whose CH2OH protons have essentially the same shift and OH can be saturated via saturation transfer from another OH line with which it exchanges slowly; also a spin density matrix was developed C system of to describe an AB,, spin-l/2 nuclei where A and C exchange while the C absorption is irradiated (A149 and references therein). Multiple Quantum Transitions. The utility of multiple quantum transitions for aiding the analysis of complex spectra has been recognized for some time, b u t used relatively little. This may be due, in part, to an assumption that conditions for their detection are more restrictive than is actually so. It has been found in two strongly coupled AA’BB‘ systems that the observed double, triple, and quadruple quantum transitions obeyed Yatsiv’s theory very well with regard to frequencies, frequency shifts, and HI for maximum amplitude, with intensities being weaker than predicted values, even though simplifying assumptions for the theory had not been satisfied (A66). Other studies of four-spin systems are noted (A64, Ad36). Experimental techniques in the use of double and triple quantum transitions for spectrum analysis are treated thoroughly in a study of some threespin thiophenes (A193). Spin tickling caused splitting of a double quantum transition in Ziodothiophene (A19.4). The geminal coupling H-C-D in acetyl

+

allene was assigned the same sign as the other couplings in the molecule using the double quantum spectrum (-42.44).

The entire single-quantum spectra of both rotational isomers of CBrF2-CBrC12, interconverting slowly, were saturated significantly upon saturation of the double quantum transition of the AB pattern of the unsymmetrical isomer

( A150). Induced

Dynamic Polarization. Strongly enhanced emission and/or absorption have been reported in high resolution spectra of compounds shortly after their formation via freeradical or triplet states. The first instances (A18, A360) and most thereafter have been chemically induced (CIDNP), appearing a t the end of a reaction and decaying to a normal appearance in a few minutes. One remarkable feature is the observation of both emission and absorption in multiplet structures, in a symmetric fashion. There is general agreement that the nuclear polarization occurs through interaction with intermediate free radical or triplet states. Some qualitative and semi-quantitative theoretical descriptions have been presented, but none published during the period of this review offers a satisfactory explanation of all observations. Optically induced D N P in solution was observed for anthraquinone as a steady emission signal upon continuous irradiation; the excited state is known to be a triplet, but a t such a low concentration that the observed spectrum must be ground state molecules ( A H ) . A rather comprehensive review of C I D N P covers these developments, with emphasis on the authors’ own work with thermal decomposition of peroxides and azo compounds (AIQO). An empirical approach to studying reaction dynamics monitors the time development of various signals as a measure of the order of events (A225). Observation of C I D N P during some halogen-metal exchange reactions on alkyl groups is interpreted as evidence for the process involving free radical (one-electron) intermediates (A361). Oriented Molecules. Analysis of spectra in oriented phase-particularly nematic-is the subject of this section. Techniques for obtaining the spectra were covered earlier in this review. The combined use of direct and moment methods is discussed fully for arriving a t the correct solution to two- and three-spin problems (A109). I n another case the fluorine second moment of a fluorinated liquid crystal was used to determine the degree of order, with which knowledge the proton second moment could be used to examine possible molecular configurations (A6.4).

The definition of

equivalence in

oriented molecules has received further attention (A267, ASOS). I n showing that the composite particle technique is useful for oriented molecules the term “full equivalence” was given t o cases where the nuclei of a magnetically equivalent group are coupled equally to one another (AS66). Deceptive simplicity in oriented molecules is discussed in terms of subspectral degeneracy; it is fairly common for two-spin and many three- and fourspin systems (A111). Analysis of the AA’BB’ system is presented (A110). The spectrum of monodeuterobenzene showed an isotope effect on the timeaverage C-D bond length relative to C-H, and a method developed for determining the deuteron quadrupole coupling constant (A108). Thorough treatment was given to some substituted benzenes with D2h symmetry (A71). Another thorough study, of acetylenes, is noteworthy ( A l 2 2 ) . The importance of performing a full vibrational analysis is stressed for accurate geometry evaluation (A68), including anharmonicity (A.49). The problem of determining chemical shift anisotropy is discussed (A28,A66, A77). Measurement of the deuterium quadrupole coupling constant in CD2C12, using liquid crystal spectrum and other data, is claimed to be the first in a methane-like molecule (A208). Determination of the molecular structure of cyclopropane utilized proton coupling to laC a t natural abundance (AS20). Enantiomers were differentiated in optically active liquid crystals, but the experimental arrangement was not straightforward (A298). 6.

GENERAL PHENOMENA: RELAXATION AND BASIC PHENOMENA

Relaxation. A formal theory of non-linear response of a n isolated spin system to a n external perturbation is developed using time-dependent perturbation theory (BS69). The multiple time scale technique was used to provide a compact derivation of the equation of motion for the spin density matrix of a spin system immersed in a heat bath; this permits more general conditions and explicitly demonstrates some key features of previous work (B95).

Descriptions of molecular motions continue to be studied and improved. A review of correlation functions for molecular motion, pertinent to magnetic resonance spectroscopy, covers calculations of functions for molecular gases (B166). I n another significant article, time correlation functions and the linear response theory are reviewed thoroughly (B108). New definitions of relaxation and correlation times for non-exponential processes eliminate arbitrary factors

usually encountered (B307 and references therein). The first quantum correction to a time correlation function is reported in the expansion of the quantum mechanical correlation function in powers of Planck’s constant (B213). The relaxation functions of Anderson has been evaluated by Monte Carlo calculation (B372) and by a functional integration technique, and the results compared (B1.44). The effect of interdependent motional terms on a relaxing perturbation has been included in calculation of the absorption line shape by the Kubo and Tomita method (B18). Calculation of spin-lattice relaxation time has been extended to axially symmetric ellipsoids with internal motion (B433).

Relaxation processes in molecules having two non-identical, scalar-coupled spins are covered in a review article (B271). Spin-lattice relaxation is calculated for a system with three spin-1/2 nuclei, of which one has magnetogyric ratio different from the other two; a relaxation mechanism based on fluctuating dipolar interactions is used (B4.40). Spin lattice relaxation in the AX system, where A has spin 1/2 and X has spin 3/2, can vary among different lines of the spin-coupled multiplet structure, even when there is only one dominant relaxation mechanism (B.412). Proton relaxation for a scalar-coupled two-spin system in paramagnetic solution is described (B102). Thermodynamics of spin systems in solids has been reviewed in great detail (B219). A very good exposition on experiments in the rotating frame is presented in terms of the thermodynamics of the spin system (BSQ). Constant entropy as a function of time, for systems of interacting spins, is verified in a theory of orthogonal operators of the density matrix (B41.4). Interaction with Electrons. Experimental proof of the strong coupling between the electron spin-spin reservoir and a nuclear spin system in dilute paramagnetic crystals is offered by the simultaneous observation of identical spin-lattice relaxation times for the nuclei and electrons, in one example (B.422). Dynamic nuclear polarization via paramagnetic impurities in solids has led to studies of the relaxation mechanism. Single-relaxation-center and multi-relaxation-center models have been used to predict TI dependence on applied field, concentration of paramagnetic centers, and the spin diffusion coefficient (B266). Relaxation of such paramagnetic centers may cause nuclear spin relaxation or DNP, depending on the circumstances (Bbl). A “sphere of influence” model for electron spin diffusion was analyzed critically and found lacking, and further consideration led

ANALYTICAL CHEMISTRY, VOL. 42. NO. 5, APRIL 1970

425 R

to a stated desirability of having more experimental determinations of the dependence of the nuclear relaxation time on temperature, external field, crystal orientation, and magnetic impurity concentration (B247). Saturation transfer from one nAl line to another in Crdoped ruby is interpreted as a direct result of thermal contact between the Zeeman systems of lattice nuclei and spin-spin interactions of the paramagnetic impurity (B19). The possible formation of H - in semiconductors in a high magnetic field can lead to nuclear relaxation (BS7). A theory for polarization of nuclear spins by direct current in semiconductors has been extended to interaction of the electron flow with acoustic phonons, impurities, and nuclear spins (B.933). Nuclear magnetic resonance in transition metals has been reviewed (B38S). A broad interpretation of T1in liquid non-transition metals is noted (BS68). A number of papers on N M R of magnetic materials appears in the published proceedings of a meeting (BS40). A discussion of the semiconductor-tometal transition in n-type group IV semiconductors includes effects on Knight shift and T1 (B6). Gases. The study of nuclear spin relaxation in gases provides a source of information about intermolecular forces. The subject is reviewed with data and theoretical interpretation thereof (B46). The theory of spin relaxation for diatomic gases can be formulated from a formal kinetic theory of gases, giving a result equivalent to the time correlation function method (B80). I n a survey of T1for gaseous methane and deuterated forms, the spin-rotation interaction is found to dominate for protons, while the quadrupolar interaction dominates for deuterium; this article presents a rotational correlation function for freely rotating symmetric top molecules (B45). Theoretical treatment is given to magnetic field interactions in a rotating molecule (Bd11). A new effect is reported as “breaks” in a plot of (T,/density) us density for protons of CHF, and CH,F, in contrast to the anticipated straight line plot. The breaks are attributed to intramolecular dipolar interactions becoming important when rotational energy levels are collision-broadened appropriately (B119). A theoretical discussion of the relation between Tl measurements and collision frequency in dense gases is presented (B166). TI measurements have been reported for gaseous acetylene (BS) and HCl, HBr, and H I (B404) over a range of temperature and pressure. I n all cases, large effective cross sections are calculated for transfer of angular momentum, implying that long-range forces are important. 426R

Surface-induced relaxation of *He has been studied. At low temperature, adsorption on the surface dominates. At high temperature, permeation occurs into borosilicate glass and quartz, but not into aluminosilicate glass (B1.43). Liquids. The microdynamical behavior of liquids is studied by N M R relaxation times. The subject is reviewed with respect to both theory and results (B194). Relaxation mechanisms (BSS5) and measurement of correlation times (B124) are reviewed, also. An instructional article on NMR in liquids and solids is intended to compare NMR with slow neutron scattering for studying motions in condensed phases (BSSQ). The Bloembergen, Purcell, and Pound theory (BPP), relating relaxation time to viscosity via the dielectric correlation time, continues to serve as the basis for interpreting most relaxation data, despite certain inconsistencies. Singularities for highly viscous liquids are proposed to be explained using a theory which already has proven successful for explaining singularities in the propagation of sound, shear, and electromagnetic waves; the model here is that of a disordered medium in which ordered regions are situated (B76). It is shown that introduction of a correlation time for rotation, different from that for translation, improves B P P calculations (Bl78). The effects of anisotropic rotational diffusion on TI are calculated (B210). A “diffusion-distribution” is introduced (BS08). The theory of spin-lattice relaxation in classical liquids is summarized and extended (BS90). A detailed study of intermolecular relaxation in liquid ethane has shown that this relaxation is described very well by Torrey’s theory of relaxation by translational diffusion, assuming jump diffusion and introducing the effect of a radial distribution function; the arguments lead to measurement of a rootmean-squared flight distance ( B l 8 5 ) . A suggestion, that molecular diffusion in microscopically heterogeneous liquids should have the same effect on NMR as chemical exchange, was corroborated by observing increased spin-echo decay, but no TIchange, near the critical solution temperature for nitromethane in tetrachloroethylene and cyclohexane in deuterated aniline (B16 ) . Cross-relaxation in pure l-phenylpropyne has been studied carefully using different pulse sequences and temperature dependence (B69). Reorientation correlation time of the same material was obtained in another relaxation study

.

(BdS0)

A method is given to estimate the correlation time for slow molecular motions in liquids from the second and fourth moments of the absorption line (B386).


Solids. Magnetic resonance in hydrogen-bonded ferroelectrics is the subject of one important review article (B4.9). The study of water in hydrate crystals by NMR also is reviewed (B52). General reviews of NMR in solids (B60) and relaxation in solids (BSSS, BS68) are noted. A collection of papers on molecular dynamics and structure of solids includes NMR work (B74). A long-standing discrepancy has been resolved in showing that the (formerly) usual first-order approximation to dipolar broadening of resolved fine structure lines in solids is too small, and that the proper calculation gives a contribution to the second moment of the line which agrees with the Van Vleck value (BS26). The general moment-method for calculating diffusion has been r e viewed, and exact expressions obtained for second and fourth moments for bcc and sc lattices (BS60). The shapes of inhomogeneously broadened resonance lines in solids are the subject of a review article (BS96). A more complete treatment of interaction between non-equivalent spins predicts possible resonance frequency shifts which could be detected (B.935). Much attention is given to the spectra of pairs of equivalent nuclei which are capable of interchanging their positions, the usual case being water molecules of hydration in single crystals and polycrystalline samples Least squares analysis of pair-powders is demonstrated as a practical way to obtain the intra-pair separation between nuclei unambiguously; the observed spectrum is matched by calculating a theoretical spectrum described by two parameters, one of which depends only on the intrapair separation (B326). This method has been applied successfully to the NH2 groups of some amides and sulfamates (BS27). A “AH method” is proposed for easier interpretation of monocrystalline hydrates; this is a new application of the Pake formula, taking into account vibrational contributions, to obtain orientation and length of interproton vectors (BlOS). Relaxation by the correlated twofold reorientation of spin pairs has been described, with extension to multispin systems indicated (B321),and applied to ethylene and trans-dichloroethylene (BS20). The effect of twofold reorientation on line shape was considered elsewhere (B268). The line shape for two interacting rotating two-spin systems is obtained by direct quantum mechanical calculations for comparison with the statistical theory for line shape (B.967). A critical review of experimental data on free reorientation in crystalline longchain paraffins and substituted paraffins includes NMR results (B363). NMR and plastic flow methods gave the same activation energy for diffusion in crystalline adamantane (BS54). The use of

the relaxation function (Fourier transform of the absorption spectrum) is examined for analysis of internal motions in solids; some advantage is anticipated in that it makes optimum use of the line shape (BS61). A procedure has been developed for determination of the locations of shoulders and singularities in spectra of polycrystalline samples, in the presence of both quadrupole and anisotropic chemical shift interactions, for nuclei with half-integer spin number; in addition a computer program was devised to calculate the derivative spectrum for such a system, including simulated dipolar broadening (BS4). A moment method has been modified so that the NMR spectrum of a quadrupolar nucleus in single crystals can be studied usefully without prior knowledge of the orientation of the electric field gradient (B424). The theory of nuclear quadrupole spin-lattice relaxation in anharmonic crystals has been developed in terms of ordinary perturbation theory and phonon Green functions; first-order and anharmonic Raman processes are shown to dominate (B406). Free induction decay (B49) and equivalent Fourier transform (BS97) have proven useful in the analysis of quadrupolar effects in solids. The existence of relaxation by spinrotational interaction in ionic solids is claimed (B44), and requisite conditions discussed (B62) for hexafluoride ions. A correct way of averaging the effect of thermal vibronic motion on chemical shift in alkali halides, in the KondoYamashita overlap model, is formulated, and a general formula for temperature variation of chemical shift derived (BSOS). Two new techniques based on 2aNa NVR are discussed for studying ferroelectric properties of Rochelle salt (B314).

A phenomenon that has received

special attention is that of methyl rotation a t low temperatures. It is observed that even a t liquid helium temperatures methyl proton absorptions are unusually narrow, implying non-classical rotation. A tunneling process has been invoked. More detailed theoretical explanations now have been proposed, which account for maxima in the relaxation rate a t low temperatures due to tunneling-assisted rotation (B9, B189, and references therein). The general problem of separating methyl rotation from alkyl chain motions may be approached best by observing lacof CD8 groups (BSO9). Nuclear spin relaxation studies of “molecules adsorbed on surfaces” have been reviewed to 1967 (BS18). The concept of the adsorbed phase as a three-phase system, with regard to N M R behavior, has been advanced (B104); each phase-strongly adsorbed mono-

layer, compressed liquid in micropores, and unperturbed liquid in macroporesis characterized by its own transverse relaxation time, and exchange is fast among the phases. Application of this model to linewidth dependence on amount of adsorbate leads to determination of molecular orientation in the micropores (B105), and temperature effects are used to evaluate incremental energy of interaction of the adsorbed molecules with the surface and with one another (BlO7). Intermolecular relaxation of adsorbed molecules in a monolayer is calculated using the two-dimensional diffusion equation to obtain the correlation function; numerical evaluation then gives the dependence of T1 and T , on the diffusion correlation time and molecular lifetime in the surface phase (BS6). Freezing points of adsorbed species can be obtained readily by observing N M R line broadening. The apparent freezing points of benzene and water are lowered 10-20 degrees in micropores of gamma-alumina but are unchanged in the macropores (B106). Freezing point lowering also has been found in monomolecular layers on fine powder surfaces (B2.38). Chemical shifts to high field are seen consistently for both protons and fluorine of organic liquids on carbon (B17 1 ), on silica gels (B160),and for protons of ethylene on a zeolite (B.299). An NMR “titration” for specific surface area has been performed by interpreting the onset of peak narrowing for adsorbed polar vapor as the endpoint of formation of monolayers (BS96). The wettability of several dispersed substances has been characterized as the ratio of benzene-to-water T1values for the substance saturated with each liquid; correlation with the heats of wetting for the same liquids is rather good (B67). CHEMICAL SHIFT AND COUPLING

Chemical Shift. A method for computing the average diamagnetic shielding a t a nucleus, from the free atom shieldings and a knowledge of the molecular structure, is presented and applied satisfactorily to a number of small molecules (B147). A semiempirical method for calculating shielding, polarizabilities, magnetic susceptibilities, and other parameters is said to avoid certain difficulties by use of an equation of weighted averages (B281). The magnetic (9) tensor, frequently asserted to be symmetric, is shown to possess an asymmetric contribution which can be detected by sufficiently accurate determinations of the angular dependence of chemical shift (S380). A theory for calculating the paramagnetic term in the Ramsey formula for shielding is based on a hydrodynamic generalization of the Thomas-Fermi model (B.266). A theoretical investiga-

tion shows that in atoms and molecules with unpaired electrons, the electron spin affects nuclear shielding in molecules but not in atoms, and has no effect on diamagnetic (temperature-independent) susceptibility of either atoms or molecules (B441). The theory of vibrational effects on the nuclear shielding and spin-rotation constants of diatomic molecules is outlined, then applied to H and F of H F ; excellent agreement was found between the calculated and measured deuterium isotope shift, and a proton shielding constant of 29.2 ppm was obtained for the non-vibrating molecule a t its equilibrium internuclear distance (B196). Gaseous H F is found to be exchanging rapidly even a t -26 “C, and chemical shifts were obtained for H and F in monomers and hexamers (B272). Experimental studies of chemical shifts of saKr, IZgXe, and IalXe in all three phases are reviewed and compared with theory (Bb7). A detailed calculation of the linear electric field effect on nuclear shielding in AB bonds employs the Born-Oppenheimer approximation; application is made to C H bond (B1S). A calculation of the magnetic shielding of a hydrogen molecule in an electric field predicts observable shift between the protons for an electric field of 106-107 V/cm in the direction of the magnetic field (B176). A second-order equation in electric field a t a fluorine nucleus, due to polar bonds (except C-C) in the molecule, is found to predict shifts fairly well (B205). Evidence is offered for the existence of an electric field effect on the shift between the vinyl methylene protons of some 4-substituted styrenes (B182). Proton Chemical Shift. A review article covers the relation of proton shifts to inductive and mesomeric effects, to IaC chemical shifts, and to charge distribution (B278). The proton diamagnetic shielding term has been calculated for a C-H fragment with digonal, trigonal, and tetragonal carbon hybridization, and discussion given to influence of free electron charges on carbon and free valence indices (B402). The influence of overlap integrals in calculations of diamagnetic anisotropy and proton shielding in aromatic molecules is studied (B7O). Calculation of the chemical shift of a series of polyenic ions by the ‘free electron model’ indicated a proton chemical shift of ca. 11.5 ppm/ unit charge on carbon (BS16). Hydrogen atoms subjected to signscant steric compression usually are deshielded by the interaction. A satisfactory explanation of steric interaction with another proton relates the magnitude of the steric shift to the component of the proton-proton repulsive force directed along the H-C axis of the observed proton (B81).


0

427

R

The correlation between proton shifts and LCAO index of free volume in some alternant and non-alternant hydrocarbons is found to be simply a fortuitous consequence of geometry, implying no causative relation of chemical shift to other molecular orbital indices of chemical reactivity (B30). A similar conclusion is reached in comparing HMO calculations of 7-electron densities in purine analogues with proton shifts, base strengths, and reactivity (B270). Correlations are reported between shifts of substituent protons (methyl and other alkyl) and CO vibrational frequencies in benzophenones, benzhydrols, and benzhydryl ethers (BS64). The proton shifts of tertiary-butyl groups in a large number of derivatives showed only small solvent effects and no close linear correlation with such properties of substituents as electronegativity, substituent constants, molecular dipole moment, and expressions involving bond polarizations and first ionization potentials (B427). l8C and H chemical shifts in simple compounds (CH,).M and (CH3CH2),t4 correlate with electronegativity and number of lone pairs on M (B65). Calculations of proton shifts and magnetic anisotropy in benzene have been made, varying effective charges in atomic orbitals, C-C, and C-H bond lengths ( B 7 l ) . Proton shifts have been compared with calculated 7-electron densities in substituted styrenes and stilbenes (B120, B418), phenanthrolines (BS29, B366), pyridine derivatives (BS30) and benzoxazole derivatives (BS15).

More complete calculations have been reported for pyridine and pyridinium ion C , H, and N (B129),C and H of pyridine (B400), thiazoles (BSSS), and all even alternant pentacyclic hydrocarbons

(B85).

Other Chemical Shifts. Absolute fluorine shieldirigs have been measured for SiFc and Fz,and the diagmagnetic and paramagnetic contributions obtained (B197 and reference therein). Experimental data for proton and fluorine shielding anisotropies have been reviewed nnd theory presented (B98). A theoretical treatment of the 19Fshifts in aryl fluorides gave unsatisfactory correlation with n-electron density on fluorine, but satisfactory correlation with the number of atoms ortho to fluorine that carry p- or 7-electrons (B112). CNDO calculations of the principal components of the magnetic shielding tensor of IgF were performed for some fluorinated benzenes and pyridines (B13O). Correlations of chemical shifts with inverse ionization potentials, in vinylidene fluorides and ethylenes, are reported, with suggestion that the paramagnetic effect operates through bonds rather than through space (B374). Increasing interest in 1aC shifts is 428 R

noted. Various average energy approximation methods have been applied to calculation of laC shifts in six-membered ring nitrogen heterocycles (B2). It has been suggested that variations in the average excitation energy are most likely to give satisfactory explanation for lac shifts in pyridine, phenyllithium, and phenylmagnesium (B232). Linear correlation of 13C shift with calculated 7-electron density is found for C ( l ) in substituted benzonitriles (B417), but only with total charge densities in azines and five-membered heterocycles with nitrogen and oxygen (B47) and monosubstituted benzenes (B48). Theoretical treatment of observed nitrogen shifts is presented (B434). CNDO calculations are given for nitrogen shielding in some small nitrogen ions (B192). An analysis of earlier theoretical treatments of phosphorus chemical shifts leads to the conclusions that tricoordinate phosphorus shifts are determined mainly by hybridization of the unshared electrons; participation of delectrons determines shifts of 4-,5-, and &coordinate phosphorus; and a change in the state of the p-electrons in the a-bonds has no appreciable effect on the shift (B216). Indirect Spin-Spin Coupling. The search for manageable approximate methods to calculate indirect spin-spin coupling constants continues. But, in the words of a reviewer of proton-proton coupling, “ I t is clear t h a t a t the present state of the art, the empirical approach, which amounts to argument by analogy, is less likely to lead to errors of interpretation than any attempt to apply the, necessarily simplified, theoretical treatments” (B39S). Even though discussion of many electron systems can be simplified by recognizing distinct groups such as bonds, U-electrons, and 7-electrons, configuration interaction among such groups must be introduced for an adequate description (B26). A similar conclusion is reached in another paper, where semiempirical methods for calculating couplings are found to be inadequate ( B l 7 ) . The perturbation-variation method has been applied to accurate calculations of coupling in HD (B16, B S B ) . A new procedure for perturbation-variation calculations accounts explicitly for singular behavior near the origin (B410). A finite perturbation method has been introduced for calculating self-consistent molecular orbital wave functions in the presence of small but finite perturbations (such as distortion of electron distribution by nuclear spin) (BSS6, B337). Moderate success in calculating Fermi contact contribution to coupling with semi-empirical MO wave functions (B338) encouraged an attempt to use single determinant ab initio wave functions, which gave poor correlation with


experimental magnitudes and trends (B317).

A parameterized LCAO-SCF ?VI0 theory which includes overlap, applied to Fermi contact contribution to coupling, gives agreement in sign and fair agreement in magnitude with experimental values of one- and two-bond couplings in a number of small molecules containing H, B, C, N, F, Si, and P (B89, B90).

An often-used expression for H-H coupling by n-electrons is shown to be wrong, and corrected ( B l l 7 ) ; the effect was to overestimate such coupling (B239).

Valence-bond formalism has been extended by introduction of a truncated matrix sum method to avoid the use of an approximate mean excitation energy (Bag), and reasonable agreement obtained with experiment in the calculation of long-range 7-electron coupling (B25). Other valence-bond calculations of coupling by 7-electrons used double perturbation theory ( B l l 4 , B116), and were unsatisfactory, without adjustment, for long-range coupling in 1,3butadiene (B96). Mechanisms of contact spin-spin coupling have been classified and described in terms of a valence-bond bond-order formulation (BW7) with discussion of “through-bond” and “through-space” concepts. Throughspace and through-bond contributions to 4 Jcoupling ~ ~ are obtained separately (B72). Valence-bond variational calculations of proton-proton couplings in ethylene, with u-7 exchange implicitly included, gave very good agreement with experiment (B79). Evaluation of coupling constants in a number of small molecules by the truncated sum approximation of Pople and Santry, using nonempirical 110 wave functions, gave trends similar to those from semiempirical wave functions (B240). The similar extended Huckel theory of Hoffmann is found t o be superior in correlating the calculated coupling constants of methane, ethane, ethylene, and acetylene with experimental values (B223). Other M O calculations of couplings in these same molecules are reported (B116, B158). A simple MO theory for H-H coupling in hydrocarbons reproduces the proper trends and reasonable magnitudes (B118).

A comparison is made between proton hyperfine interaction in n-radicals and spin-spin coupling, whose theoretical descriptions are similar, in a study of spin polarization in an aliphatic fragment (B269). The effects of lone electron pairs on coupling constants involving adjacent bonds have been examined for geminal protons of CH2 attached to sulfur, where eclipsing causes a more positive coupling by 2-3 He ( B l l ) , and to oxygen and

nitrogen (B86, B91). Nitrogen lonepair effects on C H coupling (B162) and on three- and four-bond H-H coupling in some 2-pyrazolines (B20) also are reported. The sign of HCOP coupling appears to be governed by phosphorus lone-pair orientation (B156). Large four-bond proton-proton coupling often has been observed in planar TY conformations, and ascribed to likely overlap between the small posterior lobes of the carbon orbitals in the bonds to the coupled protons. The largest such coupling (18 Hz) has been seen between the bridgehead protons of bicyclo [1.1.1]pentane and derivatives. The X-ray single crystal structure analysis of one derivative showed that the distance between bridge5ead carbons is unusually short, a t 1.89A, and the C-H bonds colinear, thus lending strong support for involvement of the abovesuggested interaction (B319). Although the sign of couplings through the planar W path is positive, it may become negative in some non-planar situations (B77). Further observations of large F-F coupling for small F-F separation lend further support to “through-space” interaction (B229, B382). Substituent effects on geminal H-H coupling in CH2 groups (B163) and in group IV hydrides (B12.3), and on vicinal H-H coupling in vinyl compounds (B161) have been discussed in terms of simple MO theory. Previous correlations indicating direct proportionality between coupling and substituent electronegativity are valid only because the range of values was small (BS75).

Some calculations of coupling in polycyclic hydrocarbons gave agreement with observed ortho coupling but not with meta or para (B273). There seems to be a linear relation between the long range ring-proton methyl-proton coupling and square of the mobile bond order (B42). Characterization of one-bond I3C-H coupling in organic molecules continues to be a popular subject. A perturbation expression for the coupling includes electron correlation, which is found to contribute significantly, and inverse proportionality to the carbon valence number is estimated (B198). This work later was applied to substituent effects, where resonance interaction among chemical bonds equivalent to the C-H bond is found to cause a reduction in the C-H coupling, in agreement with observed behavior in methanes ( B I 9 9 ) . Excitation energy from ground to lowest triplet state for a series of substituted methanes is found to agree qualitatively with those calculated using Coulomb and resonance integrals (B97). I n applications of the Pople-Santry MO theory, correlations with s-character of the carbon bond were found to be somewhat fortuitous (B164); electronega-

tivity of substituents may affect C-H and Si-H coupling mostly through variation in the Coulomb integral (B113); and calculations were made of a large number of C-H and Si-H couplings in methanes and silanes (B221). The one- and two-bond C-H couplings of protonated formic acid are opposite in sign (B203). The signs of the reduced one-bond C-H and Si-H coupling constants are the same in CH3SiHC12 (B286). An unexpectedly large apparent anisotropy in the C-H and C-F indirect coupling constants of CH,F has been reported (B250). Proton-Proton Coupling. A definitive review of the correlation of proton-proton coupling constants with structure of organic molecules covers the literature well into 1968 (B393).

The geminal coupling constant of protonated formaldehyde was found to be +21.7 Hz, and the vicinal couplings also positive, in a study of hydroxy- and alkoxycarbonium ions (B423). The geminal coupling in acetals is sensitive to solvent and concentration (B348). Vicinal HCOH coupling constants follow the Karplus relation with respect to dihedral angle, in a qualitative fashion (B163, B224). An earlier-reported apparent deviation from this relation in di-tert-butylcarbinol, based on steric arguments, is rationalized by an alternate steric argument based on general trends in related systems (B18.3). Both HSCH and HSCCH couplings are positive, being qualitatively similar to the corresponding couplings through carbons (B68). The coupling between protons of geminal methyl groups is shown to be positive for (CH3)4C, ( CHI)ZO, (CH3)2CO, and (CH3)zSO (BI,U),but negative in (CH&C: CHOC(O)CH3 (B371). Observable coupling is reported between meta methyl protons in substituted benzaldehydes (B212). The fivebond coupling from methoxyl to orthohydrogen is negative in a n anisole (B167), and enhanced by a bulky substituent ortho to the methoxyl (B92). The alpha-alkoxy protons of alkoxy vinyl ethers are coupled observably to all three vinyl protons (B138). Atypical long range paths for coupling are proposed in 1,3-thioxanes and 1,3dithianes (B159). Correlation is found lbetween coupling constants and substituent electronegativity in 2-substituted thiophenes (B66). The meta coupling through nitrogen is found to be positive for pyrazine N oxide, in contrast to negative values for other aromatic nitrogen heterocycles (B998).

The sign of the four-bond allylic coupling constant is shown to be important for conformational analysis (B177). Proton-Other Nucleus Coupling. The one-bond C-H coupling in CHaX

compounds is correlated with X electronegativity and C-X bond length (B439). Coupling constants in HCN are 274 Hz for H C and 8.7 Hz for H-N (15) (B39). Two-bond HCC couplings have been studied in terms of substituents and hybridization (B293). The signs and magnitudes of HCC couplings in some substituted ethylenes have been determined (B381, B421). All H15N couplings in formamide are negative (B83). The relative signs of all couplings in NHID have been measured, and isotopic chemical shifts observed (B226). The signs and magnitudes of two- and three-bond H-N couplings in oximes and quinoline have been given full attention in terms of solvent effects and stereochemistry (B94 and references therein). The H N coupling in anilines correlates well with Hammett substituent constants (B21, B54)

The dihedral angle and bond angle dependence of vicinal H-F coupling have been characterized well (B4SO). Studies of four-bond H-F couplings show largest values for planar W conformation (B148, B429). Stereochemical requirements for five-bond H-F coupling have been studied in a rigid tricyclooctane (B220). Examination of the literature on P-H and P-F couplings in alkyl phosphines reveals anomalies that seem to be related to changes in molecular geometry (B282). The variation of HPC coupling with angles in phosphines has been described graphically (B4, B 5 ) . The vicinal HPOC coupling in phosphates also depends on dihedral angle in a consistent fashion (B180). H-C and H-P couplings are reported for a number of organophosphorus compounds (B288). The signs of P-H and P-F couplings in triphenylphosphine derivatives have been characterized (B289). Similar information is reported for allenic derivatives of phosphine and phosphine oxide (B.392). Solvent effects on the one-bond H P coupling in (MeO)zP(H)O and related structures are attributed to dimer breakage and solvent hydrogen-bonding to the oxygen (B409). I n a study of beta-substituted-ethyl tin compounds, trends in the H-Sn coupling were attributed largely to Fermi contact interaction, with significant contribution from electron orbital term as well (B408). The HCSn coupling in methyltin halide-pyridine complexes is said to be related to bond strengthening rather than to s character of the Sn-C bond (B405). Many couplings were measured and compared in a study of coupling in methyl, silyl, and hydride compounds of Group VI elements (B.331). Signs and magnitudes of the coupling constants are reported for dimethyl cadmium (B122). The RhH couplings in some Rh(II1)-silyl complexes are 20-30 Hz


429R

(B188). Magnitudes and relative signs of H-Pt couplings are reported for olefin complexes (B227) and square planar complexes (B285)of Pt(I1). The H-Hg coupling constants of CH3HgL are related to nucleophilic reactivity and proton basicity of nucleophile L (B377). A study of the various couplings in organic phosphorus and silicon compounds compares magnitudes and signs for tetramethylphosphonium, trimethylphosphine, and phosphines with P bonded to C, Si, and Sn (B128). Other Spin-Spin Coupling. I n a study of hexafluoroethane the vicinal F-F coupling was found to be 3.50 Ha, presumably positive, and the F C and FCC couplings and 13C isotope shifts determined (Bl73). The vicinal F-F couplings in trifluorochlorocyclobutanes change sign twice as the dihedral angle varies through B O 0 , being negative for small and large angles and positive for intermediate angles (B133). The relative signs of couplings among F, H , P, and C were determined in a series of perfluoroalkyl phosphines (B274). Sign reversal is found in the geminal FCP coupling on going from CFz: CFPClz to CFz: CFPFz (B88). Evidence for a positive hyperfine interaction in the CF bond is found in the signs of couplings from ring F to CH3 protons in fluorotoluenes, being opposite to the corresponding ring H to CH3 couplings (B41). The one-hond B-F coupling constants of BFs and derivatives show a linear correlation with fluorine shift, and change sign on going from BF3 to derivatives (B140). The Be-F coupling constant and fluorine exchange were studied in BeF4(B1S7). A plot of fluorine shift us Xe-F coupling in xenon fluorides gives a smooth curve predicting the as-yetundetected coupling in XeFs to be 300 i= 200 H Z (B149). The one-bond C-C couplings in (CH3)3CX are in the range 36-40 H Z and positive for substituents CH3, "2, OH, C1, and Br (B261),and are less than half that magnitude between ring carbons of cyclopropanes (B419). C-C couplings in acetyls are studied (B174). Discussion is given to the C-P coupling in organophosphorus compounds (B283). The one-bond coupling of 13Cto zo7Pbis positive and the geminal coupling negative (B287). The coupling between directly bonded phosphorus atoms is f465.5 H Z in the diphosphite ion, and known to be negative in other cases; the variations are attributed to substituent and bulk effects on the outer shell s-electrons (SlC1). Geminal P N P couplings are unusually large (B305). Geminal P-P couplings between phosphine ligands have been reported and related to various bonding contributions (Sa9 and references therein). Other studies report direct coupling between phosphorus as phos430 R

*

phine ligands and Hg (B243), Pt (B8), and W (B176). O(17) coupling is 30 Hz to 65Mn in MnO4- and approximately 22 Hz to the attached in acetone (B64). Coupling constants and their signs for T7Se are reported (B290). Isotope Effects. A theoretical treatment of deuterium effect on C-H coupling in deuterated methyl groups predicts a n increase of 2.5 Hz for methane, due to changes in carbon 2s character averaged over the H-C-H bending vibrations (B367). A deuterium isotope effect is reported on geminal H-F coupling through carbon and through silicon (Bas). A large deuterium isotope effect on T1 for lOB and l1B in diborane may be related to' a large deuterium effect on one moment of inertia (B10). Most isotope effect studies have dealt with chemical shifts. A review of the subject presents correlations and theoretical explanations (B31). Solvent isotope effects of H20/D20 on halide and alkali metal ions in aqueous solutions range as high as 13 ppm shielding with deuteration. Similar effects also are caused by introduction of 1 8 0 into the water. Alterations of the interactions between the ions and their hydration spheres, by changes in molecular vibration amplitudes, are implicated (B264). Hydrogen bond strength also is involved (B181). Special attention has been given to 19F shifts, in H20/D20J of alkali halides (B110) and hydrofluoric acid (B52, B376). The fluoride shift of K F has been proposed as a reasonably sensitive measure of the H/D ratio in HzO/DZO mixtures (B109). Directly bonded deuterium causes sizeable shifts from the proton species of alp in phosphines (B145),of N and B in NH4+ and BH4- (B386), and 15N in ammonia (B260). Long-range deuterium isotope effects are reported over as many as three (Barr) and four (B12) sigma bonds. Examination of deuterium as an aromatic substituent, via ring fluorine shifts, led to the suggestion that the observed shifts arise from intramolecular interactions rather than medium effects (B.438). 34S isotope shifts of fluorine have been reported in 2-fluorothiophene (B363) and HS03F (B394). llsSn/ll7Sn isotope shifts were measured in the proton spectra of tetravinyltin (B267). Aromaticity and Aromatic Systems. I n biphenylene, significant shielding of the alpha carbon and deshielding of the tertiary carbon are attributed bo a para-magnetic ring current in the fourmembered ring (B231). A paramagnetic ring current is demonstrated in 2,8,10,16 - tetrahydro [17]annulenone (B61).Superaromaticity is claimed for


coronene on the basis of proton chemical shifts (B84). It has been noted that aromatic methyl group effect on shift of ortho ring protons is transmitted more strongly through the aromatic C-C bond having more double bond character (B313). Relative chemical shifts in pyramidal boron compounds are found to correlate reasonably well with a conical ring current model, although it is recognized that such ring current need not be the main factor in the total shifts (B280). I n a study of the transmission of electronic effects from substituent to ring protons, including a literature survey of Hammett correlations with NMR data, it is concluded that such correlations are too poor to have real value. Even when acceptable correlations are found, their theoretical significance usually is obscure (B4S7). Empirical relationships are reported between proton shifts and HMO chemical reactivity indices for alternant hydrocarbons; the data split into two independent groups-non-angular and angular protons (B251). An empirical procedure for predicting ring proton shift in any substituted benzene is quite useful (B351). An additivity scheme based on pair-wise parameters describes the ring proton shifts of polyhalogenated benzenes in cyclohexane and in CC4 (BS69). Methyl and ring proton shifts are reasonably correlated with each other in ring-substituted toluenes, except for halogen substituents (B310). A11 proton-proton coupling constants in mono- and dihalobenzenes have been correlated linearly with the halogen electronegativity; the substituent effects are additive and transferable for meta and para isomers, and additive but not transferable for ortho isomers (B399). I n para-substituted benzyl compounds, the benzylic proton shifts correlate linearly with Hammett u value of the para substituent, with slopes of the plots being related to the most probable conformation of the sidechain (B152). The methyl proton shifts of toluenes bearing phosphorus substituents on the ring are related linearly to Hammett u constants (B378). The hydroxyl proton shifts of orthosubstituted phenols are linearly related to the para-substituted values in dimethyl sulfoxide solution, indicating apparent freedom from steric effects of the substituent because hydrogen-bonding is strong to the solvent; as a result, Hammett u values are deduced for ortho substituents, which should afford a decent measure of electronic effects (B40S). Ortho acetyl and nitro groups did not fit this correlation because intramolecular hydrogen-bonding persists in DMSO solution. This problem was treated in another study of ortho-nitrophenols, salicylaldehydes, and ortho-

hydroxyazobenzenes, where substituent effects on the OH shift were resolved into contributions through the 0-H bond and upon the electron donor involved in the hydrogen-bond (B36). A great deal of attention has been given to aromatic fluorine chemical shifts and coupling constants as affected by substituents. Shifts and couplings in a large variety of pentafluorophenyl derivatives were examined carefully, revealing a linear correlation between the ortho-para F-F coupling and parafluorine shift, attributable to their primary dependence on r-electron donoracceptor properties of the substituent (B20.9). Close examination of the above meta-coupling/para-shift relation is given to evaluating the r-bonding character of halogens, phosphines, group IV derivatives of oxygen and sulfur, and N - substituted pentafluoroanilines (B201). Group V pentafluorophenyl derivatives have been reported (BSO4). Chemical shifts and couplings of fluorine in fluorinated pyridines are tabulated, and compared with corresponding substituent effects in benzenes and with calculated r-electron densities and couplings (B13I). Additive substituent effects on H-F coupling (B26'3) and F-F coupling (B1) in fluorobenzenes have been reported. Proton Spectra. Some larger tabulations and correlations are noted for tertiary butyl derivatives (B427), methyl and ethyl borane derivatives (B312), and cyclobutene systems, with assessment of methyl and phenyl substituent effects (B135). Correlation charts for acyclic methine proton shifts (B435) and proton chemical shifts of oxygenated unsaturated aliphatics (B78) have been published. The spectra of twenty-one amino acids in trifluoroacetic acid have been analyzed (Bas). Monosubstituted cyclopropane spectra also have been analyzed fully (B93). A detailed study of proton shifts and couplings to Si and N in silyl amines is presented (B362). Proton characterization of over eighty furan derivatives is reported (B322). Temperature dependence of chemical shifts and coupling constants in vinyl halides and ethers has been measured carefully; proton shift changes are attributed to molecular vibrations, modified by rotational isomers in the ethers, and only H-F coupling was found to change (B56). Boron-11 Spectra. A review of llB N M R spectroscopy covers the field very well (B191). NMR spectroscopy of boron compounds is surveyed ( B I N ) . Constitutive IlB chemical shift parameters are presented for substituent effects in amineboranes, and compared with similar parameters for '*C shifts (B343). The boron chemical shift of borate-boric acid solutions has been studied over pH range 2-12; extreme

sensitivity to pH between 7 and 10 is due to equilibrium between H3BOs and B (0H)r- (B208). Carbon-13 Spectra. Carbon-13 chemical shifts and coupling constants are the subject of a recent review of developments in this rapidly-growing area of investigation (B297). A model for steric perturbation of shift includes angular and distance features of H-H non-bonded interactions (B172). A valence-bond formulation for carbon shift rationalizes observed trends in simple alkanes (B82). Judicious use of the average activation energy approximation and orthogonal wavefunctions was required to calculate reasonable 13C shifts in nitrogen heterocycles (B2). Correlation of lacshifts of halogenated methanes required inclusion of pairinteraction terms (B262). Carbon shifts of five-membered nitrogen heterocycles, including their charged species (B342), have been studied (B42O). Similar attention also has been given to six-membered nitrogen heterocycles (B341), especially substituted pyridines (B355 and references therein). Chemical shifts have been examined in disubstituted cyclohexanes (B258) and a variety of other alicyclics (B328). Effects of 0, S, Ge, P, and Si substituents on acetylenic carbon shifts are reported (B365). Cis-and trans-substitution effects on lacshifts of unsymmetrically substituted ethylenes are explained on the basis of C-C r-bond order (B373). Nitrogen Spectra. Nitrogen N M R spectroscopy was reviewed recently (B296). The 15N chemical shifts of NH3 as liquid and vapor over the range -70 to f35 OC are reported (B259). The 14N shifts of a number of aliphatic amines and ammonium ions have been examined; it is noted that there is no satisfactory theoretical explanation of nitrogen shifts in saturated systems (B432).

Nitrogen chemical shifts in ureas, amides, and corresponding thio- and seleno-derivatives, seem to be governed by the extent of delocalization of the nitrogen lone pair (B184). The shifts of a large variety of sp2-hybridized nitrogens are correlated empirically with theoretical ground state properties (B431). The 14N shifts of some halogenated pyridines show a satisfactory correlation with their calculated r-electron densities (B193). Fluorine-19 Spectra. A general review covers high resolution literature of 1966 (B295). Other review articles present tabulations of literature and unpublished shifts and couplings in nitrogen-fluorine compounds (B55) and of published shifts and couplings in fluoroxy compounds (BZOO). The fluorine spectra of perfluorobutane (B187) and hexafluoro Dewar benzene (B75) have been analyzed fully.

Shifts and couplings for a number of fluorinated ethanes are correlated empirically with nature of the substituents (B100). I9Fand 29Sishifts and couplings have been determined for some fluorosilanes (B226). The fluorine spectra of a large number of Sn(1V)-fluoro complexes of general formula SnF&,Xn2-, and related, are given and described in terms of characteristics of the substituents X and their placement relative to the fluorine atom ( B 9 9 ) . Phosphorus-31 Spectra. Rather complete coverage of phosphorus N M R is given in a series of review articles on quantum mechanical theory of shifts (B257), interpretation of experimental spectra (B407), and a compilation of data (B275). The phosphorus NMR spectra of coordination compounds are covered well in a recent review (B306). The phosphorus shifts of a large number of phosphines, phosphonium salts, and diphosphinonickel(I1) chlorides have been tabulated (B146). A linear correlation is found between shifts and acid dissociation constants of substituted diphosphonic acids (B17O). Other correlations with substituent constants are found for phosphinyl compounds (B379 and references therein). The phosphorus and proton spectra of phenyl phosphonates and phosphinates, and the sulfur and some selenium analogs, have been investigated fully (B284). Other Nuclei. Chemical applications of 1 7 0 NMR and electron spin resonance are covered in a review (B391) . 25Mg spectra are reported a t 5 MHz, in aqueous solutions of MgBrz, MgC12, Mg(N03)2, and Mg(C104)2 (B127). Mixed tetrahaloaluminate ions were found in CH2C12solutions of A12X6and R4N+X- v i a nAl NMR spectra (B248). The 29Si and lI9Sn shifts of alkyl silicon and alkyl tin compounds are shown to correlate with electronegativity of substituents and presence of ( p + d ) r-bonding (B209). Rhodium-103 N M R of Rh(C&l& PzCOCl and Rh(C6H5)aP&1 has been recorded by INDOR, observing one phosphorus line while decoupling the phenyl protons (B63). m5Tl chemical shift has been studied in thallium borate glasses (B294). Contact Shift. Large observable shifts in nuclear resonance frequency occur when the nucleus experiences Fermi contact interaction or anisotropic dipolar interaction with a n unpaired electron spin undergoing rapid relaxation. Most studies have observed proton shifts of ligand molecules on paramagnetic ions Ni(II), Co(II), V(III), and Fe. A theoretical analysis


431 R

for ligand nuclei in paramagnetic distorted octahedral co*+ is given (B.2.2.2). Mechanism of spin delocalization to the ligand nuclei is not always clear. Transfer from ligand *-system into orbitals of u symmetry has been attributed to hyperconjugation (B186). I n other studies delocalization from the metal unpaired spins to ligand ?r and/or u donor electrons were examined (B14, B 1 4 q B.254, BS56, B486). Abnormal ligand proton shifts are taken to indicate strong direct interaction between sterically favored protons and unpaired electron on the metal as a likely additional mechanism (B154). Hyperfine splitting constants of radical ions in solution can be measured by contact shift (B69, B398). Large shifts of the cation with a Co complex are attributed to pseudocontact interaction with the Co, and related to models of the ion pair (B.253). All six distinguishable diastereoisomers have been resolved in paramagnetic Ni(I1) complexes with four asymmetric ligand centers ( B l S d ) . Ligand exchange rates (BSS4) and electron exchange rates (B4d8) have been obtained from NMR contact shifts and line width. Pseudocontact shifts are reported for 14N in paramagnetic complexes (B29.2). KINETIC PROCESSES

General. The most significant development during this period is the presentation of unified theories for lineshapes of high resolution spectra under arbitrary conditions of interand intra-molecular exchange processes and spin-spin coupling (B38, B167, B190). Their use is illustrated for intramolecular reaction of two AB systems (B190) and a n ABC system where A and B undergo mutual exchange (BS8). A point is made that lineshape fitting of such complex systems over a large range of exchange rates will give more accurate activation parameters than analysis of simple systems because coalescence of lines, where there is greatest sensitivity to exchange rate, occurs a t various exchange rates (temperatures) rather than a t just one (BS8). A density matrix method was developed for chemical exchange and transient effects in an AB system undergoing intramolecular exchange (B168). Spin-echo study of proton relaxation in formaldehyde allowed simultaneous examination of relaxation rates, chemical exchange, and spin-spin coupling (B136).

Lineshape calculations for exchange in AB systems have been given close attention. Exchange of AB systems between two different environments can be handled simply when coupling in the two sites is the same or when the two sites are equally populated and 432 R

differ only in chemical shifts (B2.28). Exchange of a species between identical conformations is described in one report as the sum of coalescence of partial spectra, and illustrated by the system AA'BB' F! BB'AA' (BIOI). Some examples of exchange between two types of AB systems have been reported (B155).

Spectra of two exchanging AX systems are described in a study of medium effects on the NMR and thermodynamic properties of ~,1,2,2-tetrabromo-fluoroethane (B169). A study of the comparable system, 1,1,2,2-tetrachlorofluoroethane internal rotation, was performed using high resolution and pulse spectra of both hydrogen and fluorine in conjunction with a comprehensive computer line-fitting program (BY). Iterative complete line-shape fitting was used to determine rate of trans-gauche interconversion in 1,2-difluorotetrachloro ethane over a large range; this careful study included good treatment of error considerations (BSO2). Exchange in virtually coupled systems, exemplified by A' exchange in systems XAA' and XtAA' with J A A>~> J A >~ J A ~ XisJ shown to cause the X spectrum to be a single line a t intermediate exchange rates as it goes from the triplet structure a t slow exchange rate to doublet a t fast rate ( B l S 4 ) . The matter of reliability of NMR methods for quantitative determination of rates continues to receive critical attention. Instances of apparent contradiction between rates of configurational inversion as obtained by NMR and polarimetric studies are cited, and careful attention given to possbile reasons (B7S). These authors point out that careful attention must be given to all the possible routes by which the observed NMR behavior may be accomplished, as compared to the processes which can contribute to the polarimetric behavior. A case in point is loss of nonequivalence of diastereotopic protons being attributed to fast ring inversion, without considering that solvent effects might cause accidental equality of shifts. Good agreement of activation parameters was obtained for the internal rotation of N-methyl-N-benzylformamide as determined by NMR lineshape analysis a t 90 to 170 "C and by equilibration methods from -2.5 to 20 "C (B179). Line shapes for exchange among many sites can be computed rapidly, by hand or by computer, by a simple re-expression of the Piette and Anderson equations to involve only one summation (BS11). Computer programming is given for such exchange processes (B436).

Truncated second moments of exchange-broadened N M R lines can be used to determine exchange rates to an accuracy intermediate between sim-


ple, approximate methods and complete computer fitting of the line shape (B195). Activation energies can be estimated in systems involving collapse of more than one set of signals, from an Arrhenius plot of coalescence exchange rates and coalescence temperatures (B276). Graphic three-dimensional surface representations of spectra as a function of exchange rate are an effective way to present the complete range of calculated spectral behavior compactly (B4.25).

Exchange Processes. "Chemicalexchange spin decoupling" of phosphorus from hydrogen in some P-0 containing solvents, induced by dissolved paramagnetic co*+ (B150) and Ni2+ ( B 1 5 f ) , had been studied quantita tively , Heteronuclear double resonance has proven useful in evaluating equilibria between other a-bonding ligands and dimethylsulfoxide in square-planar platinum complexes (B291). Double rate processes were ascribed to dimeric niobium and tantalum pentaalkoxides, with different activation energies obtained for exchange between two types of terminal group and between terminal and bridging groups (B5S). Two exchange processes were detected in two spirophosphoranes, the slower one presumably being pseudorotation (B207). hlethods of line width measurement are given in kinetic studies of reactions involving triphenylarsine and triphenylphosphine as ligands on I r (B41S). Intramolecular Rotation. Studies of many kinds of hindered intramolecular rotation by N M R methods have been reviewed, with a summary of numerical results (BSOO). Modulation effects in the low-temperature 19F spin-echo measurements of l,l-difluoro-1,2-dibromodichloroethane, caused by internal rotation, were analyzed to obtain rotation rates and activation energy (B411). Rotation about the C: C double bond of some enol ethers, MeO(R)C: C(CO0Me)2, is supported by NMR evidence, with barriers in the range 18 to 28 kcal/mol (BS87). A number of examples of sterically hindered rotation about the single bond connecting sp3 and sp2 carbons, with general formula Ar-CR3, and single bonds sp3-sp3, were studied quantitatively (BS60). A cogwheel effect has been offered as a possible mechanism for rotation of the aromatic ring of the highly hindered dimesityl carbonium ions, where the ortho methyl groups remain equivalent even a t -60" (B.258). Slow rotation about benzene-oxygen and benzene-sulphur bonds is indicated by observation of non-equivalence in

some aromatic ethers, sulfides, and disulfides (B246). Two exchange processes were observed in the fluorine spectra of fluorophosphanes of the type RSPF4-one is pseudorotation and the other is hindered rotation about the P-S bond (B324). A barrier of 19 kcal/mol was found to rotation about the PC-CO bond of (PhaP: C(COOMe))2, for which three slowly interconverting conformers are seen a t room temperature (B384).

Amides. Hindered rotation about the C ( 0 ) - N bond in amides continues to receive a great deal of attention. The classic example, dimethylformamide ( D M F ) , was given the most accurate treatment to date by incorporafing both spin-spin coupling and the temperature dependence of the chemical shift between the methyls; the dependence of rotation rate on concentration in CCL solution correlates with the extent of its dimerization (B346). Rotation in DMF, N,N-dimethylcarbamyl chloride, and methyl nitrite were re-examined by combined spin-echo and high resolution methods (B215).

The hindered rotation and proton exchange in formamide were studied using 14N decoupling; solvent effects are attributed mainly to hydrogenbonding to the carbonyl oxygen (B234). Solvent and substituent effects are noted on rotation in substituted N,N-dimethylbenzamides (B217). Rotation about the C(S)-N bonds in thioureas is reported, coalescence temperatures being below room temperature B60, B4Of. Equilibrium between cis and trans configurations of N-methyl thiourea was observed in various solvents (B4O1). Hindered rotation im primary thioamides was established by proton N M R of the 16N-enriched structures (B415). Some N,N - dialkyldithiocarbamate esters have 10-12 kcal activation energy for C(S)-N rotation (BaO4). The configurations of a number of substituted thioamides have been studied (B370, B416). Complexation with Sn reduces the rotational barrier in N,N-dimethyldithiocarbamates

.

(B206)

Amide self-association in ccl4 solution has been studied for D M F (B345) and DMF, dimethylacetamide, and their thio analogs (BSOI). Restricted rotation of amidines is strongly dependent on solvent (B186, B249).

Other Hindered Motions Involving Nitrogen. The coalescence temperatures of some 1,1,2,2-tetramethylphenyl guanidines lay between - 5 and -95 “C, depending on phenyl substituent (B245). More restricted rotation is found in biguanide cations, where the nonamethyl derivative has one bond

freer than the others and 3-phenyl-octamethyl biguanide has all skeletal bond rotations rather restricted (B32). A general conclusion is t h a t rotation is “fast” about the C-N bonds and restricted about the C : N bond in guanines, and fast about all bonds in guanide ion (B33). Hindered rotation about the N-aryl bond accounts for non-equivalence in anilides (B244, B389) and in 2dimethylaminopyridine derivatives ( B 2 4 f ) . Slow rotation about the Nsecondary alkyl bond in N,N-diisopropyl amides is indicated by selective broadening of the methine protons at low temperature (B388). The dimethylamino rotational barrier in compounds of the type (CH3)ZN(CH).O, n = 1,3,5, decreases with increasing chain length and decreasing solvent polarity (B34’7). Other unsaturated dimethylaminoaldehydes, and ketones also were studied for various influences on the dimethylamino rotation (B4O). Conformational and configurational isomerism have been studied in N-alkyl imines (B236) and in N,N-dimethyl hydrazones (B237). Mechanisms for thermal isomerization of the C : N double bond are discussed (B279).

The matter of nitrogen and/or X inversion versus hindered rotation about N-X bonds remains an active area of investigation. I n some aminophosphines it is found that N inversion and N-P bond rotation are fast a t room temperature and P inversion is slow ( B 2 f4). I n di-(iso-propy1amino)phosphine sulfide the loss of non-equivalence on warming may be fast nitrogen inversion (B242) or changes in conformer populations (B87). The N-S rotation is hindered significantly in sulfenamides (B266, B344). The barrier to B-N rotation in amineboranes has been studied ( B a s ) . The N-N bonds of simple hydrazines are hindered significantly in their rotation; brief discussion of this involves comparison with the other N-X rotations that have been reported, implying lone-pair involvement ( B f11). Nitrogen inversion is found to be unusually slow in N-halo-aziridines, greater than 21 kcal/mol (B58, B255). This inversion is sufficiently slow to allow isolation of diastereomeric 7chloro-7-azabicyclo [4.1.O]heptanes a t room temperature ( B f39). A re-investigation of inversion of 1,2,2-trimethylaziridine obtained values for the kinetic parameters that were higher than previous estimates (B218). C.

APPLICATIONS: CONFORMATION AND CONFIGURATION

General. By far, the greatest amount of published NMR information concerns the utilization of high

resolution spectra, primarily of protons, for determining conformation and configuration of organic molecules. Allowed molecular symmetry may be inferred from symmetry and equivalence of chemical shifts. Steric placement of observed nuclei with respect to substituents and to other magnetic nuclei may be inferred from the magnitude and signs of spin-spin coupling constants, based most quantitatively on empirical correlations that usually are supported qualitatively by theoretical arguments. Chemical shifts are characteristic of basic chemical environment. Substituent effects on chemical shifts are correlated empirically also, being related to electronic behavior and magnetic anisotropy of the perturbing substituent. Solvent effects on chemical shifts serve as diagnostic tests for particular geometries and/or substituents. Conformational preference often is determined. The energetics of interconversions among conformations may be obtained from variable temperature studies. It will be possible to mention here only some of the work of general interest that has been published. One review article on use of NMR for conformational analysis deals with the measurement of rapid interconversion among conformations and the geometrical information obtainable from coupling constants (C312). The study of carbohydrates and related compounds by N M R is reviewed also (Cf 60). The study of fluxional organometallic molecules is reviewed ((764); YMR is well-suited for such work because the activation energy for this rearrangement of bonds among equivalent structures is in a range where spectral coalescence behavior is observable. Another interesting intramolecular rearrangement was reported for (C5H& MONO, where the three nonequivalent cyclopentadienyl rings were shown to be interconverting rapidly and rotating individually a t room temperature (C65). An anomalously large methyl proton linewidth for a 4,7-dimethylphenanthroline chelate of Cr(I1) has been attributed to methyl group rotation (Cl84).

Substituent Effects and LongAdditive effects Range Shielding. of substituents on ethylene on the shifts of remaining olefinic hydrogen have received further consideration. Values good t o 0.1 ppm are obtained for -CH3, -C1, -Br, and -CN, which have relatively little bulk and bear threefold or greater rotational symmetry; a substituent with less rotational symmetry can be handled in an additive fashion when its geometry is the same in the unknown as in the model structures (C313). Additive increments for 43 functional groups were obtained by least squares analysis


433

R

of over 4,000 chemical shifts, with a number of conspicuous exceptions being found ( C 2 f S ) . Structural features responsible for deviations were examined further (C2f4). The olefinic 13C shifts of qp-unsaturated carboxylic acids also can be described in terms of incremental values for substituents, according to their placement as gem, cis, or trans to the carboxylic group (C269). Incremental substituent effects are found to be additive on tertiary methyl group shifts in over 100 limonoids (C24S). The glycine CH2 proton shifts of 26 dipeptides are additive in state of ionization of the amino acids (CM5). The methyl shifts of methylarenes correlate with the inverse cubes of the distances to aryl ring centers (C247). Bond anisotropies for C-C and C-H were obtained empirically from examination of norbornanes and other hydrocarbons; the values of +4.39 X cma mole-' and " 0 , respectively, are useful for predicting proton shifts, although they are incompatible with values from Cotton-Mouton constants (Cf57). Apparent magnetic anisotropies of C-F and C-C1 bonds have been derived from proton shifts; their use as empirical values only is emphasized, because they are dependent on location of the idealized point bond magnetic dipoles and include electric field effects characteristic of the bonds (Cf 58). Errors in an earlier calculation of C-C1 bond anisotropy are corrected (C250). A careful study of long-range shielding by triple bonds -C=C-, -C=N, and -X=C led to empirical values for their magnetic anisotropy values and a conclusion that electrostatic effects are important for -C=N and ---K=C but do not dominate the magnetic anisotropy (Cl61). An argument is given for placing conical shielding region of the carbonyl group along the axis of the CO bond and centered on the oxygen, based on proton shifts and steric arguments in aldehydes and ketones (Cf 7 3 ) . This same geometry is likely for S-0 bonds of sulfoxides and sulfones ((7169). I n anilides, however, deshielding appears t o occur over the whole amide plane ((7299).

A study of anisotropy effects of the nitrogen in aziridines shows that ring protons syn to the N-H or N-R bond are more shielded than anti; a qualitative calculation indicates that magnetic anisotropy of nitrogen bonds and lone pair does indeed cause this ordering, which overrides hydrogen-bonding shifts ( C 2 8 f ) . Nitrogen lone pair anisotropy dominates in piperidines, also (Cl89). The CS2- group appears to be a good conformational probe, as evidenced by strong deshielding of equatorial N C H 434 R

protons in cyclic amine N-carbodithioic acid salts relative to the free base (CSS). Calculated effects on proton shifts of replacing another proton with a methyl group a t various distances and angles are tabulated (C88). Introduction of methyl groups on cyclohexane rings causes changes in vicinal protonproton coupling constants attributable to substituent effects rather than to changes in bond angle (CS4). Fluorine shifts and coupling constants were determined as a function of geometry in hexopyranosyl fluoride derivatives (Cl44) and pentopyranosyl fluoride derivatives (Cl45). Rigid Multicyclic Systems. The rigid bicyclic norbornene continues to be studied as a convenient source of known geometric relationship among bonds. Deuteration and long-range couplings were used to establish t h a t the 7-syn proton is less shielded'than the 7-anti in the parent structure ( C l f 2 , C206). That assignment is reversed, however, for the endo cyclopentadiene trimer and tetramer, where both 7-protons are shielded by the adjacent double bond, but the 7-anti proton is more seriously deshielded by steric compression with the double bond ( C f 0 8 ) . The endo-endo and exoexo vicinal couplings are comparable, contrary to earlier reports (Cf5). The coupling between bridgehead protons is. 1.12 Hz in norbornene and 1.85 HZ in norbornadiene ( C l f8). Isomeric alchols having the norbornane structure were distinguished by comparing dilution curves for the OH chemical shifts with hydrogenbonding possibilities for each (C248). Large tabulations of shifts and/or couplings were applied to structure assignments for the following rigid systems: cyclopropyl ring protons of bicyclo[n.l.O] alkanes ( C 7 f ) , 16-substituted-17 (20)-pregnene adducts (C203), hydroxychromemes by acetylation and long-range coupling (Cf 8 ), and alkaloids of the atisine and veatchine family (0251) . Rotation and Inversion. Rotational isomerism in substituted ethanes has received a great deal of attention. Energy differences between rotamers are estimated from temperature dependence of averaged vicinal couplings and chemical shifts. Solvent effects on averaged vicinal coupling usually are taken to reflect a difference in rotamer populations. Accuracy of this technique is doubtful, however, in view of the finding that the observing frozen-out rotamer distribution a t low temperature does not agree with that predicted by high-temperature studies of CHBrzCFBrz (CIS.2). Also, solvent effects on coupling constant may occur without conformation change (C98, CS04).

The relation of temperature and sol-


vent dielectric constant change to vicinal coupling constant change also is used to evaluate rotamer energy differences and/or gauche and trans vicinal couplings. This approach is presented well in studies of substituted ethanes (CZ, Cf 49, C239, (2249, (2274). Vicinal P-H and N-H coupling constants for individual conformers have been estimated from temperature dependence of the fast-rotation-averaged values (C36). A simple method t o evaluate the barrier to internal rotation in 1,2-disubstituted ethanes utilizes the Karplus relation with a rotational potential function to describe the time-averaged dihedral angle between vicinal hydrogens (Cf96). Slow rotation of tertiarybutyl groups has been observed as 2 : l doublets a t low temperatures in t-butylcycloalkanes, with barriers of 6-8 kcal/mol ( C f 6 ) . All three t-butyl methyl groups were seen separately in 1-methyl-7-t-butylcycloheptatriene a t -100 "C ( C f 5 2 ) , due to molecular asymmetry. Strongly hindered rotation about sp3-sp2 and spa-sp3 single bonds has been studied for activation energy in a variety of structures (Cars), in triaryl methanes and triarylmethyl radicals (Cf 7 6 ) , and in tetra-arylmethanes (CSOO).

Detection of s-cis and s-trans configurations of l13-butadienes has been approached by observing long-range couplings in cyclic model compounds (C39). Haloprenes appear to exist largely in the s-trans configuration, while more, bulky, substituents cause skew and s-cis conformations (C37, CS8). Strong association between 1-Sn and 4-Br of a tetraphenyl-l,3-butadiene causes a skewed skeletal structure with slow rotation about the 2-3 bond (32). Complete conformational analysis has been performed for the longer-chain molecules Z14-pentanediol (Cf f 6 ) , some 1,2,3-diastereoisomers (C29), and 3phenylpropanol (CS06). Conformations of a number of aldehydes have been studied ( C f 7 2 and references therein). Vicinal H-H coupling in a number of threo-erythro stereoisomeric structures bearing t-butyl groups indicates that in some cases a t-butyl group is gauche to another large group (CSO). There is no general relation between vicinal coupling and threo/ erythro configuration assignment (C289). A lack of H-bonding solvent effect on the HCOH coupling in methanol is taken to imply that conformational changes account for variations in this coupling for other alcohols in these solvents (C267). Pyramidal inversion of phosphorous and of arsenic very likely occurs in l12-dimethyl-1,2-diphenyldiphosphine and the corresponding diarsine, with activation energies 26 and 27 kcal/mole,

respectively (C186). Sulfur inversion is reported in platinum-sulphide complexes with 18 kcal/mol activation energy, without complex dissociation (C141).

Non-planar Rings. Cyclobutanones with bulky a- and 0-substituents are shown to be non-planar (C282). I n 3-substituted-thietane-1-oxides the ring proton spectra are characteristic of cis and trans oxygen orientation (Cl67).

I n a well-documented proton N M R study of ethylene phosphites and ethylene sulfite, the five-membered phosphite rings were shown to exist in the twist-envelope conformation (Cl4O). I n another careful study, of l,a-disubstituted indanes, vicinal proton-proton coupling constants were corrected for effect of substituent electronegativity before using them in a Karplus relation to determine ring conformation (C19). Six-membered rings continue to receive the greatest amount of attention. A “double R-value” method for conformational studies of six-membered rings has been introduced, where R = J(trans)/J(cis) for averaged vicinal couplings of the -CH&H,- fragments of the ring (Cl88). Interpreted in terms of a Karplus relation, the Rvalues are a convenient measure of local geometry, so that having the values for two overlapping ring fragments should give a fair picture of ring conformation. Good correlation is found between this solution R-value and crystallographic dihedral angles in the few systems for which data are available ((2185).

Conformational preferences ( A values) of substituents on six-membered rings are determined best by measuring the relative amounts of the two ring inversion isomers, via NMR integrals under slow exchange conditions; this has been done for twenty-two monosubstituted cyclohexanes a t -80 “C (C166). Under fast ring inversion conditions, the conformer ratio is contained in observed chemical shifts and couplings as the population-weighted average of their values for the individual conformers. These limiting values are difficult to obtain, however. One popular technique was attachment of a 3- or 4-t-butyl group to the ring, assuming that it would be 1 0 0 ~ o equatorial; this would place the substituent and monitored shifts or couplings in their limiting conformations, and it was assumed that the t-butyl group would have no other effect on the monitored parameter. Unfortunately, it has been demonstrated that the t-butyl group does exert chemcia1 shift effects a t remote parts of the ring (CSS1, C l 6 4 ) , and that all chemical shifts are temperature-dependent as well (Cl6.4). By applying correction factors, the 4-t-butyl derivatives give reasonable results for H

and F shifts on C(l), but 3-t-butyl do not (C90). shifts have high potential for evaluating conformational preference, as evidenced by the 5 ppm shift between axial and equatorial -CHzOR on cyclohexane (C50). Introduction of heteroatoms into the cyclohexane ring affects conformation and interconversion rates by changing bond lengths, bond angles, and torsional barriers. Torsional barriers probably dominate the ring inversion (Cl48).A major study of 1,3-dithianes has been reported (C89). The twist-boat form of tetramethyl-s-tetrathiane was isolated in solution a t -80 “ C and examined (C5S). I n some cases inversion of the heteroatom bonds must be considered as well as ring inversion (C190, CSSS). The P H proton of phosphorinane is preferentially axial (C191). Mono- ((7165) and 1,Cdisilacyclohexanes (CSSS) have ring inversion barriers less than 6 kcal/mole. Model calculations of relative energies of ground, intermediate, and transition states of cyclohexane and methylated derivatives were performed; for cyclohexane, several unsymmetrical transition conformations had energies similar to the commonly assumed half-chair form (C290). Six-membered rings in spiro-systems have higher inversion barriers for smaller rings (ClO). Conformational preferences of several 4-substituents on cyclohexene were determined, in the range 0.1 to 1 kcall mole (CSS9). I n four 7-monsubstituted cycloheptatrienes the substituent has strong preference for “equatorial” placement (ClS8).

K M R has been used to study ring conformation and inversion kinetics for cyclooctanes (Cia), cis,&-cyclo-deca1,6-diene, and derivatives (C69, C276). Conformational mobility and rapid valence tautomerism were studied in [16] annulene (C246). [3.3] Paracyclophane exists as a 2 : l mixture of boat and chair conformations in solution ( C l d ) , and hindered rotation has been studied in some other paracyclophanes (CS22).

Non-eqiiivalence. Magnetic equivalence between nuclei, or lack thereof, is a n important feature in NMR spectra. I n a review of the subject i t is concluded that non-equivalence can be explained on topological grounds after it has been observed, but quantitative prediction is not possible ( C l S l ) . Most studies are based on the chemical shift between geminal protons or methyl groups. Nonequivalence is enhanced by intermolecular hydrogenbonding in an isopropylphosphoramidate (Cl20). Substituent effects are claimed on magnetic nonequivalence of the methylene protons of some acetals (CH,CHzO)&HZ, presumably trans-

mitted along the bond system (C270). The unusually large shift of 4.08 ppm occurs between geminal protons in a dinapthothiocin ((7168). The influence of various types of asymmetry on chemical shifts and couplings in ethyl ethers and ethyl thioethers has been studied as a function of temperature and solvent (C211). Specific deuteration of one proton over another in a geminal pair has been observed in ethylenediamine chelates (C307), and in benzyl p-chlorophenyl sulfoxide (C24O). Phosphorous served as the source of non-equivalence in a phosphine (C220). The non-equivalence of methylene protons of a benzyl group attached to an asymmetric phosphonium ion is dependent on the anion, in CDCL solution, implying ion pairing (C288). This non-equivalence also was observed in another study of benzyl methylenes on phosphonium and arsonium ions (C56). Optical purity of asymmetric phosphines may be determined by making the phosphonium salt with 2-phenyl-2methoxy ethyl bromide and observing the NMR spectrum (C56). Solvent and temperature dependencies of nonequivalence in asymmetric phosphonates are attributed to hindered rotation ( C l l l , C16S). Enantiomers. hfagnetic nonequivalence of enantiomers is observed often when they interact with optically active species in solution, presumably t o form diastereoisomeric complexes. Conditions necessary for such observations have been examined in the use of optically active sulfoxides to resolve various carbinols in optically inactive CC14or CDCb as solvent (Cl?‘), Stoichiometric mixtures of optically active acid and base in solvents of varying ionizing power dramatize the dependence on association: when the above salts are undissociated, the diastereoisomers are clearly different from the acid and base spectra, and when partial dissociation occurs only the acid signal is doubled (ClS7). A logical extension of the interaction between optically active solutes in optically inactive solvent is the expectation that the spectrum of a single enantiomer differs from that of the racemate, since the environmental asymmetry (optical activity of the net solvent) differs. I n this fashion, separate spectra for d and 1 forms of dihydroquinine were seen in a mixture of the natural (-) and racemic materials in CDCl3 solution, and the separate samples gave single but different spectra (C324). A solvent which is itself optically active is desirable because it should impart greatest separation of enantiomer signals. Alpha-aromatic amines resolve optically active carbinols ( C W l , C267). Aryl carbinols have been


435 R

used, conversely, to resolve amines (c262). S(+)-2,2,2-trifluorophenylethanol can be prepared in high purity (CH“69)and has been used to resolve enantiomeric sulfoxides (including CD& (0)CH,) (CS58),sulfinamides, sulfinates, sulfites, thiosulfinates, phosphine oxides, and amine oxides (CSSO). I n most cases one can argue absolute configurations on the basis of the results. One very interesting observation is the doubling of the ligand signals of enantiomeric hTi(I1) complexes in unequal mixtures of a-pinene enantiomers as optically active solvent; after a period of time the ligand signals tend to coalesce to a single set of broadened lines. This change is attributed to possible slow equilibration of solvent molecules in the immediate solvation spheres, from initial random distribution toward preferred diastereomeric forms (C95). Sometimes it is preferable to make formal diastereoisomers for resolution of enantiomers. NMR has become very useful for monitoring such resolutions (optical purity), agreeing within 1% with polarimetrically determined values in one case (C70). Use of Solvent Effects. Many pertinent features of solvent effects as applied to conformation and configuration analysis are covered in the next section. Some of the very many reported studies will be mentioned here for their relative uniqueness. Pseudocontact shifts caused by complexation with added paramagnetic ions have aided structure elucidation (CSSl, C337). Shift changes on going from CCl, or CDC& to benzene as solvent often are rather large and characteristic of geometry. It is found that epoxides can be distinguished from larger-ring cyclic ethers (CSSS) and proton placement relative to OH and OCH3 (CSSS) can be characterized this way. Benzene solvent effects on alkanes are given (C4). Addition of a small amount of trifluoroacetic acid or other protic substance to benzene enhances the benzene solvent effect on polar groups (CSS8, CSSS). ASSOCIATION PHENOMENA

Solvent Effects. The net observed shielding of a nucleus from the magnetic field applied to a sample customarily is expressed as the sum of its shielding in the isolated gas molecule, contributions of sample bulk susceptibility, dispersion forces, electric field, solvent anisotropy, and specific interactions. One review article is recommended especially to the reader, for its critical description and discussion of the various effects, their origins, and applications ((7193). Another review places its emphasis on specific association, especially aromatic solvent-induced 436 R

shifts (ASIS) (C277). Organic chargetransfer complexes (between electron donors and acceptors) are reviewed (C107). A brief article discusses various spectrographic methods for studying aprotic solvents (C5). The London-van der Waals dispersion forces have been described most successfully by studying the gas-to-solution shifts of non-polar magnetically isotropic solutes in magnetically isotropic solvents. Nearly quantitative values for medium shifts and their temperature dependence have been calculated by extending the binary gas collision model to liquids, incorporating appropriate density and an empirical factor; a sitefactor correction is needed because the observed nuclei are not a t the center of the molecule (CS80). Another model for calculating dispersion shifts was applied to both polar and non-polar solutes (CSS8). Dispersion forces were shown to affect geminal Pb-C-H and Sn-C-H coupling constants, by observing them in n-heptane-carbon disulfide mixtures differing markedly in their refractive indices but not in their dielectric constants; this contribution appeared to be of similar magnitude to reaction field and/or Stark effects proposed for polar solutes in polar media (C194).

A most significant observation was the splitting of the l4N resonance of nitrobenzene upon partial molecular orientation by a very strong applied electric field. Quantitative comparison of the splitting with applied field did not agree with either the Lorentz or Onsager local field models, with the latter being closer; deviation was attributed to neglect of short-range interactions in the models (C164). The usual source of medium electrical effects lies in the reaction electric field induced in the solvent by the dipolar solute. Here, again, the Onsager reaction field model was inadequate to explain chemical shift and coupling constant trends for CF2:CH2 with solvent dielectric constant (the F-F coupling depended on dispersion interactions) (C2OS). Solvent dependence of spinspin coupling is argued in terms of Fermi contact alteration by electric fields (CS72). Magnetic anisotropy of aromatic and other unsaturated solvents induces relatively large shifts in solute molecules. A statistical distribution of orientations of disc- or rod-shaped model solvent molecules is calculated to induce less shift in ideal spherical solute molecules as the size of the solute molecule increases, in agreement with general observations (C27). This model predicts shifts that are about twice as large as observed, presumably because van der Waals forces were not considered. The model was applied with quantitative success to benzene-induced shifts from


TMS, relative to values in CCL, for some methylated cycloalkanes (CS8). I n some cases, such as hydrogen bonding, the assignment of solventinduced shifts to specific solute-solvent interaction is relatively well-founded. There are many other situations, however, where variation of chemical shift with solvent and temperature can be described best by the formal model of specific short-lived “collision complexes” for which activation energies in the range of 1 kcal/mole may be obtained from the temperature dependence. I t must be emphasized that the actual existence of such complexes is given only tenuous support, at best, by other measurements. I n a careful study, it has been proposed thrtt most polar sovent effects on polar solutes are interpretable in terms of this specific complex formation much more satisfactorily than in terms of the reaction electric field alone; complex formation is demonstrated by nonlinear plot of polar solute shift ver. polar solvent concentration in a mixed polar-inert solvent system (C183). The collision complex model also has been applied to the solvent and concentration shifts of alpha-chloro-acrylonitrile, with dielectric constant reflecting dipolar cohesive energy in the complex (C292). Aromatic Solvent Induced Shifts (ASIS). Much attention has been given to the sizeable change of proton shifts in polar molecules, relative to internal TMS, upon going from a n “inert” solvent to an aromatic solvent. The usual combination of solvents is CCl4 or CDC13 and benzene. “Collision complex” formation generally is invoked as a practical working model for placement of the shifted nuclei a t preferred orientations relative to the aromatic ring. This is supported by reasonable thermodynamic parameters based on 1:1, 2 : 1 , and 2 : 2 so1ute:benzene ratios (C12). These simple complexes are viewed as a result of dipoleinduced dipole interaction. Freezing point diagrams give no evidence of compound formation, however, and solvent ordering which is geometrically equivalent to a complex is suggested (C106). Solvent ordering may, indeed, prove to be the argument which lends acceptable support t o actual occurrence of complexation. Ir’oting that 1 : l and 1: 2 solute: benzene complexes do not furnish enough anisotropic susceptibility to cause many of the large ASIS effects, one author proposes association via van der Waals forces; this would permit short-lived association with a larger number of benzene molecules oriented in the proper manner (C2lS). The solvent shift of benzene itself is predicted rather well by a locally ordered liquid state model which accounts for experimental data other than magnetic resonance (CSQS).

Steric factors have been examined in solute-solvent association. 1-Substituted-2,4,6-trialkyl benzenes in toluene demonstrate the anticipated loss of shielding of ring protons as the alkyl group size increases, and association of the toluene ring with the more positive portions of the solute molecule (CS27). XSIS of ester ethyl groups becomes greater with increased polarity of the group adjacent to the ethoxy and with coordination of the ester with tin(1V) chloride ((2175). Dimethylformamide, dimethylacetamide, and methylated benzenes also show steric effects (CS66). Other Solvent Effects. It is noted t h a t CC14 is not a n inert solvent, causing deshielding from T M S as large as 0.16 ppm relative to cyclohexane as solvent, and showing evidence of complex formation and steric effects (C182, C287). h linear correlation is noted between the SH proton shifts of n-butanethiol and thiophenol in various solvents, and other similar correlations are obtained from the literature for acetylenic hydrogens and some others (C207). Much less has been published about solvent effects on coupling constants. Preferential solvation of a polar solute by polar groups in the solvents is said to cause observed correlation of H-H coupling in 1,1,2-trichloroethane with solvent dielectric constant (C74). The direct H-P coupling in dimethyl hydrogen phosphite varies by 100 Hz with temperature and solvent, primarily reflecting solvation of the P:O group (C179). The one-bond Si-F coupling in SiF4 is additive in substituents attached to the central C or Si atom of the solvent (C271). Water and Aqueous Solutions. 1 7 0 studies of liquid water yielded TI and T z dependence on temperature and found the effective quadrupole coupling constant to be the same for H20 and D20 (C119). 1 7 0 shifts were measured from room temperature to 215 "C for liquid and vapor, with total change of 9 ppm; the OH coupling is 79 H Z in the vapor phase (Cf01). The proton exchange rates a t 29 "C were obtained for acid and base catalysis, by computer analysis of the 1 7 0 linewidths (C265). 1 7 0 X M R of single crystals of D2O ice led to quadrupole coupling constant and isotropic chemical shift in one orientation ((217). No evidence was found for proton T1 discontinuities in liquid water a t temperatures where other physical measurements show discontinuities (C43). Second moments of proton and deuteron X'MR in ice polymorphs gave H-0-H bond angle of 104" and 0-H distance of 1.01 A, indicating that the hydrogen bonds are bent, and showed that the 0-D bond distance is unchanged in the various forms (C264).

Proton T1 measurements in hexagonal ice over the range 0 to -80 "C and 125 to 675 gauss permitted separation into effects of pure ice and of impurities, and combined with other data show that T1in pure ice is determined mainly by motion of Schottky defects (CS10). The structure of liquid water may be studied by NMR examination of perturbations by dissolved electrolytes and non-electrolytes. Water ''0 line widths and chemical shifts for a number of solutions of both kinds have been presented, showing that the relaxation rate increases with increasing concentration of structure-forming salts and decreases with increasing amount of structure-breaking salts (C100). The proton chemical shifts of water containing ethers, acetone, or t-butanol show an anomalous deviation from linear dependence on concentration a t 3-6'% solute concentration, which correlates with the formation of clathrate gas hydrates and indicates water stabilization (C128). Stabilized water octahedrons are proposed for 20 wt. % water in pyridine, based on water proton shift measurements (G237). T1 study of the water-dioxane system shows a change a t ca. 20 mole percent dioxane, which is suggested to reflect a transition from a solution of dioxane in water to one of clusters of water in dioxane (C61). Rate constants for aminewater hydrogen bond dissociation were measured by NMR methods; they are measures of the rate of diffusion of water into bulk solvent, and net results gave negative evidence for any special ice-like water structure (CISS). The proton and 1 7 0 shifts of water in NH3 are interpretable in terms of a water monomer-dimer equilibrium (C7) Water proton shifts are found to correlate well with its activity coefficients in mixtures with acetone, benzyltosylates, and dioxane (C181). Molecular motion in aqueous mixtures with organic liquids was investigated by spinecho measurements of both H and D I

(C130).

Very accurate proton shifts were measured for the ternary system HC104DC1O4-Hz0-Dz0 between 0 and 60 "C to obtain water and perchloric acid dissociation constants (C81). Hydrogen Bonding. A thorough review of spectroscopic studies of hydrogen bonding contains a section on N M R work and a large tabulation of thermodynamic data (CSS4). Attention is given to the determination of equilibria in hydrogen-bonded systems. A model for monomer-long chain equilibrium is equivalent to monomer4imer description (CSO6). Situations where there was competition between inter- and intramolecular hydrogen bond formation required three species, including free monomer, cyclic dimer, and intramolecularly bonded

monomer for best description (CSZ9). Temperature dependence of the intramolecularly H-bonded proton shift is consistent with the model of excited vibrational levels as the cause of deshielding (C286). Hydrogen-bonding in the vapor phase has been studied by pressure and temperature dependence of the shift of the bonding proton in the systems HC1dimethyl ether ( C l S S ) , methanol, and methanol-trimethylamine (C60) ; values for AH and AS are obtained. Association in solutions of t-butyl alcohol in water was evaluated from temperature dependence of OH chemical shift; assuming reasonable values for the self-association energies, the authors found that the alcohol-water association energy was greater than either self-association energy (cS16). The self-association of ethanol in cyclohexane was measured from OH shifts, giving AH for dimerization of -5.04 kcal/mole, compared with -7.2 in C c 4 (C58). Adducts between dimedone (5,5-dimethyl cyclohexane-1,3-dione) and aldehydes are supported as dienolic tautomers by NMR, but substitution causes the two enolic protons to differ markedly in their exchange behaviors (C105).

Hydrogen bonding has been measured by fluorine S M R in a para-fluorophenyl label (C139) and via the 14X shifts of formamides in methanol (C171). The hydrogen-bonding behavior of compounds (C2H&MXH, where M is Si or Ge, and X is S or Se, is observed as solvent effects on the X H proton shifts and used to characterize the M-X bond (C85). Electrolyte Solutions. A comprehensive review of K M R studies of ions in pure and mixed solvents is inclusive to mid-1966 (C155). Another review of work in electrolyte solutions emphasizes diamagnetic salts (C77). Ion-solvent and ion-ion interactions are the subject of another review article (C61).

The chemical shifts of ions due to hydration have been measured from the difference between accurately measured magnetogyric ratios for the hydrated ion a t infinite dilution and for the bare ion or molecule (by atomic beam or optical pumping methods)

.

(C93)

The shielding and relaxation of alkali and halide ions in aqueous solution (C78, C79) and in methanol-water mixtures ((714%')have been studied in many ways, showing that electronic overlap with other species is a major source of halide ion chemical shift. 7Li and *H relaxation in LiCl solutions differ sufficiently between high and low concentration to imply different structures (CSSO). Cesium ion shifts were studied


437 R

in aqueous and a number of non-aqueous solutions (C146). Negative hydration (greater mobility of water molecules in the hydration sphere than in the bulk solvent) was examined and discussed thoroughly for a number of ions of different types, in aqueous solution ((792) ; this paper incorporates earlier work of one author on molecular motion and relaxation in electrolyte solutions (Cl51). Ion pairing of perchlorate with zinc ions in methanol solution is implied by the change in zinc solvation number from 6 to 4 as zinc perchlorate concentration increases, as measured by proton NMR (C6). Intimate ion pairs were not indicated for aqueous aluminum nitrate a t high concentration, where a coordination number of six is seen ( C S f 7 ) . Ion pairing of tetramethylammonium ion (C192) and butyltriphenylphosphonium ion ((299)with paramagnetic anions has been studied in aqueous and nonaqueous solutions. Fluorine relaxation in aqueous H F is affected by Cu(I1) concentrations as low as molar; the processes involved were studied, leading to derivation of a detailed structure of the liquid (C87). The NMR linewidths of 2aNa in different isotropic and mesomorphic phases in the sodium caprylate-decanolwater system are determined primarily by the number of water molecules per sodium ion (C196). Proton NMR studies of water in melts of hydrated salts show dependence on both anion and cation, and reflect lattice structure of the melt (C91, C $ S l ) . Deuteron KMR should be useful for studies of paramagnetic ions in aqueous solution because better net resolution than observed in proton spectra is anticipated on the basis that dipolar and hyperfine relaxation mechanisms dominate; linewidths will be proportional t o the square of the magnetogyric ratios while chemical shifts are proportional to the first power (C273). Hydroxylated complexes of Fe(II1) in acidic solutions were identified (C278) and complexing of Fe(II1) by 5’-deoxyribonucleoside monophosphates examined (C279) via proton relaxation. Aqueous HC1 solutions of cobalt(I1) were characterized by 170 and 35Cl shifts with temperature (CSdO). The chlorine shifts in solution with paramagnetic cobalt and rare earth ions are attributed mainly to covalent character of the bonds to the metal ion (C26). Exchange. Proton exchange between Al(OH2)e3+ and water procedes faster than exchange of entire water molecules, as found in a kinetic analysis of the process by NMR methods (ClO4). Criteria for the general detection of ionized intermediates were discussed in conjunction with a study of proton 438R

exchange in the ionization and acid dissociation of imidazole in aqueous acid

(CS68).

Kinetics of NH and CH proton exchange in amino acid complexes of platinum(I1) were studied, and it was found that nitrogen inversion and base-catalyzed CH proton exchange occur a t comparable rates (C94). Proton exchange rates in methanol have been estimated from T I and nuclear Overhauser enhancement ( C l l 5 ) . Solvation and Coordination. Measurement of solvation and coordination with an ion is simple when exchange with the bulk solvent is slow enough to give a separate absorption for the associated species. Exchange rates are measured by conventional line shape analysis. Most work deals with solvation of cations by oxygencontaining molecules, especially water. A deuterium isotope effect is seen in the preferential solvation of sodium methoxide by CH30D over CH30H The primary solvation number of paramagnetic Ni(I1) in aqueous solution was measured as 6 by integration of the observed solvent proton line a t low temperature; exchange is slow so only bulk water is seen normally, and this signal integral is compared with the total water integral obtained by adding acid t o catalyze the exchange (CSO9). A method for evaluation of total hydration number under fast exchange conditions a t low concentration in water utilizes a two-stage hydration model to interpret temperature and concentration effects on the water proton shift; a value of 13.4 was obtained for A1(N03)3 (C205). The method was applied also to the hydration numbers of NaC104, HC1, and HClOd in aqueous solution (C180) and to alkali metal salts and alkaline earth metal salts in concentrated aqueous solutions ((767). An 170NMR study of the hydration of Al(II1) and Ga(II1) ions included relaxation time and exchange rate measurements to obtain activation parameters; the results were applied to estimate the conditions under which a separate water proton resonance could be observed, with the conclusion that this would be feasible for aluminum but not for gallium (C97). This 170 technique also has beeen applied t o organometallic ions of Pt, Au, Ga, In, and Sn (Cf 27). The solvation of sodium ions of NaI in several solvents was studied by the 2aNa NMR chemical shifts; effects of solvent Lewis basicity and magnetic anisotropy were noted (CSf). Second coordination sphere interaction has been suggested to interpret proton NMR data for dimethyl formamide solutions of paramagnetic vanadyl ions (Cl70) and aluminum halides (CWO).

ANALYTICAL CHEMISTRY, VOL. 42,

NO. 5, APRIL 1970

A long-standing problem is that of detecting and measuring the hydration of anions by NMR. It is suggested that by knowing the proton shift of solvent bound to the cation (at slow exchange), one can assign the shift due to the anion, and application is made to the perchlorates of Mg, Al, and Zn in methanol (C54). The hydration of several cations has been observed as a separate water absorption in aprotic solvents, usually a t low temperature. Sulfoxides and amides are found to compete effectively with the water in this situation, while ketones, ethers, and others do not. Under low water or anhydrous conditions, the latter group of solvents often will coordinate strongly enough to give a separate absorption also. Competitive solvation is observed in mixtures of solvents. In general, the relative strength of solvation is in the same order as Lewis base strength. Ions for which the solvated species have been resolved are Al(III), RIg(II), Be(II), Ga(III), In(III), Sc(III), Y(II1) and Th(1V). The following papers cover some of this work and refer to most other work that has been done:

(ells,c156).

Complexation. This section deals with only a few of the many important applications of N M R spectroscopy to complexation studies, the remainder being covered in other sections. Halide ion-trihalomethane association has been studied in acetonitrile and carbontetrachloride solutions to obtain chemical shifts, equilibrium constants, and thermodynamic parameters ; since hydrogen bonding is the source of association, it was possible t o evaluate parameters for electric field effect on the shift of the complex (Cf 34). Halide ion complexes with acetylated p-d-glucopyranosyl derivatives also have been investigated by NMR ((210). Intramolecular coordination is proposed as an effective approach to the study of weak donor-acceptor interactions in solution by N M R ; it is based on altering nonequivalence in a structure containing a centrally located asymmetric center, and illustrated by intramolecular coordination in alkyl mercury compounds ( C f 7 7 ) . A coordinated species of CH3HgCl in dioxane was evidenced by the deviation of the proton shift and H-Hg coupling constant from their behavior with respect to reaction field parameter in other solvents (CZ48). A scale of relative Lewis basicities for inorganic halides has been constructued from the alpha-proton shifts of ethers upon complex formation (C75). The B-F coupling constant of BF4varies with solvent, anion, and concentration, leading to implication of ion pairing; collapse of the loB-l1B isotope shift demonstrates fluorine exchange

from one B to another (C123). Fluorine exchange has been studied in diamagnetic complexes of Ni(IV), Pd(IV), and Pt(1V) in anhydrous H F solutions (C216).

The donor strengths of a number of organic bases have been evaluated by N M R observation of their complexes with boron trihalides (Cll4,C229, and references therein). A study of the 2aNarelaxation times for sodium aminocarboxylic acid complexes demonstrated a chelate effect, identified the important chelating groups, and obtained the complex formation constants (‘2162). The N M R determination of the conformation of dl-propylenediaminetetraacetic acid salt in water implies a “proton chelate’’ structure (CSOS). Simple electron transfer reactions occur for alkylisonitrile complexes of Mn(1) and Mn(II), without ligand exchange, which recommends their use for the study of such processes (C215). OTHER TOPICS

Organometallic Structures. Organometallic exchange processes are treated well in a n extensive review ( C l 4 7 ) and in a shorter, instructional article (C45). Application of KMR to the general study of metal chelate compounds is discussed in a chapter (C218). The tetrameric structure of methyllithium was established rather dramatically by observation of the appropriate ‘Li-’aC coupling pattern in the ’Li spectrum (C222). Lithium bonding to the methylene groups was demonstrated also in dimethyl- and diethylmethyllithium-methylene-phosphorane (CS91). As a substituent on thiophene and derivatives, lithium has been found to cause markedly different ring proton shift and coupling changes from those anticipated on the basis of electronegativity and experience in related systems; the reason is not known yet (C135). Inversion rates have been obtained for 2-methylbutyllithium and triisohexylaluminum, by N M R lineshape analysis a t low temperature (C11O). Pseudorotation in the five-membered rings (AsCH3)s and (AsCFJFJ has been implied from study of the proton and fluorine spectra (CSSO). A review of organotin chemistry includes NMR work (C72). Rather thorough discussion is given to interpretation of methyl- and ethylhalostannanes (C201). Arguments are given for attributing the variations in tinmethyl spin-spin coupling in tetraorganotins to steric effects, by their influence on the hybridization of the tin orbitals ( C l S l ) . The signs and magnitudes of the onethrough four-bond couplings in tetramethyl compounds of the group IV elements have been reported (C80). Other

comprehensive studies include proton spectra of the vinyl and allyl derivatives of group IV elements (C84), methyl and ethyl derivatives of silicon and germanium (C83), and mixed group IV/ group VI hydrides (CISQ). Proton and fluorine spectra of phenyland fluorophenylmercurials have been studied (C178, CS53, C265, C254). lg5Ptand proton shifts and couplings have been utilized to study various platinum complexes ( ( 2 5 6 , C316), including hydrides (C73). The proton spectral parameters of ferrocene, ruthenocene, and cobaltocene chloride were obtained from the 13C satellites (C66). Methylene proton non-equivalence is studied in several derivatives of cymantrene and ferrocene (C82). Proton spectra have been reported for cyclopentadienyl complexes of paramagnetic Pr, Nd, Ho, Pu, Tm, and Yb (CQ). Cyclobutadiene-iron-tricarbonyl compounds have been studied oia their proton-proton and carbon-proton coupling constants, with the conclusion that the C-C bond order is uniform (C46). The proton spectra of chromium-tricarbonyl complexes of some condensed aromatic compounds were discussed extensively (C76). The 59C0 shifts of a number of tervalent diamagnetic complexes have been related to the electronegativities of the directly bonded ligand atoms (CSSS). Polymers. T h e application of N M R techniques t o polymer studies has been treated extensively. A major survey article covers both high resolution and broad-line applications (C4O). The use of high resolution NMR is reviewed, with useful discussion and referencing by structure (C3.25, CS32). Developments in high resolution studies from 1963 through 1966 are reviewed (C298). Some shorter general survey articles are noted (C8, C42,CSOS). The NMR and E P R section of a review on analysis of high polymers presents developments in many systems from 1966 to 1968 (C2.26~). A significant development is the high resolution observation of lacin polymers a t natural abundance, using complete proton decoupling to simplify the spectra and increase sensitivity. Initial work dealt with ethylene oxide-maleic anhydride copolymers, detecting triads and pentads of monomer units (CS84), and with poly(propy1ene oxide) (C285). The reactivity of vinyl and alkenyl ethers in cationic polymerization is related to the chemical shifts of the p carbons (C15S). Polymers of selectively deuterated monomers have given some very useful information regarding the proton spectra. It was shown that long-range coupling is a major source of broadening

in isotactic polypropylene, whose natural linewidth was less than 0.5 Hz with deuterium decoupling ((7296). Other structural information has been obtained also on deuterated poly(pr0pylene) (CSS8) and polystyrene (C297). I n both cases above, very high steric purity was demonstrated for isotactic materials. Some comments on nomenclature may be helpful to the NIMR spectroscopist who finds himself dealing with polymers. There is objection to the designation of units in vinyl polymers as having d or 1 configuration, because such distinctions are not observable experimentally; instead, a nomenclature based on sequences of meso (m) and racemic (r) diads is proposed (CdO). A terminology has been introduced to describe rotation conformations about bonds in vinyl polymer chains (C102). A structure-based nomenclature for linear polymers has been presented (C204). Detailed consideration of the likely conformations of tetrads in vinyl polymers is applied to predicting the CH2 proton chemical shift variations in poly(propylene), using reasonable anticipated shift values for individual conformations ( C f O S ) . The appearance of NMR spectra of polymers is influenced by gross molecular conformation as well as by the immediate environment of the observed nuclei. The solvent in which the polymer is observed can affect the spectral pattern by the usual local interactions, which collectively induce conformational changes as well. Chain folding of polyethylene oligomers is invoked to explain the observation of two absorptions due to internal methylene protons for chain lengths greater than C16 in “bulky” aromatic solvents, but only one line for shorter chains ((7198). This would seem to correspond to the onset of characteristic polymer structure. “Near end” groups are detected in oligomers of poly(ethy1ene oxide) (C197) and poly(dimethy1 siloxane) (ClUS),for both of which materials a characteristic polymer absorption appears when the chain length is seven or more units. Potassium ion complexation with poly(ethylene oxide) is reflected in the polymer proton T1 value, which is reduced in methanol and unchanged in water solution (C2OO). The rotating frame proton spin-lattice relaxation in poly(ethy1ene oxide) of various high molecular weights has been examined well for the sources of relaxation; spin diffusion appears to be important (C6S). Benzene proton relaxation in the benzene-poly(methy1 methacrylate) system was evaluated in terms of the various intermolecular contributions ( C I S ) . A 31 helical structure was deduced for the sodium salt of isotactic poly-


*

439 R

(acrylic acid)

in

aqueous solution

(CSSS) *

Hindered rotation about amide bonds in polymers has been studied for placement in the backbone (CSS5) and side chains (C.226); evidence for a preferred helical conformation was found in poly(succinylpiperazine). The pressure dependence of molecular motions-to 680 atm-has been studied for some elastomers by pulsed NMR methods ( C l l ) . A linear correlation has been established between the "mobile" fraction of an oriented polymer and its modulus of elasticity ('286). Biopolymers. The judicious use of N M R spectroscopy has proven to contribute unique, valuable information t o the general understanding of biopolymer structure. Most studies have involved high resolution techniques about which work a very instructive review article is recommended (C44). Another article discusses the contributions KMR has made to the study and structure of nucleic acids (CSl4). I n general, the high resolution proton spectra are able to resolve useful features of smaller molecules or unique portions of large molecules whose shifts differ from thoseof themajor portion. The chemical shifts and breadths of absorptions are used to characterize sites of interaction with other molecules, mobilities of the groupings, and, sometimes, helix content. These studies have generated a number of rather sophisticated experimental procedures and techniques. Complete (C68) and selective (CSOS) deuteration of proteins result in muchsimplified proton spectra, allowing better evaluation of their environments. Halide ions have been used as probes to titrate accessible bound metal ions, by quadrupolar broadening upon association with the metal. Most work has involved Hg, but Zn and Cd show promise, also. A study of the technique used chloride ions with bovine mercaptoalbumin to explore the potential and sensitivity (C47). Another potential ion probe is 43Ca, whose line width suffers quadrupolar broadening upon complexation with ATP in aqueous solution (C.48). Deuteron TI in specifically labelled polynucleotides gave some information on motional characteristics (ClS4). Rotating frame T I values of fluorine in trifluoroacetyh-phenylalanine were used to measure the binding rates with the chymotripsine (CSlO). The various angular correlation functions and correlation times for reorientation of observable labels on macromolecules are discussed (CS18). A spin-labelled analog of nicotinamide adenine dinucleotide acted to enhance proton relaxation in water and other small molecules complexed to liver alcohol dehydrogenase (C.224). 440 R

Proton NMR markers bearing aromatic rings have been used, also, to detect certain close-range interactions via magnetic anisotropy effects (CS96). Chlorophyl-ligand interactions have been evaluated using aromatic magnetic anisotropy arguments to estimate shifts for obtaining equilibrium constants (Cl74)*

Significant binding studies of adenosine (CS4) and purine (C.25) with polyuridylic acid in aqueous solution showed base-stacking and intercalation of purine at 29 "C; and adenosine intercalation and adenine-uracil base-stacking above 26 "C. Below 26 "C adenine-uracil hydrogen-bonding and adenine-adenine base stacking occur to stabilize a triplestranded complex. Studies of inhibitor binding and kinetics are noted as examples of the work in this area (CS.23, C.236). Polypeptides serve as models for proteins, to study helix-random coil transition. A review of polypeptide chain conformation studies includes NMR work and compares it with other methods ((741). The 220 MHz proton spectra of random-coil proteins and polypeptides in neutral aqueous solution can be predicted rather well using a large tabulation of descriptive features for the various amino acids in the chain (C.219). The helix-coil transition of poly-L-glutamic acid a t pH ~6 has been observed by pulsed-field-gradientspin-echo as a smaller self-diffusion coefficient a t higher pH, due to unfolding from the helix to bulkier random structure (C8.27). Temperature dependence of the proton NMR of yeast transfer RNA in 1 molar NaCl solution has been shown to be a result of salt-induced aggregation rather than melting of secondary molecular structure (CSOS). Solvation of proteins by water can be studied profitably by 170 (Cl.25) and deuterium (Cl26) relaxation times. Spin-echo measurements of TI and TZ for water in living tissues show that the self-diffusion coefficient is less than in pure water, as expected, and that the activation energy for self-diffusion is the same ( C l ) . NMR studies of erythrocyte membranes have been performed by observing the well-defined spectra obtained from sonification into fragments (C59).

Organic Ions and Radicals. There have been many papers describing spectra of carbonium ions. A book on the general subject includes a chapter on NMR methods ('2109). The superacid systems in which these ions are formed are reviewed with respect to chemical and physical properties (CSS, C l S 2 ) . Rotation of the NOzH+ group (CS46) and inversion of trivalent oxygen (Cf 87) are described. The aromatic 2 n-elec-


tron system, tetramethylcyclobutenium dication, has been assigned on the basis of proton and 13C NMR data ((244). Rearrangement of methyl groups in the t-amyl cation is shown to occur with 15.3 kcal/mol activation energy, and the three- and four-bond coupling constants seen have like sign (C.283). NMR in radicals and radical reactions is reviewed through 1967 (C49). The liquid radical di-t-butyl nitroxide has proven to be a useful solvent for relaxing solute radicals fast enough to give reasonably sharp lines; phenoxy and aliphatic radicals have been studied in this (CSS4 and references therein). Analytical Studies. Several brief survey or review articles have been written on specific areas of analytical interest : hydrocarbon characterization (C57), paints and coatings (CS, CS52), oils (ClSQ),and local analgesics (C.238). Broad-line applications to analysis of fats, oils, and water also were surveyed ((735, C.263).

The addition of 5% SbCl3 to chloroform, CSz, or CCL increases solubility for many materials ( C S I l ) . Determination of the deuterium content of organic molecules by NMR is discussed (C.209). Two or more samples can be examined simultaneously, in melting point tubes placed as a cluster in a thin-wall five millimeter sample tube (C.23). Alcohols can be resolved by type as their dichloroacetate esters ( C M )or as chloral alcoholates (C.2.21). Amino acids may be distinguished as alpha or not by comparing N-trityl esters (CSl9). A rapid determination of isomer ratios is described using combined NMRVPC (CSO). Quantitative determinations a t low levels is described for hydroxyl groups in a lignin (Cl17). Broad-line apparatus has served to measure purity of a material as the slope of a plot of temperature against amount melted (Cl6O). Similar measurements are reported using spin-echo (C5.2). Rehydration of foodstuffs is measured by observing bonded water with an R F level that saturates free water proton signal; this is done in a process control instrument (Cl43). Hydrogen adsorbed on the surface of a metal can be measured non-destructively by NMR absorption (C96). Quantitative use of nuclear quadrupole resonance has been described for measuring elements in organic and inorganic materials (CS94). LITERATURE CITED

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