Nuclear Magnetic Resonance Spectrometry Ernest lustig, Food and Drug Administration, Washington, laboratory, Washington, D. C. 20390
N
UCLEAR
JlAGSETIC
RESONASCE
(SAIR) and the literature con-
taining S M R material have kept u p their es1)losive gron.th. IC’AIR was referred to in one way or another in every sixth I)aper appearing in 1964 in the chemical literature of the United States. (See “Chemistry: Opportunity and Xeeds,” ?u’ational A\cademy of Sciences, h b l . S o . 1292, Washington, D. C.) 1)tiring 1)eccmber 1965, publicat’ions touching on S l I R came out a t a rate of about 13 lmprrs a day, their total number ticsing aliout 6000 for the period of this review (July 1, 1063, through June 30, 1965; tlie-e dates are based on the reception of iniblieations by libraries in the Washingtonl I). C., arcs). Fortunately, matei.ia1 wl)plied by abstract services (202, 547) obviated, for ])art of this period, the liage-by-liagc search through joiii,iiali ii(~ccssary for the writer of previouh rcbvicxivs (228). Tlir author of “Gas I siiiildy too much material. Fiirthoriiioie it i:, outdated by the time of ~)ublicatioii,and thc wealth of niateria1 ivliic*li ran only be :illuded to in any c a w , is not readily accessible for lack of a wliject iiitlcx. He who is looking for infoi~iiiatit~ii on a particular subject can rcfvr to (‘horiical Abstracts, since by that tiiiic tlict .sul)jcct index covering the l)ci,iotl of tliih review will then be available.. Still, the “information gap” exists; but tinwly summaries in depth, of a \vcll-cii,ciiiiiccribed talk, offer a better iiicaiis to fill it. I n fact, several publications of this t y p ha\.e appeared; a partial li4t of the-c as \vel1 a. of articles giving Iiackgi,ound information on S N R \vi11 be i’oinitl preceding the list of refcrc~llccs. 31o.t of the material contained in, or w l ( w ~ dto hy, such publications (66, 84, 20‘6, 651, G‘58, 859) \vas left out here,
D. C., 20204, and William B. Monk, U. S. Naval Research
erating at 60 M H z have been t h e most popular type since 1961. T h e first such instrument was improved in various ways (695), such as higher sensitivity, better magnet-cooling water control, and recorder performance. Sister models were developed which by the flick of a switch can be converted to fluorine spectrometers (335, 695). Whereas these spectrometers are single coil nonbridge type instruments with electromagnets, another type (534) is on the market which has a permanent magnet and a bridge-type detection system; conversion to F19, Bll, and P3l operation is possible with the aid of extra rf units and probes. The manufacturers of these spectrometers also supply homonuclear spin decouplers. A multipurpose spectrometer was built for operation a t 28.4 MHz a t the ‘Iechnical University in Dresden (East Germany) (550), with a resolution of 1 in 1.5 X lo7; “the Caechoslovakian 40 JIHz NMR spectrometer” is reported to have a resolution of 1 in 5 x 107 ($67). Commercially available instruments INSTRUMENTATION in the 90-100 AIHz class (333, 695, 681) (for protons) are all equipped with proDevelopment of new and especially ton field-frequency stabilization, two of R equipment has been them (333, 6.95) only when operating at very brisk in the past two years. Inthe proton frequency, and the third formation on instrumentation available (681) a t 30 and 60 AIHz as well as at in the United States can be obtained via 90 lIHz. Some of them have special a list of manufacturers (37). A review features: only one rf unit for operation article (262) describes S M R spectromat several frequencies (681); automatic eters exhibited at the ACHEMA y-axis homogeneity control (695); no fair in Frankfurt (West Germany). cycling of magnet (681), whatever the I n the folloning section we shall treat field strength. Two of them (333, 681) biiefly, first, complete spectrometersalso have broad-line capabilities. All coininercial or not-, and then party these spectrometers work in the fieldmodifications, and accessories adaptable sweep mode, but only with two of them to e\i,ting instrumentaion. Magnets as such, including Hall effect devices, (333, 695) is frequency-sweep possible will not be mentioned, except for the as far as we can tell from available catalogs. announcement by the Kational Magnet Laboratory (36) that s s e r a l magnets Operation at higher fields, that is, with field strengths between 65 and 260 about 40 kG and above, requires the kG may be used there free of charge by use of coreless superconducting soleany qualified scientist. Furthermore, noids (492). Spectrometers with such it must be said that very often important magnets are still in the experimental details on instrumentation are given in stage. Two of the problems in the dethe experimental section of publications; velopment of high-resolution spectromethis is particularly true for papers dealing ters of this type are to keep increasing with multiple irradiation (cf. section on the field homogeneity at the same pace multiple irradiation under “Obtention as the field itself, and to niake its operaof Spectral Parameters.” tion economically appealing through the Spectrometers. HIGH-RESOLUTIONdesign of a helium recovery system. SPECTROMETERS.Proton spectromHigh-resolution high-field spectrometry eters with pre-calibrated charts opof nuclei other than protons (540),
as duplication of the comprehensive work of their authors is unwarranted. We have also omitted EPR, most material on solid-state physics and on metals, and the vast amount of structure elucidation of natural products by XNR. I n the light of the foregoing we ask the reader to consider this review only as a partial and complementary introduction to the S I I R literature for the period under consideration. Explicitly, we did not attempt to be critical. We have tried here to treat mainly new techniques, equipment, and theory, as well as results novel from the N l I R point of view. Only a few examples were included which illustrate applications based on known techniques, and thus, paradoxical as i t may seem for a review appearing in this journal, ahnost all analytical applications of MIR were omitted. I n conclusion, we must repeat the perpetual apology of review writers for incomplete coverage and personal bias in the selection of material.
VOL. 38, NO. 5, APRIL 1966
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where enhancement not only of 6’s but of S/N are major advantages, is expected to benefit greatly, especially since the requirements of field homogeneity are often not so stringent as in proton spectroscopy. Since this is also the case for broad-line and pulse work (%), research in these areas will very likely receive a major boost. BROAD-LINE SPECTROMETERS. Commercial instruments of the single-coil type have been available; but many spectrometers in use are homemade, a t least as far as their nonmagnet part is concerned. A frequency-sweep spectrometer was described (50),as well a> a low-frequency superregenerative oscillator (490). A low-noise autodyne-type spectrometer with solid-state components was designed (566), which operates with frequency discrimination, displays the dispersion mode, and is said to be especially suitable for saturation measurements. PULSE SPECTROMETERS.Developments until 1963 are summarized in a review paper (421). Since then several commercial instruments have been marketed. I n one of them (98),various fixed frequencies are available between 1 and 200 RlHz, as well as fields up to 11.7 kG. Another type (438) supplies several fixed frequencies up to 60 RIHz and two units generating HI pulses up to 5 and 50 G respectively, and is equipped for flowing-sample experiments. A third commercial instrument (500), of the bridge type, also supplies fixed frequencies and is being used in combination with a superconducting magnet. Modifications (288) of one of these spectrometers (98) operating a t 48 KHz, and a new quite flexible type (50 KHz-160 MHz) were described briefly (290). Another low-frequency spectrometer (142) (153 KHz) was built especially for T1 measurements in organic liquids. 4 2-30 MHz apparatus (118) was developed for pulses u p to 5 kw, and a receiver recovery time in the microsecond range. The problem of receiver paralysis is reported to be avoided entirely by a “zero-time resolution” technique (546) developed for the study of solids. Fully described is the apparatus designed for pulsed double resonance by the adiabatic demagnetization technique (412). Solid-state circuit modules were used in a pulse pattern generator which enables one to switch easily from a T I to a T 2 measuring mode (198). A gated (“boxcar”) integrator using standard analog computing amplifiers was developed for the improvement of
S/N (564). Spectrometer Modifications and Accessories. Additional details of a field-frequency locking system described earlier were given, along with illustrations of double resonance (DR)
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(186). Another such system (62) was designed for broad-line operation in connection with C059 shift measurements. A 100 MHz proton spectrometer was modified so that DR at the frequency of C13 can be performed (13). A stable and simple oscillator capable of operation between 3 and 50 MHz was designed (317) for heteronuclear DR. For homonuclear D R , a system (501, 568) was developed which is suited for field- and frequency-sweep modes and Khich includes a field-frequency control system. Equipment for electronnuclear D R experiments was described in two papers (563, 572), and is commercially available (533). The modulation frequency for fluorine spectrometry was increased to 15 KHz so that most of F19 resonances can be observed at 56.4 MHz without interference between centerband signals and side bands; the details involving modification of the integrator and the probe were thoroughly discussed (529). For P31 spectroscopy the 2 KHz modulation circuitry was modified for suppression of the centerband (610). Devices to produce signal enhancement by repetitive scanning were adapted to high-resolution spectrometers (333,695). The SNAIL technique (141) developed for the same purpose was mechanized with microswitch circuitry (463), and itq use illustrated in papers dealing with the C13 satellite spectra of vinyl fluoride (463) and methylthiophenes (459). Reminivent of this technique is another S/N enhancement method employed in broadline spectrometry (685), where by the use of a gating circuit the actual absorption curve is obtained and not the usual first derivative. Frequency synthesizers have been used in several instances of D R experiments (145, 231) where the second frequency must be produced with high accuracy and precision, and also indirectly, where the magnetic field was to be swept in a very reproducible way by the use of a phase-lock system applied to a superregenerative crystalcontrolled oscillator (76). Marginal oscillators were studied from various angles and were further developed. The spectral noise distribution of the Pound-Knight-Watkins oscillator-detector was thoroughly examined ( % I ) , the results being particularly relevant in connection with fast-scanning experiments by the timeaveraging technique. Frequency pulling was found to be proportional to the resonance frequency and inversely proportional to the line width (700). A variation of the Hopkins-type oscillator was developed which, under the conditions of use (fluid flow through porous media), is largely free from microphonics (677). Frequency modulation
was introduced to suppress spurious amplitude modulation (67, 576); a further refinement concerns automatic frequency control (67, 723). A simple marginal oscillator was designed with a probe volume of 8 mm.3 and a resolution of 2 in lo6, to measure fields in hard-toreach locations (539). Another new broad-line spectrometer component is a synchronous-detecting x-y recorder, containing an electrodynamical wattmeter (689). The measurement of T I and T2 was described at length in two papers (686, 687), one of which (687) contains instructions on how to modify a commercial high-resolution 60 RIHz spectrometer for this purpose. Relaxation times between 10-3 and 102 seconds can be determined advantageously by adiabatic passage, where the field is swept in a non-recurrent manner (42). The calibration of field during sweep can be accomplished by a device which produces markers every 2 G over a range of 2 kG (from 7 to 9 kG) (499); a similar field-tracking system operates over a 0.5-kG range (from 3 to 4 kG) with an error of 1 in 3 X 106 (442). For lack of a better place we mention here a method whereby the sweep scale for proton high-resolution spectra can be precalibrated (354) : the composition of a mixture of substances was indicated, theje substances giving singlet peaks a t known intervals. Two aspects of low-field spectroscopy (175), which were given attention are the avoidance of spurious effects by shielding with mu-metal (592), and field homogeneity control (at 36 G) (142).
A large variety of probes for variable temperature studies (73, 389, 606) were developed, most of them with some type of flow-thermostating. Work at He3 temperatures (0.3’1.0’ K.) is possible with a “one-shot” glass cryostat (136); another device (293) operates a 4’ K. and between 20’ and 273’ K. Very high temperatures make it impractical to work with the crossed-coils system; thus several single-coil systems were developed where either the sample only (276) or the sample plus coil (276, 506) are heated; in one such system (606) temperatures over 1400’ IC. can be reached. Corrosive fluorine compounds must be handled in inert sample tubes. A method is described for making and sealing Teflon tubes which can be inserted in glass tubes and spun (567). Kel-F tubes are used in a system equipped with corrosion-resistant components so that liquids and gases under pressure can be examined (707); but here sample spinning is precluded. Details of a system for gases under pressure, mentioned earlier in the literature, are given in a thesis (156). Moisturesensitive powders can be packed in a
specially developed container which includes the rf coil (450). Finally, by the use of a tube assembly designed for the measurement of magnetic susceptibility, adjustment of the y-gradient for each new sample is obviated because of a favorable shape factor characteristic of this design (480); cf. also: (233, 237). OBTENTION OF SPECTRAL PARAMETERS
Analysis of Spectra. HIGH-RESOLCTIOK SPECTRA.I n NLIR as in other branches of spectroscopy, spectral analysis for the correct set of parameters-here the 6’s and the J’s-is more often than not overdetermined. Nevertheless, the correctness of a solution can often be decided by intensity arguments. Since the question of uniqueness still remains, theorems were developed so that critical assessment of unequivocality can be made (878). Relative and absolute signs of J’s need to be determined. Comparison of experimental and computed spectra is still a popular method for finding relative signs of J’s, often in conjunction with some method of increasing the ratio (J)/(S). I n evtreme case of this is the reduction of 6 to nearly zero by the use of the earth’s field as the only Ho. The observation that some geminal proton J’s decrease (become less positive) as the dielectric constant t of the medium increases, led to the suggestion that the absolute sign of such J’s might be determined by varying t of the medium (642). Methods other than measurements of intensities (in terms of peak heights) and line positions have been known, such as multiple irradiation for the determination of relative signs of J’s and orientation of molecules with respect to Ho for absolute signs. Line width measurements were proposed for special cases (417), and the conclusions based thereupon depend on the predominance of certain relaxation mecahanisms. The general problem was discussed for absolute signs ( 2 3 ) . The presence of large anistropies in the chemical shift tensor n 2 6 ) , where relaxation is due to interaction of the quadrupole of Iz with its surroundings (664), and the extension of the concept of molecular symmetry t o molecules with several conformations (410). Full use of symmetry properties results in considerable simplification of the description and analysis of spectra through the concept of subspectra (158, 159) which are sirnpler than the composite spectra and whose parameters
can be related to the molecular KMR parameters. These subspectra can be found by the use of group theory and ‘‘good” quantum numbers, and the often tedious application of the Hamiltonian can be entirely avoided. Chemical shifts in strongly coupled spectra, such as those of -k2B3systems, can sometimes be found by consideration of satellite spectra, should these systems be coupled in turn to magnetically active nuclei-(e.g. Cd, Sn, Hg, and Pb)-giving rise to satellite bands (246, 598). Particular Spin Systems. There are pitfalls in the interpretation of -1BX systems (104). The ABC system still presents problems and challenges, such as the matter of discarding imaginary solutions of the secular equation (108). -1n approximation method for this system was described (585). Multiple irradiation and/or C13 qatellites were used for unambiguous d B C analysis (231, 716). Under suitable limiting conditions, ABCD systems become manageable (55). Expressions for ABCDE and XBCDX were discussed (263). Another system still being given considerable attention is .lzB2 (3, 306, 612). Perturbation methods were developed for special cases [,i2Xz,(alX)zand (A1B)2 approuimations] (247, 248), but also for the related ;12BLX(249) and =izBzC systems (311). Extensive use of subpatterns was made in the analysis of the spectrum of 1,3,5-trifluorobenzene, an A 1 3 X , system mith Cav symmetry (S45), which had served as an early example in the application of group theory to K N R spectral analysis (721). The ten-spin system .1BKLX3Y3 was analyzed as a superposition of .1KLX3, BIILY3, and d B K L systems (239). For JXX= 0, the Xn-1a4’X’ncase can often be solved rather simply (284). Computer Techniques. Highresolution spectra arising from three or more spins are often so complex that a high-speed digital computer is needed for their analysis. Even in cases which look “first-order,” a beet fit is most conveniently obtained by making use of the computer programs written with iterative routines; a case in point is the five-spin system of indazole (7Z). d program originally designed to handle up t o eight-spin systems was extended to larger systems possessing symmetry and only one coupling constant with any two sets of nonequivalent (chemically shifted) nuclei (203). h n other modification concerns setting equal to zero certain off-diagonal elements in the Hamiltonian matrix, which involve J’s between nuclei with large chemical shifts (565). Iterative methods based on leastsquare refinement were developed (105, 477), one of which is described in de-
tail (105). I n this latter one, a simple statement in the input data allows one to introduce large chemical shifts without generating large errors; thus the computer automatically distinguishes betn-een, say, an *1BC and an ABX case. Programs which calculate an KMR spectrum without iteration (“computer simulation”) were written for the common case of isotropic coupling (657) and for the case where direct dipolar interaction terms must be introduced into the Hamiltonian which is encountered in the spectra of oriented molecules (644). S o t invented yet is the ideal program which would accept as input the spectrometer output, and yield as output reasonable sets of 6’s and J’s. Two programs constitute steps in this direction, both written for the ABC case (83, 107); they generate all sets of parameters compatible with line positions used as input data. The correct set will presumably be that whose line intenqities match most closely the observed ones, and which seems most “reasonable.” A unique position is occupied by a program (316) written for calculating Qpectra arising from nuclei with I > 1/2; it is also capable of handling up to ten spins of I = 1/2. BROAD-LINE SPECTRI. -1technique realizable by digital and analog computers is described which enhances the reiolution of spectra made up by superimposed lines, through the admixture of higher derivatives of the spectrum (14). hiodulation of field or frequency used in the detection of broad-line spectra causes distortion from the true line shapes. Modulation effects on the amplitudes and widths were discussed for two common cases, Lorentzian and Gaussian lines (6‘35), and corrections to the spectral moments of any order were derived for modulation-broadened lines detected a t any harmonic of the modulation frequency (722). The effects of intermediate rf field strength and of field modulation were considered with regard to the true line shape as well as T1 (257). Broadening by paramagnetic impurities can sometimes be accounted for semiquantitatively, as in the case of Mn-doped LiF (656). Such impurities often produce asymmetries which can be best expressed in terms of the third moment (692). Asymmetries arise also through bond anisotropies ( 2 7 ) , such as in F19 spectra (1,2,4-trifluobenzene and fluorohaloethaneq) in which the second moment is field-dependent. Methods were given to evtract magnetic anistropy parameters and quadrupole coupling constants, when these affect line shapes (346). The density of dislocations was related to the quadrupolar contribution to the line width (355). VOL. 38, NO. 5, APRIL 1966
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The effect of inotion in spin systems was dealt with in several cases. An exact expression mas derived for the second moment, where isotropic motion takes place (162). Line narrowing due to o 4 l a t i o n s of the ethylene part in Zeise’s salt was discussed in detail (449);in the same paper a computer program is mentioned which calculates the second moment of a spectrum from the observed line shape and the applied time constant. I n hydrate crystals, “fast” 180” flips of water molecules must be taken into account when second moments are used for structural determinations (532); the quadrupole splitting in spectra of D20-hydrates is a function of such flips (116). For cubic solids, vibrational corrections to the second moment vanish up to secondorder terms, unless such vibrations are anisotropic (513). I n hydrates and ice, line narrowing by vibrational (424, 533) and librational (424) motions of small amplitude were accounted for quantitatively. Multiple Irradiation. Before novel developments are described, a number of examples will be given vhich illustrate the power of multiple irradiation techniques already known. li complete analysis of t h e AzBzC system in 4-chloro-1,2-butadiene was greatly facilitated by spin decoupling (202). The same holds for the spectrum of D-glucal triacetate; in this case the additional use of 100 1 I H z spectra aided the analysis (278). Strong and weak irradiation with a second rf field was employed to obtain all proton parameters, including the relative signs of J’s, of propylene oxide (dBCD3 system), and of the nonaromatic portion of indene oxide (=1BCD system) (186). With the aid of double irradiation at H’ and C13 frequencie5, all the 6’s and J’s were determined from proton spectra of acrylonitrile with C13 in natural abundance, which are a superposition of .lBC and -4BCX spectra ( 2 S l ) . Calculation of frequmcy-sweep double resonance (DR) spectra in first order cases AmKm.. . . .X, and extraction of relative signs of J’s can be accomplished by a graphic procedure (48); relative signs of J’s between H-H, H-F, and F-F in several fluorocarbons were determined in this way. By transitory selective irradiation (TSI) experiments involving a third rf frequency, relative signs of J’s in several olefins were determined (509); line intensity changes aided in building u p the energy level diagrams. h similar technique produces selective enhancement of ill-resolved doublets, interpretable in terms of relative signs of J’s in ABX systems (216, 310). Moderately rapid exchange reactions can also be studied by TSI methods (217). D R pulse techniques are useful in the detection of resonances of Li6 (7.4%
334 R *
ANALYTICAL CHEMISTRY
natural abundance) by the “adiabatic demagnetization” method (412), and for the analysis of spin population redistribution in the study of relaxation mechanisms (262). The effects of pulse duration, amplitude, and repetition rate are discussed for various degrees of decoupling (469). llodulation methods in D R , proposed earlier, mere given a thorough theoretical treatment (252), especially regarding the transfer of modulation among coupled nuclei, and several important avenues seem to be opened, such as detection of usually obscured portions of spectra of protons coupled to other nuclei in low isotopic abundance. Thus proton signals involving such nuclei can be observed directly with the advantage of good S/N inherent in proton spectroscopy. Relative signs of J’s can also be determined in this manner. Fundamental studies of multiple resonance dealt with the application of stochastic (191) and of t v o or more discrete rf fields (624). It was indicated in the latter study how otherwise inaccessible 6’s can be measured and relative signs of J’s determined. Earlier treatments of the symmetry properties of D R spectra and of operators other than those for spin inversion were extended (190). Introduction of the Redfield formalism greatly simplifies the density matrix treatment of D R spectra and make? it possible to compute D R apectra by digital methods (45). An extension of this study dealt with the effects of HI and Hz and shows that even under off-rewnance conditions spin populations will be disturbed ( 4 7 ) . Explicit use of the Overhauser effect (1) was made in C13 spectroscopy (528) and in proton spectroscopy with H-TI decoupling (29). This effect can allparently be tranqmitted intermolecularly through dipolar interactions, as demonstrated by D R experiments with chloroform-cyclohexane mixtures (554A). Comparison with qteady-state Overhauser enhancement and TSI techniques shows that their differences reside mainly in their dependence on relaxation rates (508). If in D R experiments H, iz small, relaxation can be neglected in the interpretation of field sweep spectra (24). Relaxation effects in D R are discussed for a single spin r i t h I = 1/2 (554) and strongly coupled systems of two spins with I = 1/2 (555). Multiple Quantum Transitions. Analogies between multiple quantum transition ( X Q T ) spectra and D R spect r a were pointed out, with the AX case as a n illustration (125). -1general theory of MQT was developed (225). -inother theoretical treatment dealt with spin syatems having long correlation times; two-quantum transitions were
found to occur (127). A shift of 1IQT lines was found and a simple theory mas formulated for it and for the width of MQT lines (514). Two-quantum lines were predicted to be sharp in the case of a spin with I = 1 in molecules undergoing anisotropic motion, when the one-quantum lines are broadened beyond observation (126). Pulse Methods. Although inforination on new developments in this area is mainly to be found in the sections on instrumentation and relaxation, a few facets merit separate mention. The dead time of the receiver is a limiting factor in the observation of the total decay pattern. The “zero-time resolution” method permits observation of the whole Bloch decay, so that reliable second and fourth moments of spectra of solids can be obtained (646). Additional interpretation of decay patterns was developed in connection with studiez on crystallinity in polymers (273), and of rate processes in liquids, involving moderately f m t (545) and rapid (414) exchange. The line-broadening effect of dipolar interactions in solids can more simply be suppreb-ed by rotating magnetic fields (rather than by sample rotation) so that the shape of the free induction decay pattern can be interpreted advantageouqly in terms of small shifts of chemical origin or of the Knight type (49,255). Various Methods for the Modification of Spin Systems. Under this heading several topics are included which deal with the ways-intentional or not-in which spectra are modified by other than spectrometric means and temperature. The nature of solvents is well known to affect almost any feature of N l I R spectra through association and kinetic effects. How the -CH20H portion of the benzyl alcohol spectrum can change in this way, is described in a paper introducing the subject (425). There and in other instances, such as in bridged biphenyls (472), information on 6’s and J’s can uniquely be obtained by an appropriate choice of solvent. X practical application of solvent induced shifts is the differentiation between -CH20H and -CH20R resonances, where only -CHzOH resonances are affected by the solvent (pyridine) through complex formation betlveen pyridine and the -OH group (520). Vicinal ethylenic proton J’s (54, 394) as well as geminal proton J’s (207, 642) are sometimes solvent dependent; different averaging over conformer populations can hardly be invoked in these cases. That not one single cause, such as the “reaction field,” can be made responsible for changes in a’s, is illustrated for the case of some sub-
stituted cyclohexanones (395) and other ketones (214). Comparison of spectra taken of CHCl3 and CCL solutions underscores the fact that the very useful CHCI, is not a "neutral" solvent and that conclusions based on 6-correlations niay be erroneous, as in the case of stereochemical assignments in various situations (392). Strong medium effects appeared in the study of hindered rotation in X>*Vdimethyl formamide (718) and thionamides (408). Metal ions were put to use in the investigation of the site of metal binding (460): paramagnetic ions produce selective broadening, whereas diamagnetic ions cause deshielding; sometimes such effects help in the elucidation of complex spectra. Vhereas the prectding examples dealt n i t h proton spectra only, solvent effects are known with other nuclei such a5 C13 (430) and F19 ( 2 ) . Xore drastic spectral changes, of course, rewlt from modifications of molecular structure and, if performed in a tailored way) produce notable simplification+ in the interpretation of spectra. Contact shifts in paramagnetic molecules are often so large as to reduce proton spectra to rather simple patterns and facilitate the extraction of J's (177). Xore generally used is isotopic d x t i t u t i o n , such a5 studies with isoto1)ically pure 13" compounds, but especially Fubstitution of H by D. Esample. of thib m-ell-known technique (soinetime~ in conjunction with Ddecoupling) are : unequivocal assignand ment-: of proton 6's in purine (609), the study of rate processes, such as hindered rotation of the -CHO group in p-substituted aromatic aldehydes with D in the 3- and 5-positions (SO), and ring inversion in dll-cyclohesane (31,89).
The detection or identification of the often elusive hydroxyl resonances is indirectly made possible by esterification. One way is to form an acetate by direct acetylation (404); in another method acetates are produced by reaction with a ketone, or carbamate? by reaction with an isocyanate (259). In the latter method (259) no confusing side products arise. The derivatives show not only characteristic lines arising from the new functional groups, but also characteristic changes in the resonances of protons alpha to the ex-hydroxyl group (468). Another technique is the conversion of hydroxyl compounds t o triniethyldyl ethers, which was used in the study of flavanoids (706) and glycrrides (372). Carbonyl groups are made "visible" in proton K l I R by conversion to 0-methorimes (196). Changes in p H , which must' often be regarded as more than solvent effects, may alter a spectrum profoundly, as in proline derivatives ( 5 ) , and allow
observation of otherwise undetectable J's as in imidazoles (446). Partial orientation of molecules (100) can be achieved by a n external D C field, or by imbedding t,hem in the nematic phase of a liquid crystal (585, 590) or in a clathrate (283). -4 wealth of new information can and has thus been obtained, as predicted in the first published report on high-resolution spectra of oriented molecules (591), that is, on: average orientation and thus on intermolecular forces in liquids; relative internuclear distances and thus bond lengths; quadrupole coupling constants; and the isotropic as well as anisotropic parts of nuclear coupling constants, including their signs. Direct analytical applications were pointed out as arising from the fact that a molecule's orientability depends on its shape (87). Referencing. I n proton N l I R , t h e Si(CH3)4signal has now become t h e most xidely used internal reference marker. I n concentrated sulfuric acid solvtion, Si(CH3)4reacts with the solvent (568) and is therefore worthless. For these solutions other standard substances were suggested, such as Cf13HSO4 (286) or (CH3)IN13F4( 1 9 7 ) . Aqueous solutions sometimes present problems with a relative of Si(CH3)4, 3-(triniethylsilyl)-l-propane sodium sulfonate, which is used because of its solubility in water: aromatics niay produce unwanted high-field shifts (280) of the (CH3)3Si- protons, and shifts (6) are reported for acid solutions. Secondary reference substances for nonneutral solutions were discussed ( 5 ) . An alternative is the use of spherical sample containers (237), because no susceptibility correction need be applied. For phosphorus K N R , P406 was proposed as a reference substance (113). I t appears to have several advantages over the customary 85% &PO4: because of its low viscosity it gives a sharp signal; it contains more P 3 I per unit volume, and its resonancc peak is 112.5 1i.p.m. below that' of H3P01, that is, away froin most P resonances. -1lthough it will probably be employed mostly as a n external reference, those properties make is most useful in providing a locking signal for fieldfrequency stabilization purposes. CHEMICAL SHIFTS
General Theory. The chemical shift ranges for nuclei, which are proportional t o their atomic numbers, were explained by t h e paramagnetic shielding contribution and its dependence on t h e average distance between the nucleus and the bonding electrons (331). Expreq-ions were derived for the paramagnetic term fiom valence-bond and LC.10-110 mode1.s
(331). The xenon chemical shifts in xenon fluorides were shown to be dominated by the paramagnetic term (336). By a variation-perturbation method, the shielding tensors for several diatomic molecules were calculated (369). The diamagnet'ic and paramagnetic contributions to nuclear magnetic shielding may be determined from knowledge of the chemical shift and the spin-rotation interaction (43, 211, 212). Perturbed Hartree-Fock wavefunctions mere used in calculations of the shielding constants for H, F, and B in HF (653) and B H (654). Experimental chemical shifts were used to correct charge distributions calculated by a valence-bond method for allyl, butadiene, and pentadiene cations and anions (629). It was concluded that the first hydration shell must be included in the calculat'ion of the proton chemical shift of the hydroxyl ion if the experimental value is to be explained (265). Correlations. The subject of diamagnetic anisotropy of electron groups was recently reviewed in depth (84). References cited therein are not repeated here. The relationships among ring current,, chemical shifts, and aromaticity of heterocycles have been actively pursued (4, 148, 152, 187, 194, 244, 391, 596, 609, 655). dnisotropy effects were considered for the C-H bond and the methyl groups of aliphatic molecules (157), proton-containing end groups in polysulfides and polyselenides (693), and 6"s in silane, disilane, and trisilane (180). The unusual proton shifts in (tris-acety1acetonato)silicon chloride hydrochloride were attributed to the delocalization of electrons in the chelate ring (302). Anisotropic contributions to the chemical shifts of methyl derivatives \yere deduced with the aid of an observed linear relationship between selected J C 1 3 - = and 6"s (171). Low-field shifts of substituent and o-alkyl group protons in derivatives of 1,3,5-tri-t-butylbenzene were attributed to intramolecular van der TVaals interactions (250). The chemical shifts in substituted benzenes have been variously correlated with electron densities, Diehl's additivity rule, Taft/Hamniett u constants, and group dipole moinents (176, 448, 452, 638, 663, 732, 739). It was suggested that changes in bond hybridization and bond angles are responsible for the unequal isotope shifts of 6 ~ ' sobserved upon progressive deuteration of methane ( 6 6 ) . The deuterium isotope shift for molecular hydrogen was calculated by a semiempirical method bmed upon a RIorse potential (588). The 6"s in alkyl groups were shown to be proportional to spin-densities derived from ESR splittings of related radicals (208). - i n electronegativity VOL. 38, NO. 5, APRIL 1966
335 R
scale based upon internal 6”s in ethyl compounds and ,JHH’sin mono-substituted ethylenes was proposed (481). The hydroxyl proton shifts of p-substituted phenols in dimethyl sulfoxide solution linearly correlate with the Hammett u- substituent constants (515). Correlations between IR parameters of the = C-H stretching band and acetylenic 6 ~ ’ swere found (3483-51). The changes in the hydride 6”s for silicon hydrides, R3SiH, mere found to be approximately additive for substituents R, including those capable of n-conjugation (363). 6”s in polymethine dyes confirmed the theoretical prediction of alternating electron density in polyene chains (144). Additional proton chemical shift tabulations and correlations have appeared (161, 473, 505). The number of compounds in which nonequivalent methylene or methyl groups have been observed grows rapidly. Unusually large ASH’S were reported for the methyl groups of 3-isopropyltetrachlorophthalide (99) and the benzylic protons of l-phenyl-2(402), trans-1benzylphthalimidine benzyl-4-methyl-5-phenyl-2-imidazolidinone, and the cis- and trans-imners of its thiocarbonyl analogue (645). Lesser shifts were noted in other exampleq (41, 181, 314, 371, 465, 470, 474? 476, 525,646, 726). A brief general review of Flg XLIR has appeared (59), as well as a review of noble gas compounds which includes FlQdata on the xenon fluorides (327). A theory of 6 ~ ’for s conjugated molecules was presented (732). From the FIQ spectra of SF3 and CHF, in 8-quinol clathrates at low temperature, the anisotropic part of the fluorine chemical shift tensor for each was calculated (283). The Taft inductive and resonance parameters were determined for the substituent change CHI to CD, in p- and m-fluorotoluene (432). The m- and p-chemical shifts for pentafluorophenyl derivatives were correlated with the Taft parameters (SQO), and with charge densities and bond orders (77). &’s in fluoroalkyl substituted fluorobenzenes were explained by postulating interactions between the fluorine p orbitals and the n-system of the benzene ring (620). I n substituted 4-F-trityl cations, the ~ F ’ S were found to linearly correlate with stabilization energies (666). The Bll, as well as the proton, chemical shifts of p-substituted phenylboronic acids were linearly correlated with Hammett substituent parameters (56). For several tetrahedral BX4- ions, the 6 ~ ’ swere attributed to inductive and delocalization effects (614). The chemical shifts and relaxation times for the alkali metal and halogen in alkali halide crystals were explained by a model combining electrostatic 336 R
0
ANALYTICAL CHEMISTRY
and overlap effects (including seconddetermined from measurements on a n nearest neighbors) (733). oriented single crystal (388). Anomalies in the P31 cheniical shifts The isotope shifts of the K&O(CN)~ of some reportedly trivalent phosphorus C05Q resonance in aqueous solution, compounds have been eliminated by produced by C13 and XI5 substitution, the finding that they are in fact pentawere interpreted in terms of the changes in vibrational energy of the ion (387). valent derivatives (601). It was proposed that hydrogen-bonding is reThe temperature and pressure dependences of the Co5Q resonance in sponsible for the observed shifts of 6 ~ ’ s of triphenylphosphine oxide in [C0(”3)6] +3, [ c o ( c x ) and [Coseveral proton-donating solvents (428). (N02)e]-3in aqueous solution were de6p’s for P--N compoundb were tabutermined (62) and interpreted theolated (498). d n unusual shift was obretically (62, 189). A theoretical calculation of the served for the phosphorus resonance chemical shift of Xe12Qin gaseous xenon, in phosphacyanines (163). From P31 and H1 NMR data, bond angles and where exchange interactions between electron densities in tri-t-butyl phoscolliding nuclei were considered to be phite were deduced and related to its the major contributor to the shift, apreactivity (451). peared in reasonable agreement with experiment (6). The temperature deN14chemical shifts for a large number of organic compounds were presented pendence of 6’q in solid and liquid xenon which were characteristic of the nitrowas investigated for Xe12* and Xe131 gen-containing functional groups (297). (94, 709, 735), and interpreted (735). From the X14 chemical shift in metalthiocyanate coniplexes, it may be deSPIN COUPLING termined whether the metal is bonded to wlfur or nitrogen (319). Primary, There have been several major deqecondary, and tertiary nitro compounds velopments in the general theory of and nitramine- are distinguishable by spin-spin interactions. (nJ,, denotes a their N14 shifts (724). 6h14’s of pyridine coupling constant between nuclei i and pyridinium ion were investigated and j , and n is the minimum number of (261). Divrepancies between W 4 and bonds between them. The subdivision S15chemical shifts were observed in of this section into homo- and heteromolecules having barrier3 to internal nuclear coupling is made for convenience only.) The Fermi contact contribution rotation (557). C13 ”AIR waq recently reviewed in was calculated for interactions through depth (66s). While the 6 ~ ’ sin alkyl, three and four bond., with an emphasis on intra-atomic electron spin coupling allyl, and benzyl derivatives vary little (368). The general problem of esfroin the calculated constitutive cheniical shift>, the aromatic and carbonyl pressing the parameters for the spin systems show significant deviations Hamiltonian was simplified by the use (593). h thcoretical treatment of 6 ~ ’ s of electron density functions (437). in carbonyl groups appeared (426). The special case of coupling through one bond was treated by molecular orbital The relationships between FlQand C13 chemical bhifts in substituted p-fluoro(M0) theory (543), with these main benzenes were discussed (429). 6c results: the Fermi contact term contributes most heavily; signs of 1J,,’s data and/or correlations have also been reported for aioniatic amines and can be positive or negative; when F couplings are involved, the “average imines ( 4 Q l ) , aliphatic and aromatic compounds (21S ) , chloroniethanes (134), energy approximation” is poor, and lJFx’s are predicted to be negative; vinyl compounds (427), l-substitutedobserved lJ,,’s fall in line with predicI-hexpeh (679), and oyythiobenzoyl tions, especially the small values of chlorides (364). ‘JFB’s. I n the 0’7 N l I R spectrum of Cr&&-*, Homonuclear Coupling. HPDROdistinct signal. from the bridging and GEN. JHD in HD was found t o be terminal oxygen4 xere observed (206, independent of p r e w r e (151). d theo330); from the concentration dependretical value of -59 Hz was considered ence of the spectrum, information was to be in reasonable agreement with the obtained about the chromate-diobserved one (=!=43Hz) and to account chromate equilibrium. Xenic acid, for the predominance of the Fermi conwas studied by 0 1 7 N X R tact term (485). (671). The Tl*O5 chemical shifts for molten ‘JHH’s in >CHz groups were treated by a valence bond method (523, 624), and solid thallium zalts (274) and for and their dependence on the H C H bond molten thallous halides to which alkali angle was studied. That, in general, halide wm added ($7’6) were intcrpreted factors other than this bond angle in term> of overlap effects. prevail, was established by an 1\10 The covalent character of lead in treatment (642). Trendi of variations perovskite ferroelectrics and antiferroof ‘JHH’s were explained in term< of electrics was investigated by means of hybridization on the carbon atom, iubthe 6 of Pb‘o7 ( 2 1 ) . The Pb2O7 chemical stitution in the Q( and p positions, and shift anibotropy in lead molybdate \vas
-,,
the presence of neighboring *-electron systems. The whole body of ‘JHH’s which range from $52 Hz in CHzO to -21.5 in sym-cyclopentenedione, were satisfactorily interpreted. Later findings and independently established trends in ‘JHH’S support the treatment just mentioned, especially with regard to the electronegativity of substituents in three-membered rings (444, 594). Additivity of substituent effects was shown to hold in CHzXYtype methanes (497). It would be interesting to see whether the above nlc treatment can be extended to >XH2 groupings with X # C (179, 599).
The fhct that within fragments of the type -CH2-CHX(with X = H , ~ or f H ) the signs of ‘JHHand 3 J are opposite, was complemented by the observation that this sign relationship albo hold5 for a -CH20H fragment (392). Furthermore, in S-methylformamide the two 3J“’S involving H C N H paths were shown to be of the same sign (86). Relative signs of ‘JHH’s and ’JHH’s,nhere P is an intervening atom, were studied in a variety of bonding situations (445). Correlations of structure with 3 J ~ ~ are based on the Karplus relation, mostly in the form of 3J” = A cos’+ B p Expressions of the type A cos2p I3 sin2p were proposed (24S), where the sine term accounts for contributionb from 7-bonds. I n olefins the assignment of cis or trans structures based On C i S - 3 J ~< ~ tTUnS-3J~~ long ago became routine. Still it cannot be over-emphahized that the 3 J ~ ~ in both isomers must be known for such assignments; the case of 1-propenyllithium illustrates this point: 3JHa (olefinic) is 17.4 Hz for the cis- and 22.2 H Z for the trans-isomer (613). The statement that 3JHR’sinvolve three bonds and hence present a more complex situation than ‘JHH’s is almost a platitude. Here again electronegativity (E) effects were established (481), but in some cases of six-membered rings no simple correlation between E and 35”’s could be found (326). These E effects often cannot be disentangled from geometrical ones, since the precise angular relationships in HCCH fragments are not always known. Reliance on angular factors only may explain why sometimes “the” Karplus relation appears to fail (636). The orientation of substituents seems to play a role (81), a factor which may or may not make it easier to arrive a t a compresimilar to the hensive theory for 3Ja~’S, one developed for 2 J ~ ~ I’ n~ the . absence of such a theory, reasoning was done in the opposite direction (481), namely an E-scale was established on the basis of trends in cis- and trans3JIIH’sin olefins. I n this situation it is not surprising
+
+
that molecules whose geometry can be assumed fixed have been studied intensively (166). Three-membered rings belong to this class. E - 3 J ~relation~ ships (398, 444) with a heteroatom in the ring as well as with certain substituents on the ring appeared so linear that the prediction of hitherto unknown ‘JHH’S seemed to be warranted (444). In cyclopropanes an increase of E of a substituent X was observed to parallel an increase of 3J”’s of protons not bonded to the same C as X ; but if one proton of the pair is geminal to X, 3 J ~ H ’tend s to decrease with increasing E of X. In both cases cis- and trans3 J ~ ~followed ’s the same trends (694). I n certain cyclopropanes, substituents may affect 3J”’s in such a way that ~ application of the original Karplus relation would lead to wrong assignments of 3JHH’~for cis- and transcoupling (720). This is especially true with larger ring systems. I n cyclobutanes and even cyclobutenes the rings may be twisted, and thus additional uncertainties of the geometry of -CH-CHfragments arise. Conversely, departures from the ‘hormal” ~is-~J”’s--that is, from values to be ’ sexpected for eclipsed C-H bonds-were interpreted to be due to a skewed cyclobutene skeleton (219). That the Karplus relation was applicable a t all in benzocyclobutenes, was questioned in the light of a C i S - 3 J ~of~ only 3.5 Hz in the monoacetate of benzocyclobutenecis-diol (503). In summary, steric assignments on the basis of the magnitude of ’JHH’s ’ s must be made with caution and are only warranted where well defined geometries can be expected and bonding situations are known so that bounds set by existing theory are not overstepped. This holds all the more for “longrange” coupling, that is, for 725”’s with n 2 4 . Material on this topic accumulates much faster than theory develops, and reviewers of the subject cannot keep pace either. By the time a comprehensive summary (661) covering 216 references went to print, an addendum treating 90 more references had to be written. Two studies dealt specifically with H-C=C-C-H (“vinyl-allylic”) coupling (46, 243). Here, again as with %JHH’s, contributions from u- and 7-bonds were suggested to account for a n A’ cos2q B’sin’pdependence of 4 J ~on~dihedral ’ ~ angles. Since the two terms may be comparable in magnitude, but are opposite in sign, the interpretation of ‘JHH’S in terms of molecular geometry is difficult. Thus it is not surprising that CiSOid-4J~~’s are not always smaller than transoid ones (45.4). Other four-bond systems, where some of the uncertainties caused by free rotation are absent, are cyclic com-
+
pounds with one or more exocyclic double bonds. Here 4 J ~ ~were ’ s found to be observable only if the four bonds were coplanar and the H-C bonds in a transoid configuration (242). The knowledge of the signs of 4 J ~ ~ ’ s relative to each other and/or to ‘JHH’S or 3 J ~ ~mill ’ s hopefully go a long way in the establishment of meaningful correlations of some kind. To cite here only one of the host of such sign determinations: in a condensed dihydrofuran one vinyl-allylic 4J” was found to be negative with respect to 3 J ~ ~ ’ s , and another J” where the shortest coupling path is H--C-0-C-H turned out to be positive (241). Such a case is, in principle, a favorable one for the construction of theory: the cyclic structure and the double bond make for a rigid geometry. Again, the meaning of an alternation in the long-range JHH’S between CH3 protons in tetrachlorotoluenes, and the ring protons ortho, meta, and para, remains ambiguous, as long as the relative signs of these JHH’S are unknown (689). The magnitude of JHH’Sbetween protons on benzene rings and on a-carbon atoms was reported to depend on the hybridization of that carbon atom (494). The obvious anomaly of JHH’S (in Hz) in a benzocyclopropene where ortho-J- = 6.6, meta-J = 0.7, and para-J = 1.0, may have its cause in the suggested nonplanarity of the benzene ring (32). Electronegativity of substituentsattached along the coupling path or not-has its effect, as shown in the case of metal (M) compounds of the type Ph,--RI-CH2-C=CH, where the 4 J ~ ~ increase ’s with the electronegativity of M (630). .4n opposite trend was observed in 1-substituted acetones (668, 669); but here the dihedral angle between the C-H bond and the T orbital of the C=O bond seems also significant, and a further complication arises through a marked solvent dependence of these 4J~H’sin some cases. FLUORINE. As of now, there is no theory of JFF comparable to what is established to varying degrees for JHH’S.Features were noted which have no analogies in H H coupling, such as the much smaller magnitude of ’JFF’S than that of ‘JFF’Sin fluoroethanes, which was explained as a consequence of the high E of fluorine ( 2 ) . Temperature dependence of ‘JFF’S in fluoroethanes does not appear strongly related to differences in averaging over conformer populations, which may be a consequence of the much smaller variation of %JFF’s with dihedral angles than in the proton case ( 2 ) . I n contrast 3 J ~appear ~ ’ ~to be intrinsically temperature dependent (2, 93, 496, 653), as shown for olefins where rotation is impossible, as well as for ethanes; excitation of molecular vibrations was VOL. 38, NO. 5 , APRIL 1966
337R
invoked as the cause (496, 553), and medium effects also seem operative (496). The controversy about “throughspace” coupling (495) is still in full swing ( 2 , 78); it is complicated by the fact that the contributions of this coupling mechanism are hard to distinguish from other factors, such as number and type of intervening bonds, and because the signs of the J’s must be taken into account as well. Correlations of electronegativity of substituents ’ sonly roughly linear (2). with 3 J ~ ~are ‘JFF’Swere found to increase with the oxidation state of the intervening atom R, where R was iodine and sulfur (252). OTHER NUCLEI. The sign of ‘JCC in CH3CN was proven to be the same as that of ‘JHC(435). A calculated I J C C of 33 Hz for CzH6agreed well with the experimental va!ue of 33.6 Hz (272); a similar agreement was observed for l J ~ nin~ [SN(CH3)3]L n (4600 us. 4400 Hz) (172). IJNN(for Y4)in the azide ion were concluded to be smaller than 30 HZ (296). The use of XI5-enriched compounds made it possible to detect a 1JXNof about 14 Hz in trans-azoxybenzene (70). Heteronuclear Coupling. For various reasons we do not claim to cover even the most important experimental and theoretical developments in this area. New information has appeared a t a n extraordinarily rapid pace, and the field as such is so vast t h a t i t deserves one or several special monographs, more perhaps than any other field, mainly because of the recent resurgence of inorganic and organometallic chemistry. Treatment of H-X coupling will be modest, and only superficial attention will be given to Y-X coupling (X z Y z H). The statement made in the introduction about the inherent obsolescence of this review applies particularly to the present section. H-X COUPLIKG.A number of papers have appeared on the general topic of ‘JHXand ‘JHx,which deal mostly with coupling in tetrahedral hydrides and tetramethyl compounds (173, 560, 636, 714). Correlations with atomic number and s-character are still the subject of intense discussion. Vinyl metal compounds form another class studied to some extent (109, 439, 715). H-F Coupling. ‘JHF’S follow a better linear relationship with electronegaS if their confortivity than 3 J ~ ~ ’do, mational average is considered ( 2 ); but several anomalies in their temperature (t) dependence remain to be explained. For instance in CHClzCHFCl either a change in the sign of the slope of (3JHF(t)),, or intrinsic temperature dependence of t r a n s - a n d gauche-3JHF’s may be the cause ( 2 ) . Solvent dependence of *JHF in CHBrC1F was attributed to hydrogen bond-
338 R *
ANALYTICAL CHEMISTRY
ing (192); this JHFdecreases from 52.1 Hz in cyclohexane to 50.6 Hz in dimethylsulfoxide. New examples of long-range JHF’s are 2,4,6-t-butyl-3fluorophenol (574) where F is coupled unequally to the protons of the 2- and 4-t-butyl protons; and l-fluoro-12methylbenzo [c] phenanthrene (493), where F is coupled to the methyl protons seven bonds away. Here again the case of through-space us. throughbond coupling is open to debate. H-C Coupling. ‘JHC’S have been in the focus of study, discussion and controversy ever since their correlation with s-character of the H C bond was proposed. The correlation has been widely used for the rationalization of bonding situations, especially in strained systems (121, 455). Cyclic systems (165, 471) mere discussed from this point of view in terms of ring qize and C’C”C”’ angle (471) when C2\, synimetry prevails about C”, and the concepts developed there were extended to C3v situations. An isotope effect in several -CDHfragments, related to H C us. H D distances, went the wrong way (220), if the s-character wa? considered to be the only factor determining the magnitude of ‘JHc’s. The effect of halogen substitution on C was treated by the mavimum overlap orbital method (258), with certain assumptions about hybridization and ionic character of C-X bond.; two of the results were that the C-F bond should have very little ionic character, and that ‘JHc’s cannot be used to calculate bond angles about substituted C’s. Known values of ‘JHC’S served to adjust hybridizations of orbitals in three-membered rings for ionic character (111). Halogen substitution on the same C, not just spatial provimity of halogen to the C-H bond, was reported to be necessary to affect ‘JHC’S (483)* Deviations from additivity in polysubstituted inethanes were accounted for by the introduction of interaction terms characteristic of substituents (169); deviations per se from expected ‘JHC’S were discussed in ternis of a deviation parameter (328). Correle tions of ‘JHC’Swith other molecular parameters were given, such as with 2 J S n - ~in ~ 3organotin compounds (1l 7 ) , group electronegativities ( 1 3 4 , ionic state of the molecule (d7‘9), and ‘JHB’s in heteroaromatics (678). Either in the light of the foregoing or on the basis of other considerations, doubts were voiced whether ‘JHc’sare a direct measure for s-character under any circumstances, (356, 857) or in the presence of substituents “capable of extraelectronic effects” (444). In the ~ ’ s case of ‘JHcC’s and 3 J ~ ~no~ single factor was found which could satisfactorily be correlated with existing data (357). Various ab initio calcula-
tions by a self-consistent-field model produced ‘JHC’S of the right order of magnitude, but with a n extreme sensitivity to H C distances and to the 2u AI 0 coefficients chosen (52). H-X Coupling (X z F,C). I n quaternary ammonium salts ‘J~H’S were found to be smaller than ’JHN’S, and ~ J H Kwith ’ S n > / 4 to be unobservable (223). A similar situation was also noted for ethyl lead compounds (236). I n proton D R studies of NI5enriched N,N-dimethylformamide (85), and N-methylformamide (86) the two ‘JHx’s were shown to be of opposite sign. Correlations of ~ H ’ S and 3 J H ~ ~ p ’ s were discussed in terms of s-character of P-0 bonds and electronegativity of groups bonded to P (697). T h a t often ‘JHP< 3 J and ~ n~J H p ’ s , with n 2 4 , =SO, with only single bonds and carbon atoms between H and P (cf. JHS’S and J ~ p b ’ s ) , was noted in conjunction with the observation that JHp’s are smaller in neutral molecules than in phosphonium ions (295). ”JHP’S with n up to 6 were measured in acetylenic phosphines (115). I n triethyl phosphite, the signs p the same, but in of 3JHpand 4 J ~ are triethyl phosphate they are different (175). Deviations from previously suggested rules on the additivity effects on ‘JHs,’s were corrected for by the inclusion of interaction terms in the corresponding and H-H expressions (443). H-Si coupling constants were found to be of the same sign in molecules containing only single bonds, but not in those containing -Si-kC-H fragments (146). Here as elsewhere comparison of signs (and often of magnitudes) of coupling constants must be made on the basis of reduced coupling constants which are defined by K,, = 2 ~ J , , / g y , y , , so that the signs of the magnetogyric ratios y are taken into account. A trend toward higher (more positive) reduced coupling constants was noted with increasing s-character of the Si-C bond. H-Sn coupling data were reported for a variety of compounds and correlated with other data, such as ‘JHc’s, electronegativity of substituents on Sn, covalent character of Sn--N bonds, and I R spectra (90, 117, 255, 376, 441, 544, 701). JHTI’S were measured in EtzTIX compounds (287) and in several series of the type R2Tl+, and RTl+’ (439). I n these latter and in TI adducts of norbornene and norbornadiene (29), long-range JHTI’S were observed and discussed in terms of bond character and stereospecificity. H-Hg coupling was found in RzHg (556) and RHgX (287) compounds, where R was an organic radical and X a halogen. Y-X COUPLING. The strong dependence of ‘JFBin BF4- on electrolyte composition of the solution was a t -
tributed to ion association effects (374). Coupling was reported (and discussed) between F and C (443), Hg (170, 201), N (478), K b (518), Pt (82), Se (71), Si (145),and T1 (201). 2JFBgin(CF&Hg depends strongly on the solvent used (170). General correlations of JFX’S were made in terms of the atomic number of X (561, 562). N-Pt couplings were determined in Pt-SCN complexes (319),and N-C couplings in several “5-enriched organic compounds (70). SPIN RELAXATION
Theory of Relaxation. Without the usual assumptions concerning correlated spin and bath temperatures, a quantuni-statistical-mechanical theory of spin resonance and relaxation was derived which gives a non-Narkoffian equation of motion for the spin system (39). The behavior of a spin */*system in the presence of spin-lattice relaxation processes was reconsidered (245) with results differing from those of the Bloch or modified Bloch equations. A general theory of spin relaxation was developed which gives insight into the relaxation process and long correlation times (240). A generalized Pauli equation was the starting point for treatment of Tzand spin temperatures (536), and Redfield’s master equation was used for a calculation of T iin high and low fields (338). The truncation of the Hamiltonian after transformation into the rotating frame was shown (122) to be responsible for the anomalously large linewidths calculated for the Redfield rotary saturation lines. The equation of motion of the density matrix in the general case was derived from Korringa’s theory with consideration of decoupling effects (476). A theory of relaxation for gaseous orthohydrogen in the strong collision limit was given (269) as a generalization of the Schwinger-Uloembergen theory. A series of three papers was concerned with the general theory of cross relaxation (267). Cross-correlation effects were considered for the specific case of thiee-spin systems (304, 587, 602) and for multispin systems (603); significant deviations from simple exponential decay of the magnetization can occur. The elements of the relaxation matrix in a multilevel system are derivable from the saturation recovery curves (381). Detailed calculations indicate that variation in the distance of separation of two spins, such as occurs in flexible molecules, retards rather than increases the relaxation rate (730). A computation of intermolecular dipolar relaxation, due to rotational as Eel1 as translational motion of the molecules, and placement of nuclei a t the periphery of the molecule (32/t),gives a T1shorter (by about 7’70 in the typical case) than that predicted by
the usual model. Equations were derived which show that measurement at a single temperature, of the transient and steady state nuclear Overhauser effects and of the relaxation rates of each type of nucleus in a system of two different kinds of spin nuclei, can yield the separate contributions from intra- and intramolecular relaxation, including spin-rotation ($23, 434). This method was tested experimentally (434). Expressions for T1 and T 2 were derived for dipolar relaxation via atomic diffusion in solids (182). Correlation Times. The detailed time dependence of the rotational correlation function which governs intramolecular nuclear relaxation for molecules of certain symmetry classes was shown to be given by the Fourier transform of a Raman spectrum (261). I n a comparison of dielectric and NMR relaxation measurements, specific distributions of correlation times were used in a consideration of theoretical T Ivalues (132). An apparent phase transition effect, occurring just before the onset of rigid-lattice behavior, was attributed to a broad distribution of correlation times (569), which may be quantitatively assessed in some systems. il new model for the calculation of KhIR and dielectric correlation times, applicable to light molecules in the liquid state, was constructed (74). The dependence of the relaxation time of a quadrupolar nucleus upon molecular shape was calculated for an ellipsoidal body undergoing anisotropic rotational motion (621). Techniques. Diffusion rates u p t o lo5 times slower than those formerly accessible can be determined with a new pulse technique which measures relaxation times in the reference frame rotating a t the Larmor frequency ( 9 , 632). By the use of a time-dependent field gradient, it is possible to obtain diffusion coefficients much smaller than those measurable by the steady field gradient spin-echo method (650). “Solid echos,” which depend upon the relative phasing of the two rf pulses and are due to dipolar interactions in solids, were observed and treated theoretically (447, 546); second and fourth moments, as well as the zero time free induction decay, can be obtained directly. The ?r/2 phase shift in the GillMeiboom/Carr-Purcell method for measuring TZcan be replaced by a small resonance shift (689). Relaxation Times. Jump-diffusion (615) and modified jump-diffusion (614) models for the T1 of liquids were developed and applied to water. The calculated Ti's for gaseous and liquid xenon with momentum-dependent interactions in the constant-acceleration approximation were found to be in reasonable agreement with the experimental values (510). Proton and fluorine Ti's were measured for benzene and several ben-
zene derivatives over the accessible liquid range, and interpreted in terms of molecular motions, correlation times, and the probable relaxation mechanisms (268). Additional proton relaxation time measurements on benzene (713) and benzene-benzene-de mixtures (641) were reported and discussed. Deuterium Ti's in D?O and benzene-de were related to molecular reorientations (728). For the purpose of separating the translational and rotational contributions to TI,the relaxation times of n-paraffins (7,8,729) and camphor (683) were studied as a function of concentration in an inert solvent. The experimental and calculated spinecho responses from flowing w t e r samples compared well; except a t high flow rates, the diffusion term may be neglected (40). The line width of the resonance together with the known quadrupole coupling constant rvere used for the determination of rotational correlation times (296). Rotational correlation times are also obtainable from N14 nuclear quadrupole resonance relaxation times (12); comparisons with results from proton KhIR measurements were made (12,637). Deuterium quadrupole coupling constants were computed by the use of proton T1 data from benzene/benzene-ds and acetone/acetone-de mixtures, and deuteron T1data from the deuterated species alone (79, 80). The T1of water in highly viscous solutions of tylose and gelatin is not very different from that of pure water (103). The correlation times of the relaxation processes in organic solvents containing free radicals were evaluated by measurement of the frequency dependence of the relaxation time of solvent protons (288, 290). Expressions were developed for nuclear relaxation induced by paramagnetic ions with anisotropic g factors (652). The properties of ionic solutions continue to be investigated, through relaxation measurements on protons (195, 271) and solute ions (573, 682). Solids studied by relaxation measurements include branched polyethylene (419) 1,2-dichloroethane (proton and chlorine Ti's) ( 5 5 l ) , ice (6S6), and n-alkanes (22, 342). The T? of 0 ’ 7 in hInO could be accounted for by Moriya’s theory, while that in COO could not (512). RATE PROCESSES
The study of rate processes by N l I R was reviewed in t n o articles, one of a general nature (163) and the other directed to ligand exchange in complex ions (531). The applicability of the spin-echo method for studies of chemical exchange has been explored, and the general features given (16, 75). Specific aspects of the method, including closed formulas VOL. 38, NO. 5, APRIL 1966
339 R
for two-site exchange (17) and studies of ring inversion in cyclohexane (15) and 1,l-difluorocyclohexane (18), were presented. Spin-echo techniques were used to elucidate the mechanism of proton exchange of neopentyl alcohol in acetic acid (124). A theory for the calculation of N M R spectra of coupled spins changing sites intramolecularly was given (339). d multiple resonance procedure for the study of exchange of a nucleus among three nonequivalent sites was developed theoretically and applied to basecatalyzed keto-enol isomerism in acetylacetone (217). Longer residence times of hydrogens can be quantitatively measured by H2NMR than by H1KMR a t the same applied field, due to the smaller chemical shifts (in Hz) of H2 (160). An applied magnetic field may decrease exchange rates if the difference in resonance frequencies of the two exchanging species is comparable to the zero-field rate constant, expressed in the corresponding units; the calculations are applicable to exchanging electrons and nuclei, providing the latter are undergoing quantum-mechanical exchange or tunneling, and are not chemically exchanging (623). Equilibria in molecular scrambling reactions were treated by stochastic graph theory, with results applicable to the elucidation of thermodynamic properties, ring-chain equilibria and gel points and to the interpretation of S M R spectra of real systems (462). Proton N M R spectra of equilibrated linear and cyclic polymethylsilazane mixtures were analyzed by the use of this technique (694).
T h a t delocalization of electrons toward the periphery of the ligand is likely, was shown in a study of 6 ~ ’ sin paramagnetic complexes of tetraphenyl porphyrin chloride and structurally similar pyrromethenes (178). Electron transfer reactions, for example, in quinone-semiquinone radical equilibria, produce changes in the N M R line shapes, which can be related, sometimes quantitatively, to the proton relaxation mechanisms (340). Proton exchange rates of amines in acetic acid can be determined from measurements of exchange broadening and appropriate 6”s of the N H and COOH signals, and from knowledge of J s H and the T I of S14(272). From such rate data, the equilibrium constants for metathetical ion-pair reactions (in which one of the cations does not have rapidly exchangeable protons) were accurately determined (548). Although the determination of activation energies by KMR has now become almost routine, the spread in reported values for AH, AS, or AG for a given molecule almost invariably exceeds the probable limits of error given by individual authors. Considerable effort has 340 R *
ANALYTICAL CHEMISTRY
been devoted to the analysis and elimination of systematic and conceptual errors in the determination of activation energy parameters. I n a study of barrier heights to internal rotation in Ar,Xdimethylamides, factors which introduce significant errors were found to be field shape and homogeneity, line height ratios, and the assumption that chemical shift differences are temperature independent (238). Along with an introduction to the subject, a critical discussion of the limitations of S M R methods used for the conformational analysis of substituted cyclohexanes was presented (199). Systematic errors in both the high resolution and the spinecho techniques were discussed (16). Computer methods of line shape analysis and spectrum fitting were used in the study of ring inversion in cyclohexyl fluoride (88)and 1,l-difluorocyclohexane (341). The former paper (88) al-bo contains a compilation of activation energy data for several six- and eight-membered ring systems. The temperature dependences of the &’s indicated that ring inversions, and therefore puckered conformations, exist in some substituted 1,l-difluorocyclobutanes (383). Chair-boat interconversion of cis-2,3-dichloro-1,4-dioxane is slowed a t temperatures below -120’ C. (19). The activation energy for ring inversion in cycloheptatriene was obtained (28,337). Inversion was studied in o,o’-bridged biphenyls (379, 472, 607). For nitrocyclohexane, room temperature populations of the conformation having an equatorial nitro group were calculated to be 100% (311) and 85% (WOO). Free energy differences between axial and equatorial conformations of cyclohexanols were determined from infinite-dilution hydroxyl 6 ~ ’ sof the subject compounds and their cis- and trans-4-t-butyl derivatives (516). Conformer interconversion in some fluorosubstituted trityl carbonium ions was found to have a n activation energy of ca. 9 kcal./mole (580). The commonly accepted notion of hindered internal rotation in alkyl nitrites and nitrosamines was challenged on the grounds that facile exchange of the NO group with the OH proton of alcohols occurs (95); the previous assignments of cisand trans-isomers were reversed on the basis of solvent studies, and steric hindrance and shielding effects. The discrepancies observed between the W5and N14 chemical shifts in compounds possessing barriers to internal rotation were attributed to incomplete averaging of electric field gradients, and it was proposed that by use of other pairs of nuclei, one of which does not possess a quadrupole moment, barriers to rotation in many other systems may be investigated (557). The activation energies for internal rotation in two N,N’-disubstituted thionamides were found to be
much higher than those in the corresponding amides (408). It was suggested that in bis(ethy1ene)r-cyclopentadienylrhodium(I), the ethylene group coordinated to rhodium rotates around the coordination bond axis, with a n activation energy of 6 kcal./mole (137). I n contrast, i t was found that the duroquinone molecule does not undergo rotation in a nickelduroquinone-cyclooctatriene sandwich compound (605). The F19 spectrum of IF, is a broad doublet attributed to coupling to the iodine nucleus (I = 5/2), with the expected quadrupole relaxationinduced line broadening (479). The seven fluorine nuclei are rendered equivalent by a postulated time averaging of the iodine-ligand bonds. The problems associated with distinguishing among the possible conformations of chain isomers have been discussed in several papers (133, 363, 436, 51 7 ) . Because considerable changes are observed in the J’s of dibasic acids upon cyclic anhydride formation, it was suggested that AJ’s be used to determine the relative amounts of linear and cyclic species in ring-chain equilibria (326). Valence tautomerism studies by NMR are numerous; the following serve as examples. bicyclo (4.2.0)octa-2,4,7triene/cyclooctatetraene (702), norcaradiene/cycloheptatriene (derivatives) (703)’ bicyclo( 6.1 .O) nona-2,4-diene/ cyc1onona-lJ4,7-triene (680). Degenerate Cope rearrangements of bicyclo[5.1.0]octa-2,5-diene (167), bullvalene r607), and bullvalene derivatives (464) were studied. It was demonstrated that valence isomerization in bullvalene ceases upon its complexation with iron carbonyls (604). Transannular hydrogen jumps were studied in cycloheptatriene (673), cyclopentadiene (579), idenes (63, 579, 71l ) , phenylcycloheptatriene (672) and cyclooctatriene (681).
Correlations between the 8”s of NH and IR parameters were found in a study of Schiff bases (174). Amideimino1 tautomerism was discussed in a review article (631). Keto-enol tautomerism was investigated in phenylated aldehydes (516) and in a large group of $-dicarbonyl compounds (577). The equilibria among the keto form, the hydrated form, and the anions of pyruvic acid in aqueous solution were quantitatively assessed (67). Tautomeric equilibria of phenylazonaphthols were deduced from 0 1 7 chemical shift measurements (407). The protolysis kinetics of glycine (619) and of the peptide hydrogen of glycylglycine (618) were determined. Protonation of a substituted 2-pyrazoline was shown to occur at N1 (184); changes in the spectra with temperature were attributed to varying rates of N1 inversion. Pyrid-2-one (684), and poly-
amine and aminocarboxylate chelating agents (877, 661) were also the subjects of protonation studies. The acidic or basic behavior toward sodium methoxide of the nitro-substituted aromatic amine type indicators was characterized in dimethyl sulfoxide solution (158). The single broad line of field-independent width in the F19 spectrum of XeFBin H F solution is possibly due to exchange with a third species having a large F19 shift, or to the existence of a low-lying triplet electronic state in XeFe (96). Unusual isomerization or exchange processes were reported for N trimethylsilyl-carbimid acid trimethylsilyl ester (6@), 3-thioacylmethylene1,2-dithioles (596), and isobutyraldehyde tosylhydrazone in basic DzO ($99). Measurement of the NI4 line width allowed determination of the activation energy and rate constant for the electron transfer reaction between ferri- and ferrocyanide ions in aqueous solutions
.
(628)
ASSOCIATION PHENOMENA
Solvation of Ions. Selection of the appropriate paramagnetic cation to shift the 0 1 7 resonance of the free water signal permits the determination of the amount of bound water, and hence the hydration number of a second cation (11, 130). One technique provided accurate data from 1.5% 017enriched water (11), while the other required 11.5y0 enrichment (130). From spinecho self-diffusion data on aqueous solutions of electrolytes, hydration numbers were deduced (420). How the structure of water is perturbed by dissolved electrolytes is of great interest. Self-diffusion studies (420) indicate that most, but not all, ions reduce the diffusion rate of water molecules. I n general, the larger a n ion, the lower the stability of the surrounding water structure (195). The reorientation times of water molecules in the vicinity of ions are longer than those in pure water (147, 300). Hydrogenbond modifications are concluded to accompany water structure changes (298). If the radial distribution function of molecules near a n ion is taken into account, the mobility of water molecules near that ion is predicted (682) to be greater than that given by the treatment of Hertz. Deuterium N M R of DzO-solvent mixtures permits straightforward study of selective solvation of cations because signals from solvent protons do not interfere (160). The negative hydration of [Cr(CN)6]-3 (271), the second hydration sphere of [ C r ( H 2 0 ) ~ ] +( I3O ) , inner and outer complexes of h/Ig+2 in MgS04 solutions (4O6),the effectof D 2 0 content upon proton relaxation in solutions of (649),and proton lifetimes in the hydration shells of several cations (409)
are examples of studies in this field. An increase in the proton T 2 upon contraction of living frog muscle has been attributed to the release of bound water (91).
The exact relationships of salt concentration and solution viscosity to relaxation processes of protons and other nuclei in the solution have not been completely determined (299, 843). Solvent systems in which solvation of ions was studied include 50% dioxanewater (227), aqueous tetrahydrofuran (228), 50% pyridine-water (226), and methanol (415, 648). Ion Pairing. Symmetrical ions containing a quadrupolar nucleus coupled to spin nuclei facilitate the study of ion association (466, 519). Ion association shortens the TI of the quadrupolar nucleus, tending to wash out its coupling to other nuclei. I n polar solvents, the ions are dissociated, the electric field around the quadrupolar nucleus is symmetrical, causing its TI to be longer, and its coupling is felt by the spin 1/2 nuclei (456). Thus, coupling of the quadrupolar nucleus As75 (I = 3/2) to fluorine in AsF6- produced a quartet of narrow lines in polar solvents, but broadened lines in less polar solvents (519). I n the proton spectra of tetrasubstituted compounds of Group I11 and V elements, corresponding effects were observed (456). Line shapes were calculated for a spin nucleus coupled to a quadrupolar nucleus as its relaxation time varies (664). The distance of closest approach of neighboring ions in the melt can be determined from the chemical shifts of the liquid and solid a t the melting point (486). From TP5 chemical shift data, it was concluded that the ion separation in TlCl decreases by 0.18 A. a t the melting point (486). Involvement of ion pairs in the proton exchange mechanism of methyl-substituted ammonium salts in t-butanol was deduced (123). Coordination in Complex Ions. The large high-field shifts observed for protons bonded to transition metals in diamagnetic octahedral and squareplanar complexes were theoretically considered (101). I n both cases, d-electrons are principally involved, and anisotropies in the proton shielding are expected. The temperature dependence of chemical shifts in paramagnetic octahedral complexes was calculated (256).
N M R and other data support the contention of Grinberg and Nikol’skaya that ligand exchange in complexes is faster the more stable the complex, because of the dominance of local interaction energetics (617). The study of ligand exchange rates in complex ions by NMR was reviewed (631). Thermodynamic parameters for ligand exchange of [Ni(NH8)6]+z in
liquid ammonia were determined from iV4 line width measurements (253). The different lifetimes of the various bonds of a multidentate ligand are reflected in its N h I R spectrum (150). From N l I R measurements on the central ion of a coordination complex, exemplified by C059 studies in several cobalt complexes (286), kinetic and structural information was obtained. Ligands which do not contain hydrogen or fluorine atoms may be profitably studied through VMR of nuclei suchas W4 or C13 (627). Relaxation induced by a paramagnetic ion may wash out spin-coupling within ligands (616). Other studies of coordination chemistry include the following: determination of the relative donor strength of a ligand (4O6), stereochemistry and covalency in hexamethylphosphoramide complexes (710), the structure and/or exchange processes of various complexes (129, I @ , 204, 522, 652, 678, 671, 688),and the effect of complexing between Mn+2 and macromolecules, such as proteins, upon the relaxation time of water (128, 183). Hydrogen-Bonding. Proton N M R has been used in the study of a myriad of hydrogen-bonding systems. Typically, the lack or presence of a chemical shift upon dilution is taken as evidence for intra- or intermolecular hydrogen- bonding, respectively. Quantitative conclusions are more difficult to reach in many cases because of simultaneous equilibria. Equations were derived for systems of two competing intermolecular equilibria in which the solvent may be inert or constitute one of the hydrogen-bonding species (585).
Variations in the chemical shift with temperature are often interpreted strictly in terms of the making or breaking of hydrogen-bonds. However, regardless of bond-making or -breaking, temperature dependence of the chemical shift in a hydrogen-bonded system can be expected because of variations in the vibrational character within the 0H- -0 bond (484). Electric field and neighbor anisotropy effects adequately accounted for the N M R shift upon complex formation for weak hydrogen-bonds (64). The relative importance of the repulsive overlap and attractive electrostatic energies of hydrogen-bonds between chloroform and nitrogen bases was studied (65). The equilibrium constants for hydrogenbond formation between chloroform and various solvents weredetermined from the 8”s of chloroform (647). A qualitative relation wasobserved between the change in JCIQof chloroform and the hydrogen-bonding ability of the solvent (192). The El4 chemical shift changes of pyridine as a function of concentration in methanol were interpreted in terms of a hydrogen-bonding model (589). It was confirmed that involveVOL. 38, NO. 5, APRIL 1966
* 341 R
ment in intramolecular hydrogen-bonding shifts the resonance of aromatic carbonyl functions downfield (431). The chemical shifts of the intramolecularly hydrogen-bonded hydroxyl proton in nitro-substituted phenols were correlated with the infrared hydroxyl stretching frequencies (143). Hydroxyl 6 ~ allowed ~ s classification of phenol (521). Hydrogen-bond derivatives complexes of 2-propanol, benzenethiol, and t-butyl mercaptan with N-methylacetamides were investigated (461, 667). Hydrogen-bond studies by EMR were the subject of a review article (662). Lewis Acid-Base Complexes. The equations used to determine thermodynamic parameters in hydrogenbond studies are readily adaptable to the study of Lewis acid-base equilibria (281, 610). Equilibrium constants were obtained for thenoyltrifluoroacetone/organophosphorus ester adducts (610) and 7,7,8,8-tetracyanoquinodimethane/aromatic hydrocarbon adducts (281). Expressions were developed for the determination of exchange rates in a system undergoing restricted three-site exchange (A s B s C) , and applied to iodine/2,4,6-trimethylpyridine equilibria in solution (386).
Since shifts in the N M R spectra of aromatic organolithium compounds upon complexing to Lewis acids depend on the acceptor strength of the acid, it was proposed that a quantitative measure of Lewis acid strengths can be derived from suitable chemical shift data (704). Similar observations were made for the following systems: Me3N/BH3 and the boron halides (46‘79, p-fluorobenzonitrile/boron-containing Lewis acids (F19 shifts) (665), N,N-dimethylformamide/metal halides (375),and polycyclic phosphites/BH3,BMe3,BF3 (698). Potentiometric measurements confirmed (559) that line broadening of the Na23 NMR line is a valid method for estimating formation constants of weak complexes involving sodium ion. Adducts of fluoborates with BF3 (97) and of fluoroalkyl borate esters with various donors (384)were studied by F19and B” NMR, respectively. Proton and Li7 NMR data for fluorenyllithium adducts with tetrahydrofuran and diethyl ether indicate that both the Li+ and the donor molecule are situated above the x cloud of the fluorene ring system (166). The nature of the adducts of N,N-dimethylformamide with oxychlorides was investigated (453). Intermediates in the bromination of 9-hydroxyanthracene and 2,6-di(t-butyl)-phenol were shown to be u-complexes (366). In the same paper, the possible contributions to the shifts observed upon *-complex formation were discussed. Surface Phenomena. Some success has been achieved in the application of IihfR to the study of micelles. At 342 R
ANALYTICAL CHEMISTRY
the critical micelle concentration in the system water/benzene/surfactant, the benzene signal shifted upfield markedly; the lifetime of benzene in a micelle was estimated t o be less than second (488). Based upon the observation of two distinct water signals, it was concluded that exchange of water molecules is slow between emulsified and solubilized water in emulsions consisting of cyclohexane/carbon tetrachloride/ water/Duomeen T dioleate (139). Changes were observed in the 8~ of water upon micelle formation by dissolved sodium alkyl sulfates (119). In limited temperature regions, a broad distribution of correlation times for an adsorbed species can result in apparent phase transition effects in spin-spin relaxation times (569). The phase transition may be interpreted in terms of a spread in activation energies for surface diffusion, and therefore provides a method for determining surface heterogeneities (569). The theory was applied to relaxation time data of water adsorbed on charcoal, for which a broad distribution of correlation times had been observed (570). Two-phase behavior was also observed for water adsorbed on silica gel; relaxation time data taken over a wide temperature range allowed extraction of the activation energy for interphase transfer and the approximate transfer rate (727). Line width transitions attributed to the successive loss of motion of parts of the molecule were reported in broadline studies of methanol, ethanol, butylamine, and water, adsorbed on thorium oxide (92). NMR data were employed to determine the amounts of free mater, that in small and in large pores, and that adsorbed on type 4A molecular sieve (696). Chemical shifts and line widths indicated that formic acid chemisorbs more strongly on a dehydrogenating type of silica gel than on a dehydrating type (221). The water-sorption properties of nylon-6 (361), and the behavior of nylon-66 toward water and methanol (normal and deuterated) (254), were studied by broadline EMR. The different proton T2’sobserved for degassed water, as the sample geometry and surface-to-volume ratio varied, were interpreted in terms of a new diffusion theory (466); the effective surface of the glass was assumed to be increased by micropores. The mobility of water between the silicate layers of vermiculite and montmorillonite was studied (264). Hydrochloric acid and perchloric acid do not equally invade ion-exchange resins from aqueous solutions of identical concentrations (164). From A127 and proton N M R studies of alumina containing Cr, oxides of Cr, Ni and Co, and/or water, information on surface configurations of the adsorbed species was obtained (511).
APPLICATIONS
This section contains a few examples of structural and quantitative results which are applicable to specific classes of compounds. I n large part, the applications of NMR during the past two years are dealt with in the more than 5000 NMR-related papers which were omitted from this review. Information on particular applications can easily be retrieved through abstract periodicals. d list of topics immediately following this text was compiled with a two-fold purpose: to provide a source of leading references to literature on various KMR topics, and to point out several articles in which the subject of NMR is discussed as it relates to a particular field of study. These latter references should prove useful to those whose specialty is not KMR. Structural Determinations. Solvent shifts may be ubed t o advantage in the characterization of N-methyl groups (418), phenylhydrazones, semicarbazones, and thiosemicarbazones (358), substituted indoles (335), and methyl groups near the carbonyl function of ketones (131). The constitution of complex phenols may be determined by comparison of the spectrum of the phenol with that of its anion (303). I n the case of N-alkyl amines, consideration of the spectra of the protonated and unprotonated forms provides stereochemical information (305). The spectra of mi-nitro anions are of use in the determination of the stereo-chemistry of the parent nitro compounds (270). Comparison of the 6 ~ ’ of s the carbonyl carbon of protonated organic acids and esters with available C13 data led to the conclusion that the ions are carbonyl protonated (433). Observed similarities in the 8=’s of N nitrosamines and carbonium ions prompted the suggestion that properties of intractable carbonium ions be estimated from measurements made on the N-nitrosamines of corresponding structure (680). Structure-spectra correlations were presented for paraffinic chains (53), aliphatic azoxy compounds (230), diazoalkanes (397), and methyl groups in polymethylnaphthalenes (736). The utility of correlations is illustrated by the following studies. An ortho J” of only 3.6 Hz in 2-methylbenzotriazole was attributed to localized double bonds, that is, a quinoidal structure (575). The chemical shift of the 6-proton of the disulfonyl derivatives of 2-aminopyridine was used to distinguish 2-(disulfony1amino)pyridines (“aromatic”) from 1-substituted pyrid-2-onimines (“quinoidal”) (58). The structure of cyclononatetraene was confirmed by the observation of a single line, the position of which is in good agreement with that predicted by a previous correlation of charge density with shielding in reso-
nating R-systems (360). Cyclic trisilazanes do not have a ring current, as concluded from a comparison of their Kh4R data with those of a related ring system known to be nonaromatic (403). Cyclohexene-3,3,6,6-d4 was shown to be a useful compound for the study of additions to double bonds (726); analysis of the spectrum of the product is greatly simplified because of the absence of coupling with protons cy to the exdouble bond. One advantage of high magnetic field strength was illustrated by the essentially complete analyses of the aromatic region of the 200 Mc. proton spectra of alkylbenzenes, whose 60 ?\IC.spectra are ill-resolved (90). I n a unique study, the H 3 NMR spectrum of 1% side-chain tritiated ethylbenzene was obtained (675). Tritium N h l R has utility, but the radiological hazards must be recognized (675). Signal enhancement techniques allowed a study of aqueous nucleic acids to be carried out (423). Helix-coil transitions of poly(7-ethyl-L-glutamate) in CF3COOH-CF3CH20H mixtures were detected by N M R (260). The proton spectrum of water in a preparation of the microorganism N . asteroides showed broad lines with some structure; upon displacement of H 2 0 by D 2 0 , line narrowing was observed (110). SiUR spectra of the anomers can be observed in dimethylsulfoxide solution because mutarotation of sugars is slowed; from JHCOH’S and ~ H ’ s , the cyand p-forms may be distinguished (33, 106).
The determination of polymer tacticity by NMR is well established. Some classes of polymers, however, are amenable to more comprehensive analysis. An example is the phenol-formaldehyde system in which a variety of structural features were elucidated (307, 731). The early stages in the curing of an epoxy resin were followed by broadline
NMR (633). Configurational analysis by NMR is, of course, not limited to organic molecules. For example, positional and/or cis-trans-isomers can be characterized in the following systems: dimethylaminophosphonitriles (215), P- and As-bridged complexes of metal carbonyls (292), tphosphine metal complexes (336), and iron carbonyl-diene complexes (440). V51 N M R permits direct observation of several vanadium-containing species in aqueous polyvanadate equilibria (318). Stereochemistry and exchange phenomena of phosphorus(V)chlorofluorides (315), and of heptavalent fluorides and oxide pentafluorides (61) were studied by F19 NRIR. Quantitative Determinations. K h I R comes very close to the ideal tool for nondestructive quantitative analysis, because under proper instrumental conditions hardly any
empirical corrections need be made for poorly understood physical phenomena, such as oscillator strength in optical spectroscopy. These instrumental conditions were thoroughly investigated for two commercial spectrometers in H1 and F19 analysis (629). The most obvious application of N M R is the determination of isotopic purity (643), especially in H-D cases. The precision and accuracy attainable compare very favorably with mass spectrometry, as demonstrated in the case of dlc-biphenyl (347). A case of B’O-Bll analysis was also reported (294). A novel method of isotope analysis is based on the use of the relatively large H-D isotope shift (0.6 p.p.m.) of F19 resonances in molecules with geminal hydrogen and fluorine atoms (25). The degree of isotopic substitution can thus be readily determined with an accuracy of i1%. Y M R often is the tool of choice; sometimes it is the only possibilitye.g., small amounts of cryptopine can be quantitatively determined in the presence of the alkaloid thebaine (684). Unseparable mixtures of isomeric phenylbromopropanes were successfully analyzed, after the spectral patterns of the pure isomers had been established (541). Extremely wide use of quantitative NMR spectroscopy has been made in the petroleum industry (112), as witnessed by the number of NMR spectrometers in its laboratories (626). The classes of materials amenable to h’NR (441) encompass crude oils, distillates, petrochemicals (209) of various types, such as surfactants (210), and, of course, polymers. Acetylation of hydroxyl compounds can yield the ratio of primary and secondary-OH groups (458). But since by an appropriate choice of solvents (114, 690) primary, secondary, and tertiary alcohols may give distinct -OH signals, these can be used directly for the analysis of mixtures of these alcohols (690). I n carboxylic acids where the -COOH groups are attached to primary and secondary carbon atoms, the protons bonded to these often allow determination of the ratio of such acids (690).
The determination of active hydrogen has been the subject of two papers (362, 550); in one of them (362) the use of a known amount of an added reference was mentioned as an internal standard. Such standards were employed in the study of aerial oxidation of benzoic acids (468), and of mixtures of toluene and ethylbenzene (670). The idea illustrated in these examples can be adapted to almost any situation arising in actual practice. A more unusual internal standard was found in the satellites of acetone, each of which represents 0.55% of total acetone (609). Only in this manner could small amounts of
water in acetone be determined with accuracy; the direct comparison of the intensity of signals from CH3- in C12acetone and H 2 0 would introduce a large error. A method for the quantitation of APC mixtures, proposed earlier, was the subject of some controversy (401). New applications of quantitative broad-line N M R are measurements of fluid flow in porous media (676), where n-heptane was displaced by CC14,and of oil content of seeds (135). The latter case highlights the utility of N M R as a nondestructive method, inasmuch as the same seeds selected for their high oil content can be used for propagation. LIST OF SELECTED PUBLICATIONS Analysis of spectra (266, 526) Biochemistry (334, 370) Carbon NMR (658, 737) Chemical shifts (68. 266., L16) . Compilations (320)’ Conformational analysis (185, 224, 329) Deuterium NMR (160) Double resonance (186, 193, 202,232,526,
TOPICAL
I
625)
Flowing liquids (168, 738) Fluorine NMR ( 6 9 ) High-pressure NMR (60, 61) High-resolution NhlR, general (68, 69, 266)
Inorganic chemistry Fluorophosphoranes (600), general (188, 639), heterosiloxanes (697), hydride metal complexes (269), transition metal complexes (234) Instrumentation (37, 222, 266,527, 660) Introductions to NMR (68, 69, 87, 175, l76A, 322, 344, 370, 487, 502, 608, 695,
N.A\
l l Y )
Meetings (Abstracts and Reports) (34, 33L. 63L)
Pl‘atuial p;.bducts Alkaloids (301, 538), carbohydrates (205, 277), chlorophyll (120),,glycosides (4OO),lipids (582),steroids (68, Organic ’chemistry Aromatics (44), carbonium ions (154), r-complexes (156),diamagnetic anisotropy (84), heterocyclics (717), strained polycyclics (611 ), structure (359,489),valence isomerization (608, 691, 699)
Organometallic chemistry (291) Oriented molecules (100, 583, 590) Pharmaceutical chemistry (557) Petroleum chemistry (112,209, 441) Polymers General (422, 504, 708), microtacticity 1373). rubber (705) Rateprocksses (153, 531) Referencing (362, 413) Solids (26, 282, 365, 712) Solvent effects (68, 425, 457, 734) Spin coupling General (266, 416, 457, 482), H-H 68, 393, 542), Long-Range H-H 68,243,651)
Spin relaxation (266, 421, 686, 687) LITERATURE CITED
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