(444) Sommer, L., Hnilickova, M., Anal. Chim. Acta 27, 241 (1962). (445) Sommer, L., Jin, T., Chem. Listy 55, 574 (1961). (446) Sousa, J. A., Weinstein, J., Rev. Sci. Instr. 34, 150 (1963). (447) Sugii, A,, Dan, M., Bunseki Kagaku 12, 368 (1963). (448) Sugii, A., Sumie, H., Ibid., 364 (1963). (449) Svehla, G., Pall, A., Erdey, L., Talantu 10, 719 (1963). (450) Swietoslawska, J. (Editor), “Spek-
trofotometria Absorpcyjna; Praca Zbiorowa,” Penstwowe Wyd. Naukowe, Warsaw, 1962. (451) Tachibana, K., Mem. Fac. Sci., Kyushu Univ. Ser. C, 4,221 (1961). (452) Talipov, S. T., Dzhiyanbaeva, R. K.. Anickina. V. S.. Usbeksk. Khim. Zh. 6, 25 (l962).’ (453) Talipov, S. T., Sigai, K. G., Zh. Analit. Khim. 18, 178 (1963). (454) Tanker, M., dst T i p Fak. Med. 24, 542 (1961). (455) Tertipis, G. G., Beamish, F. E., ANAL.CHEM.34,623 (1962). (456) Testa, C., Anal. Chim. Acta 25, 525 (1961). (457) Tishchenko, I. G., Skorokhod, 0. R., Shedov, N. V., Zh. Obshch. Khim. 32, 3808 (1962). (458) Tokvo Shibaura Electric Co.. Ltd., FTD 70G5, 1962. (459) Tolmachev, V. Pi., Ukrain. Khim. Zh. 27, 559 (1961). (460) Tolmachev, V. N., Lomakina, G. G., Serpukhova, L. N., Ukrazn. Khim. Zh. 27,‘584 (1961). ~
(461) Tomic, E. A., Bernard, J. L., ANAL.CHEM.34, 632 (1962). (462) Tonosaki, K., Otomo, M., Bull. Chem. Sac. Japan 35, 1683 (1962). (463) Tsuji, K., Yakugaku Zasshi 81, 1655 (1961). 14641 Turner. V. L.., Jr.., U . S . 3.032.401. , , , Mav 1. 1962. (465) “Tyler, J. E., Proc. A;atl. Acad. Sci. U . S . 47, 1726 (1961). (466) Uesugi, K., Katsube, Y., Yoe, J. H., Bull. Chem. SOC.Japan 35, 516 (1962). (467) Umbreit, G . R., Anal. Chem. 33, 1572 (1961). (468) Unicam Instruments, Ltd., Cambridge, England, Bulletin on SP. 800 (1963). (469) Unicam Instruments, Ltd., Cam-
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SP.
1400 (1959). (470) Usatenko, Y. I., Arishkevich, A. M., U . S . S. R . 149,252,Aug. 4, 1962. (471) Usatenko, Y. I., Arishkevich, A. M., Dopovidi Akad. Sauk. Ukr. RSR 1962, 504. (472) Vandenbelt, J. M., J . Opt. SOC.Am. 52, 284 (1962). (473) Vasil’eva, L., Ermakova, M. I., Zh. Analit. Khim. 18, 43 (1963). (474) Volodarskaya, R. S.,Derevyanko, G. Pi., U . S. S. R. 143,785,Jan. 27, 1962. (475) Wallace, R. M., Dukes, E. K., J . Phys. Chem. 65,2094 (1961). (476) Watkins, K. O., Jones, M. M., J . Inorg. Nucl. Chem. 24, 1235 (1962). (477) Wayman, C. H., U. S. Geol. Surv. Profess. Papers No. 450-B, 117 (1962).
(478) Wawrzychzek, W., 2.Anal. Chem. 184, 191 (1961). (479) Wierzchowski, P., Wierchowska, Z., Chem. Anal. (Warsaw) 6, 579 (1961). (480) Williams, D. D., Miller, R. R., ANAL.CHEM.34, 225 (1962). Wilson. R. F.. Daniels. R. C.. 2. (481) ‘ A&. Ch&. 194. 190 (1963’). (482) Wilson, R. F., Lester, G. W., Ibid., 193, 260 (1963). (483) Wolfe, W. C., ANAL. CHEM. 34, 1328 (1962). (484) Yamamura, S. S., Wade, M. A., Sikes, J. H., Ibid., 34, 1308 (1962). (485) Yamana, T., Sato, T., Yakuzaigaku 22, 189 (1962). (486) Yaphe, W., Nature 197, 488 (1963). (487) Yoshida, H., Yamamoto, M., Hikime, S., Bunseki K a ~ a k u 1 1 , 197 (1962). (488) Zak, B., Cohen, J., Clin. Chirn. Acta 6 , 665 (1961). (489) Zak, B., Holland, J., Williams, L. A., Clin. Chem. 8, 530 (1962). (490) Zeiss, C., Inc., Oberkochen Wuit. 50-660/III-e, 1962. (491) Zhivopistsev, V. P., Chelnokova, M. V., Zh. Analit. Khim. 18, 148 (1963). (492) Ziegler, M., Z. Anal. Chem. 188, 335 (1962). (493) Zimmermann, M., “Photometric
Metal and Water Analyses,” Wissenschaftlich, Verlagsgesellschaft m. h. H., Stuttgart, 1962. (494) Zitomer, F., Lambert, J. L., ANAL. CHEM.34. 1738 (1962). 4495) Zscheile, F. P., Jr., Murray, H. C., Baker, G. A., Peddicord, R. G., Ibid., 34, 1776 (1962).
Magnetic Resonance Spectrometry Harlan Fosfer, Central Research Department, E. 1. du Ponf de Nemours 8 Co., Inc., Wilmingfon 98, Del.
T
review follows the pattern of its immediate predecessor (99) in placing major emphasis on the high resolution aspects of nuclear magnetic resonance spectrometry, conveniently abbreviated to NMR. It covers the period from August 1961 through July 1963. Somewhat less attention is given to broad-line K M R and to electron spin resonance, similarly abbreviated to ESR. This may be regarded as a reflection of the relative utility of the techniques in chemistry. In the previous review, the writer expressed apologies for the incomplete and highly subjective coverage which the rapid increase of the magnetic resonance spectrometry literature made necessary. This flood has not abated in the least but instead has steadily risen. I n addition to the proliferation of the literature, a further serious drawback to carrying out complete coverage is the normal but nonetheless unfortunate extent to which significant resonance work fails to be noted in a title or even in the summary of a paper because the technique was used as a tool and was not a subject of primary interest. This is understandable from the viewpoint of the investigator, but HIS
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it means that a literature reviewer, to be thorough, must go through the journals page-by-page rather than risk himself solely to titles and abstracts. When preparation of a review is of necessity a spare-time enterprise and one wishes to stay on speaking terms with his family, page-by-page coverage of the technical literature is fantastically impractical. A compromise has been adopted here. The subject matter has been watched broadly by way of titles and abstracts with page-by-page examination of selected key journals. From an initial selection of over 1500 references so assembled a final, very personal selection has been made. When such compromises must be made, it is highly probable that some contributions will not receive a proper evaluation. No mention will be made of the numerous short reviews of NMR, roughly equivalent to covering the entire subject in a one-hour lecture, which continue to appear. Six longer reviews of a different character have been published, however, a complete mastery of any one of which would go far toward giving an individual using N M R as a tool a reasonable command of the field. Strehlow’s (255) small
monograph of 179 pages is the most elementary of the five. The treatment of Stothers (253) is intentionally slanted almost entirely toward the elucidation of organic structures. The series of articles by Ranft (224) constitutes a rather highly theoretical text on nuclear resonance. Jardetzky and Jardetzky (156) present a neatly balanced treatment of theory, technique, and applications. Their examples are drawn almost entirely from the biological field. Fluck has produced a monograph directed entirely to the utility of X M R in inorganic chemistry (96). This is noteworthy because most of the applications have been to organic chemistry and the usefulness of the technique elsewhere is easily forgotten if it is realized at all. The chapter due to Muetterties and Phillips also emphasizes inorganic chemistry while covering fundamentals thoroughly (198). EQUIPMENT AND TECHNIQUE
il detailed description has been given of a spectrometer system which incorporates the use of magnetic field modulation for the purpose of simul-
taneously stabilizing .;he base line and the resonance condition. Alternative modulation techniques are included in the discussion (I6 ) . Several years ago Baker and Burd described a high resolution frequency-swept spectrometer stabilized by the error signal from an auxiliary proton sample (22). A modification has been made in which the use of modulation in the measurement channel is abandoned but proton stabilization is retained. Most significantly, this spe1:trometer can be changed from operation with one nucleus to another by the flip of a switch, so to speak. il frequency synthesizer is used to divide down a control channel frequency to that needed for any other nucleus. Frequency fluctuations in the control channel due to magnetic field oscillations are divided down proportionally. The synthesizer output is suited to the production of precalibrated spectra (IS). Adjustment of spectrometer magnets to peak performance is facilitated by the use of a sideband oscillator system due to Kaiser (164). The phase detector ticheme described by Flautt can be ads,pted to either a bridge or cross-coil spectrometer. Modifications to the spectrometer include an attenuator fo,*control of probe excitation voltage, leakage control independent of probe balance, and a differential d.c. amrlifier to cancel leakage changes due to spurious modulation of transmitter power (94). In using nuclear resonacce to determine 1-methylnaphthalene in 2-methylnaphthalene, deviations from linearity were detected. The discussion of these deviations, while familar to a practiced spectroscopist, would be of value to a neophyte (112). With current instrumentatim, quantitative work based on integraldon of intensities has become routine. In almost any field involving physical measurements one is continually fighting the problem of weak signals, or more properly, a poor signal-to-noise ratio. Nuclear resonance is no exception. The CAT (acronym for Computer of Average Transienh) , manufactured by the Mnemotron Division, Technical Measurements Corp., White Plains, N. Y., is an instrument which has been brought to bear on this problem. The is spectrometer output, voltage digitalized, stored, and summed in digital counters while providing for continuous visual mon toring and readout to either a digital device or a recorder. The signal buildup is proportional to the number of passes through the signal while the noise accumulates as the square root of the number of passes. This is equivalent to improving the signal as the square root of the number of passes while maintaining a constant noise level.
Jardetzky and coworkers have reported the successful use of the technique with enzyme systems which a t biologically meaningful concentrations are too weak for successful observation by ordinary methods (15 7 ) . -4short article by Slomp calls attention to some mistakes that have been made in reporting N M R data and indicates the proper ways in which it should be done (248). I t is unfortunate that such an article should be needed, but the examples are drawn from the published literature. Tetramethylsilane has all b u t preempted the field as a reference for proton resonance peaks but is not ideal because of its immiscibility with many .polar materials. A variety of water-soluble internal references have been suggested. Jones and coworkers have evaluated acetonitrile, dioxane, and tert-butyl alcohol (161). Sodium 2,2-dimethyl-2silapentane-5-sulfonate, a suggested reference for use with polar solvents, is now available from Distillation Products Industries, Rochester 3, N. Y. Kothing approaching unanimity has been achieved regarding preferred reference materials for use with nuclei other than protons. Resonance spectroscopists have not been able to agree upon a scheme for reporting the chemical shifts of proton spectra. There is rather universal accord on the use of a dimensionless parts-per-million scale measured from tetramethylsilane. The point of disagreement is whether to assign the references a value of zero or some arbitrary integer, such as 10, as in the 7-scale. Sign conventions are also unsettled. In general, it seems that when structural data of organic chemistry are reported the 7-scale is favored. When the data are from physical chemistry or a field of primarily theoretical interest, there appears to be no clear choice. Because of their bearing on the accuracy of N M R data, solvent effects may be regarded as properly falling under the subject of technique. Typical of studies in this field are those of Hatton and Richards on the shifts of the proton resonances of mesityl oxide, &@dimethylacrylic acid, acetaldoxime, 7-picoline, and N,N-dimethylphosphoramidic dichloride ( f 4 f ) ,and dimethylformamide, diethylformamide, and dimethylacetamide (140) in a variety of solvents. An attempt has been made to interpret the effects, particularly with aromatic solvents. Aromatic aldehydes have been used by Klinck and Stothers to study solvent effects. In chloroform the chemical shifts are but slightly altered, but in benzene and in acetone there are very considerable effects. In benzene the shift is to higher fields and in acetone to lower fields. The shifts in benzene are strongly substituent-de-
pendent and this is attributed to the formation of weak complexes. Polar rather than steric factors appear to be dominant (17 1 ) . THEORY A N D ANALYSIS OF SPECTRA
In this section contributions are included which apply to K M R in general or to the hydrogen nucleus. The latter is far and away the nucleus of most significance. Items which apply largely or entirely to other nuclei are considered elsewhere in this review under the specific nucleus The proposition that the intensity of a line arising from a group of n equivalent nuclei is proportional to n seems an obvious one on the surface but is in fact rather subtle. Proof has been given by Grimley (112). In a series of para-substituted phenols the hydroxyl chemical shift, when extrapolated to infinite dilution in benzene, chloroform, and acetone solutions, was found to be largely independent of the substituent but to depend strongly on the solvent. Agreement with accompanying theoretical calculations of the ring proton shifts was not good (212). From the spectra of a series of anisoles and phenetoles it was concluded that for meta- and para-substitution there was good correlation between the methyl resonance and the corresponding a-parameter (149). With anisole itself there is a preferential solvent effect suggesting a specific interaction between solute molecules. The chemical shifts of the aromatic protons of the isolated molecules of a series of hydroxybenzenes were obtained by making measurements in two different solvents and extrapolating to unit dielectric constant. The electric fields from the C-0 and 0-H bond dipoles were calculated and their contribution to the shift was removed by the relationship of Ruckingham. The remaining part of the shift was assumed to be due to changes in the s-electron distribution and Huckel MO calculations were perforTed (241). A theory of proton chemical shifts based on perturbation of molecular orbitals by the proton magnetic dipole and external magnetic field has been proposed by Fixman. The theory gives results in good agreement with experiment for such molecules as simple hydrocarbons, group VI hydrides, and hydrogen halides (93). The linear variation of shielding with electric field has been calculated semiquantitatively. The fields on protons may approach lo6 stat. volts per cm. in ions or in molecules containing polar groups (205). The major portion of the paramagnetic contribution to proton shielding can be obtained from electrostatic considerations alone and without explicit knowledge of the electronic wave functions, VOL. 36,'NO. 5, APRIL 1964
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according to Chan and Das (57’). Diamagnetic anisotropy effects are estimated empirically from correlations between proton chemical shifts and C13-H coupling constants. The method applied to molecules such as benzene, thiophene, acetylene, and allene gives results supported by internal consistency and agreement with independent estimates (117). Musher has given a method for quantitatively treating long-range shielding in molecules in which the effect is shown to arise principally from the diamagnetic part of the susceptibility of the individual bonds (203). The chemical shift of the formyl peak in substituted aromatic aldehydes depends on both the character and the orientation of the substituents. Polarization of the C-H bond, reducing the electron density on the proton, leads to the lower shielding found in ortho compounds. This may be modified, however, by electron-releasing groups. Major contributions also arise from the currents of the ring and the anisotropic bond (170). The quantitative nature of the empirical correlation between the proton resonance shift and the electron density on the carbon to which the proton is attached has been investigated. The reverse application to determine the electron densities in a variety of aromatic compounds and the limitations inherent in such a procedure are included in the same investigation (239). The magnetic anisotropy of X has been found to contribute to the relative shift of the CH,- of C H S and both the CHI- and the -CHZof C Z H ~ X . After allowing for this contribution, correlation with the electronegativity of X may be obtained. Inductive effects together with anisotropy account for most of the relative shift (260). One of the more important building blocks for N M R theory is Pauling’s semiclassical model for aromatic molecules in which diamagnetic susceptibility is ascribed to ring currents. This model has been re-examined and found to give currents much too large in systems having more than two or three rings, and better results are obtained if one simply used the benzene value for each ring (183). I n a series of condensed ring hydrocarbons the ring current shifts have been estimated by London-type MO calculations. hgreement may be considered as satisfactory if one is satisfied with a correct prediction of the relative positions of the proton shifts (159). Calculations of the field due to ring currents have been shown to account for the observed chemical shifts in porphyrin systems (1). I t has been found that substituents which act as electron acceptors may increase the (3’3-H coupling constant by as much as 20 cps. (150). Schaefer has examined an extensive 268 R
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series of substituted vinyl compounds and found a strong correlation between the electronegativity of X and the three vinyl proton coupling constants (238). Similarly, Mavel reports coherent variations of the spin-spin couplings with the electronegativity of several groupings-for example, the CI3-H in various molecules and HCC H in ethyl and isopropyl. A large number of examples indicates the order to be rather general, with the coupling increasing with electronegativity (192). Hiroike attributes the magnitude of the coupling constant between two protons almost entirely to the ionic character of the bonds between the protons and the atoms to which they are joined (146). Valence bond calculations of the C13-H and HC-CH coupling constants in substituted ethyl, n-propyl, and isopropyl compounds have been carried out by Ranft (222). In the N M R of paramagnetic molecules interaction between the nuclear spin and the spin of the unpaired electrons can lead to very large hyperfine contact interaction shifts. A favorable system for studying these shifts has been found in the Si(I1) aminotroponeimineates, and contact shifts of up to 200 p.p.m. have been observed in these compounds (34, 79). The results have been analyzed in terms of the spin density distribution in the molecule, and the technique has been used to study problems of Conjugation (80) and hyperconjugation (78). Fluorine contact interaction shifts have also been observed and have provided information on fluorine bonding in aromatic systems (81). Gutowsky and coworkers have called attention to the assumption of a constant H-C-H angle in previous calculations of the coupling constants in - CHz- groups. Because J H -is~not a linear function of the angle, the value based on an average angle should be greater than the value based on a static angle, a difference which should be manifest either in experiments with variable temperatures or isotope effects (130). The relative effects of molecular asymmetry and differences in conformer populations in causing nuclei to be magnetically nonequivalent have been subjected to an algebraic analysis which emphasizes the nature of the asymmetry factor (127). Reddy and Goldstein have proposed a correlation wherein the sum of the coupling constants of monosubstituted ethylenes falls into one of two groups. I n the first group the sum of the coupling constants is 19 to 21 cps., the J,,, is negative, and Jtrana/JCls is approximately 2. I n this group there is a lone pair to interact with the vinyl *-electron system. In the second group the sum of the coupling constants is 29 to 31, J,,, is positive, and Jtlans/JO,S approximates
1.7 (228). If this correlation should hold up in a reasonable number of cases, it could be very useful in making preliminary assignments, particularly for the calculation of spectra, whether it is theoretically sound or not. The dependence of molecular relaxation processes upon molecular shape has been examined by regarding the molecule as a rigid ellipsoidal body with anisotropic rotational and Brownian translational motion. Several relaxation mechanisms have been considered (2.69). -1 method of analyzing complex spectra has been given by Swalen and Reilly which utilizes an iterative technique on the spin energy levels. Techniques which are helpful in assigning the experimental lines, together with sum rules for the energy levels and transition intensities useful in the analysis, are given (269). A computer technique has been described in which the assignment of the lines to transitions within the energy level diagram is consistent with equal spacing and intensity sum rules. Eigenvalues are calculated and introduced into equations derived from the spin Hamiltonian secular determinant. The cases A B , ABz, iiZBz,and 4,B,, AA’BB’, dzBC, and ilBB’CC‘ lead to equations explicit in the spin parameters, while cases ,4BC and ABCD are implicit (276, 276). Corio has discussed the calculation of spectra for systems of the type (A),,BX,, where n. and n., are arbitrary numbers of spin l / z nuclei. Recursive frequency and intensity formulas were given and it was shown that the general case reduces to familiar simpler systems AzBX (63). such as A B X , ABXz A method which the authors describe as “direct” for calculating frequencies and intensities as solutions of an eigenvalue problem has been presented by Banwell and Primas. The mathematical methods used are relatively unfamiliar and are therefore explained in detail together with extended examples (28). Least squares forms the basis of a method useful for the refined analysis of high resolution spectra. A rough assignment must first be made and aids to doing this are discussed briefly. Then the spectrum is analyzed systematically with high precision. The method is applicable to any spin system and appears to be suitable for an analysis involving superposed lines (19).
The technique of Chapman and Harris for handling AB4 systems simplifies the secular determinant by treating identical nuclei as a composite particle with a fixed total spin (68). Application has been made to the analysis of the Fig spectra of -SF5 systems (136). Ranft has given extensive formulas for the types &B and L4,J3Xand applied them to the analysis
of the spectra of some isopropyl compounds (22s). Assistance in decomposing complex high resolution spectra made up of many interleaved and overlapping lines may tie found in the systematic correlation method of Allen (11). There are empirical additive rules which may be of considerable help in making an inspectional assignment of the protons in ortho, meta, and 1,3,5substituted benzene derivatives or in connection with getting started on a rigorous mathematical analysis (21).
SPIN-DECOCIPLING
The course of developments in nuclear resonance oher the last few years has been such that normally the subject of spin decoupling or double resonance would occupy a major portion of this survey. The task of covering this aspect of newer developments, however, has been immensely lightened by the recent appearaiice of a thorough and authoritative review by Baldeschwieler and Randall (26). Anyone interested in looking into this subject would do well to start with this review. For the most part, the additional references noted here constitute a selection of papers which the writer has found to be particularly informative. Proton-proton decoupling has received most attention, as would be expected, and instrumental developments reflect thik Turner has given a reasonably complete description of a single side band modulation system designed to accomplish this type of decoupling (267). Bttker and his coworkers have added a second frequency synthesizer and suitable power amplification to their frequency-swept, proton-stabilized spectrometer (24) to carry out decoupling. One advantage of this instrument lies in its yielding frequency-swept, devoupled spectra which are easier to interpret than fieldswept ones. A decoupler circuit which has been described in some detail by Elleman and Manatt (87) closely resembles the familiar k'arian integrator. Varian Associates, Palo Alto, Calif., have available instructions for using their integrator as a side band source for decoupling. Equipment intended specifically for decoul: ling is available commercially. Space Avionics, Inc., Alexandria, Va., supply an instrument which has, according to the writer's understanding, a cir1:uit closely resembling that of the Turner unit. Nuclear Magnetic Resonance Specialties, New Kensington, Pa., are most active in this field and marke., what they term a "Homonuclear" spin decoupler for decoupling like nuclear species and a "Heteronuclear" decoupler for use with nonidentical nuclei. Foth are modular units intended to give maximum flexi-
bility in changing from operation with one pair of nuclei to another. A h d e m o n and Freeman have published two rather theoretical papers on double irradiation in which the technique was used to determine the ordering of the energy levels of a system and was followed by an iterative computer program to determine the shift and coupling parameters. Types A,X, (16) and ABC (104)were considered. A very informative discussion of spin decoupling applied to A X systems has been given by Freeman and Whiffen (107). Of particular value is the comparison of field sweep and frequency modes of observation. I n cases of greater than minimum spectral complexity, use of a strong decoupling field may perturb more than one transition and lead to complication rather than simplification of the spectrum. I n these cases a very weak field may be used so as to perturb only a single transition (105). The article of Turner (267) noted above gives a number of examples of the use of spin-decoupling to simplify spectra and to aid in their interpretation in connection with structure proofs. The examples given include both the trivial and the complex. I n elucidating the structure of the tricarbonylbicyclo[5,1,0]-octadienium cation, spin decoupling was used to determine which groups of multiplets were coupled to each other (72). Spin decoupling may have special merit in showing the relationship of two sets of lines in a badly interleaved pattern of almost equal coupling constants ( 3 ) . Spin decoupling may be employed to determine chemical shifts in cases where the direct measurement gives ambiguous results for reasons such as complexity of the patterns or overlapping multiplets (186). The chemical shifts of "4 in pyridine and in the pyridinium ion have been determined by this technique (27). An exact calculation of certain types of high resolution spectra can be made to yield relative signs of some of the coupling constants. The method depends upon changes in the pattern of the calculated lines with changes in sign. The extensive calculations required are usually done on a computer. There are several reasons for concern about the signs. There is theoretical interest in the possibility of negative constants in view of recent valence bond studies. I n the calculation of theoretical spectra to match experimental ones the relative signs of the constants must be considered as one of the variables. I n some cases the relative signs may be obtained experimentally by observing the order in which the components of a multiplet collapse as the decoupling condition is traversed. The contributions of Freeman (102) and Freeman
and Whiffen (106)are both devoted to cases of three coupled protons. Extension to the A K X , system shows the basic treatment to be sound (103). APPLICATIONS AND RESULTS
This review assumes that those who may read it have some familiarity with magnetic resonance and how it is used. However, in the event this assumption is not justified, it has been regarded as wise to include a few examples in which the technique has been used as a significant contributor in proving the structure of an organic molecule, to the present the primary field of application. The examples have been chosen because they represent fairly complex molecules and thereby further serve to counter the notion that XMR can be used only with simple structures. With each a very informative discussion is also included. The illustrative examples chosen are otobain (If@, pisatin (214), palmarin and its congeners (as),and the steroid helvolic acid (12). From the wealth of the current organic literature many equally rewarding examples could have been taken. Followers of this literature will have their own choice. The fact that a recent issue of the Journal of Organic Chemistry contained more than 25 papers giving N M R results in some detail points up the impracticality of noting here the hundreds of data on individual compounds that have found their way into the literature over the past two years, usually as part of a larger problem such as a structure proof. I n addition to these, there have been a number of publications in which a group of materials, which may be related by some characteristic such as a particular functionality, are treated. This section is devoted to a number of such studies. To stay within reasonable space limits, the descriptions given for listing are highly abbreviated and admittedly not as informative as is desirable. The breakdown is illogical in places and sometimes redundancies appear. These deficiencies are forced by the overlapping nature of some of the papers. Saturated aliphatic types for which spectra have been reported include methanes which are mono-, di-, and trisubstituted with - 4 1 , -CX, --F, and -CHO (268), halogen-substituted ethanes (84),polysubstituted ethanes (fa@,and disubstituted n-butanes (42). Brown and coworkers in a single paper have treated halogenated propanes, fluorinated butanes, pentanes, hexanes, and octane (85). Coverage on aliphatic unsaturated compounds includes halogenated propenes and butenes ( S S ) , halogenated ethylenes (274),alkylethylenes ( 4 S ) , vinyl ethers (go), fluorinated propynes ( S S ) , and acetylenic ethers and thioethers (7'6). Other aliphatics which VOL. 36, NO. 5, APRIL 1964
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have been studied include a-glycols (11l ) , enolized aldehydes (98), the formyl proton of aromatic aldehydes (17O), alkoxy proton of anisoles and phenetoles (143), methyl and methoxy of hydroquinone derivatives (17 2 ) , and the nonaromatic portion of 2,4-dinitrophenylhydrazones and semicarbazones (166).
Alicyclics to which some attention has been given include cyclopropanes (12O), methylenecyclopropanes (f 4 , tetracyclobutanes (278), monosubstituted cyclohexanes (207), cyclohexanols (82), bicyclohexanes ( d r y ) , cyclophanes (64-67, 246), halogenated cyclopentene (SS), indan and indene derivatives (88, 182), pentacyclic triterpenes (243), and annulenes and hydroannulenes (164). Recent N M R data are available on the aromatic protons of monosubstituted benzenes (251), disubstituted benzenes (121, 188), condensed ring hydrocarbons (159), mesitylene and durene derivatives (49),dinitronaphthalenes (273),hydroquinones (172), indan and indene derivatives (88, 18O), and veratole derivatives (10). Studies have been made of a number of natural products and their derivatives including carbohydrates, such as furanoses (4, 6) and pyranoses (SO, 1S1), porphyrins (Sf,66), and steroids (284). Nowhere is there a richer lode of S M R material buried than in the steroid literature. Heterocyclic systems have proved to be fascinating subjects to many N h l R spectroscopists. The previous review cited numerous papers which illustrate the point and to these a rich new harvest may be added. Nitrogen heterocycles perhaps have been the most investigated and systems examined include pyrrolines and pyrroline oxides ( 4 l ) ,monoand disubstituted pyrroles (124), pyrazolines ( I % ) , azoles, including specifically pyrrole, imidazoles, pyrazole, and 1,2,4triazole (162), the N-acetyl derivatives of some imidazoles, benzamidazoles, and purines (ZSO), piperidines (272), pyridines (46), quinolines (242), pteridine, quinoxaline, and their methyl derivatives (191),pyrazines (SZ), and benzofuroxan (135). The much sparer list of oxygen heterocycles includes 2- and 3-substituted furane ( l a @ , benzofuran (88), 1,4dioxanes (55), 1,3-dioxolanes (55), flavones, and flavanones (190). In attempting high resolution work with polymers, one is commonly frustrated by poor solubility coupled with high viscosity, resulting in broadened signals of low intensity. I n some favorable cases it is possible to get the viscosity sufficiently low to permit tumbling to average out broadening dipole-dipole interactions and to obtain a signal-to-noise ratio sufficiently high to obtain spectra on polymers whose 270 R
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quality and interpretability approach those of simple micromolecules. These spectra are treated precisely as any other high resolution presentations. High resolution techniques have been applied to the study of methyl methacrylate polymerized with organometallics as catalysts (46). The effects of solvent, temperature, and cation on the proportions of isotactic, syndiotactic, and heterotactic phases were examined. Among the several differences in the spectra, the key to the interpretation is the appearance of the -CHz- resonance of the isotactic form as a clearly resolved A B quartet. It has been found possible to get good spectra of methyl methacrylatestyrene and methyl methacrylatemethylstyrene copolymers, which can be explained with reasonable assumptions which are brought over from the analysis of the spectra of low molecular weight compounds (44). A partial description of the stereochemistry can be given. Other polymers whose structural details have been studied by N M R include polypropylene (23?'), poly(viny1 alcohol) (YO), and poly(viny1 chloride) and poly(viny1idene chloride) (60). The effect of aromatic solvents on the proton chemical shifts of systems having internal hindered rotation was studied by Moriarty. The two N-methyl peaks of iV-methylcyclohexylacetamide can be made to shift unequally and to the extent of undergoing crosscwer with solvent changes. The behavior has been discussed in terms of specific complex formation (195). The shift of t h e methyl protons of substituted o-nitrotoluenes has been related to the rotational angle around the C- X bond and has been attributed mainly to the molecular anisotropy of the -NOz group. After making some reasonable assumptions the susceptibilities have been calculated from this dependence (281). The position of the proton resonance signal of a gas is pressure-dependent. Four factors are believed to contribute to altering the chemical shift with changes in density: the bulk magnetic susceptibility, a field-square van der Waals term, a n electric dipole effect due to the field along the bond, and a contribution from molecular magnetic anisotropy of near neighbors. Theoretical expressions have been developed for all these factors and experiments to check the very small predicted effects carried out on such gases as methane, ethane, and hydrogen chloride, either pure or in mixtures (226). The same theoretical considerations apply to gaseous fluorine compounds but the effects are more than an order of magnitude larger, as has been found with tetrafluoromethane, silicon tetrafluoride, sulfur hexafluoride, and fluoroform (215). Petrakis and Sederholm have re-
ported that the proton resonance of a series of compounds, principally of the hydrocarbon type, varies with temperature. The magnitude of the change is small and is usually negative in sign. The effect has been attributed to the presence of vibrational modes of excited molecules which may be differently shielded from protons in the ground state (116). Buckingham has added the molecular interaction in imperfect gases and the excitation of rotational states as factors to be considered in the temperature effect (48). Nuclear resonance has been employed to study the interconversion of forms in cyclic structures. The separation between the axial and equatorial peaks as a function of temperature was the approach used with cyclohexane (168). The rate constants for interconversion of cyclo-octatetraene were derived from observations on the CI3 satellites over the range of temperature from -50" C. to room temperature (18). I n studying the photolysis kinetics of sarcosine hydrochloride and its ester, the mean lifetime of the amino group in acidic aqueous solution was measured from changes in line shape (244). The rate of proton abstraction from substituted acetylenes in aqueous tert-butyl alcohol has been measured from line broadening. Some new equations relating peak heights to exchange rates are given (59). The conclusion that the X + D bond is stronger than that of X + H has been reached from an ?JMR study of systems characterized by bonding with chloroform and deuteriochloroform (68). I n investigations of weak interactions in solution, Abraham has measured the chemical shifts of cyclohexane, methyl iodide, and iodoform in a number of solvents and calculated the contribution of solvent anisotropy to the solvent shift. The actual contribution is greater than calculated and there are differences between aliphatic and aromatic solvents which are discussed and reasons for their existence assigned ( 2 ) . Various halogenated hydrocarbonbasic solvent mixtures have been used to examine the extent to which the chemical shift depends upon the intensity of the hydrogen bond and the basicity of the acceptor. I n this same study weak interactions in the pure materials were also included (189). From N M R data the thermodynamic association constants of the haloforms have been determined. The magnitude of the enthalpy of association decreases in the order C l > B r E F > I (69). Hindman has made an extensive study of N M R effects in aqueous solutions of univalent diamagnetic salts. The factors influencing the contributions of the separate ions were considered and a model was developed which treated the ion-water complex as a molecular species. Effective hydration numbers
were calculated and found to decrease with increasing radiut . Larger halide ions seem merely to break down the water structure and only the fluoride forms a hydrate. Lithium ions appear to exert a structure-forming effect (145). The solvation number of the magnesium ion in methanol was determined by comparison of areas a t low temperatures, since the “solvated” and free peaks were stxparated and not rapidly exchanging and the cation concentration was known (260). From the spectra of the conjugate acids it has been confi-med that monoprotonation of a number of azulenes occurs in the five-memtiered ring and the unit electron deficiency is mainly localized in the seven-membered ring of the ion (71). The rourse of the 1,2- and 1,4addition of thiols to conjugated olefins such as 1,3-butadiene 2nd isoprene has been followed by S h I R (210). Two studies of Grignard cnompounds have been reported which agree that the compounds are best rcbgarded as made up on RzMg and MgX,, building blocks. Questions remain regs rding the exact nature of the moleculai. unit into which these components enter and the role of the ether (89, 256). An improvement over a n earlier technique for determining magnetic susceptibilities by NrvlR has been described (108). I n favorable cases N M R can be used for the determination of molecular Eeighti (29). Among he requirements is a peak or group of peaks of known proton count which does not overlap any of the other parts of the spectrum. Conformational studies of the cyclooctane ring system have been carried out taking the appearance of the axial proton resonance a t a higher field than that of the equatorial as a basic premise (15). Cyclohexanol conformations have received attention, again utilizing the axial-equatorial proton shift difference. Lewin and Winstein i i working with 4-alkj lcyclohexanols (178)reached conclusions opposed to those previously enunciated by Musher (203). The origin of the difficulty stlems to be in the assignment of certain broadened lines which are critical tcl the analysis. Lewis and Winstein refined the assignment and by deuteration eliminated the spin-spin splitting u hich produced broadening and h e w e uncertainty. In. a varied group of cylohexanols the carbinol peak has been found to range over 100 cps. and there is appreciable overlap of the shift ranges of axial and equatorial protons (82). Rluch larger cis- and trans-coupling constants than previously reported have been found in a serLeq of a-acetoayketones from the steroid group. Consistent dihedral angle:, may be calculated if larger parameters are used in
the dihedral angle-coupling constant relationship. I n some instances equatorial proton resonance lines may occur a t higher fields than axial ones ( 2 7 9 ) . The influence of substituents u i t h diamagnetic anisotropy on the N M R spectra of steroids, including the features characteristic of conformation, has been investigated (155). The spectra of 30 flavones and related compounds have been reported and the conformations determined (290). Abraham, Hall, and coworkers have given attention to the SRlR of carbohydrate derivatives ( 4 , 5 , 131). Xssignments of the shifts t o precise positions in the molecule are made and the stereochemistry required by the coupling constants is clearly defined. A paper which is short but gives full data and a thorough analysis treats of the use of N M R to determine the link configuration in the glycosides of glucose and galactose (269). Deviations in the S M R spectra of 14 a-glycols have been attributed to differences in the steric configurations of the hydroxy groups (12 1 ) . Nuclear resonance was the technique used to investigate the stereoisomerism of a series of 2,4dinitrophenylhydrazones and semicarbazones (166).
RESULTS WITH NUCLEI OTHER THAN PROTONS
A series of papers by Williams and coworkers contains a large body of shift and coupling constant data, particularly on fluorinated paraffins (33, 84, 86, 86). The last paper in the series summarizes the numerical findings and gives some generalizations on shifts and splittings, drawing attention to certain unespected variations with environment. Entirely because of the normal improvements in instrumentation and techniques between the time this work was done and the present, the accuracy falls somewhat short of present standards, but the data are still valuable for correlation purposes. A number of polysubstituted ethanes have had their F19 spectral parameters studied over the temperature range 250’ to 450’ K. Reorientation was rapid enough to average out the three rotational isomers (128). Muller and Carr give the fluorine shifts of 52 compounds of varied types (201). Correlation of the shifts with C13-Fl9 coupling constants indicates a relationship between the two which is discussed elsewhere in this review. The pentafluorosulfur group has an interesting spectrum which has been recorded and analyzed in a number of different compounds (136, 137, 194). The addition of trifluoromethanesulfenyl chloride to halo-olefins can be used to prepare a variety of different products, a number of which have been
characterized by their fluorine spectra (232). As would be espected, in a series of fluorocarbon sulfides only the resonance peaks of the fluorines on the carbons directly attached to sulfur are appreciably shifted from their positions in more conventional compounds (264). Packer has reported the shifts and couplings of some phosphorus-fluorine compounds (211). For nuclei such as fluorine, in which the chemical shift is dominated by changes in the paramagnetism due solely to the electrons on the fluorine nucleus, LCAO theory may be used to express shieldings in terms of localized bond parameters such as ionic character and hybridization. The double bond character is important in the shifts of fluorines on aromatic rings (168). Along similar lines, the quantitative analysis by MO calculations of the fluorine shielding parameters i n conjugated molecules (282) extends the previous ideas of Saika and Slichter (236). The polarity of the carbonfluorine bond and the r-electron density on the fluorine are regarded as dominant in determining the value of the paramagnetic term. -4ndreades has determined the cis and trans fluorine-fluorine coupling constants in a series of perfluoroalkanes B where A, B, of the type 6
\
/
/
\
c=c
A and C are -F
C
and -CF8 in various arrangements (17 ) . It has been possible to get a firm assignment of the respective fluorinefluorine coupling constants in cis- and trans - 1,2 - dichloro - 1,2 - difluoroethylene by observing the splitting of the C13 satellites of fluorine by the fluorine on the C12 at the opposite end of the double bond (266). Assignment of coupling constants, including signs, for cis- and trans- is usually made by comparison methods and previously had to be based upon theoretical calculated couplings in reference compounds. Linkage between theory and experiment has been extended by analysis of cis- and trans-2-chloroheptafluoro-2-butene (268). I t is a well-known anomaly that coupling constants between 1,2 fluorine nuclei are frequently near zero, while couplings between 1,3 and 1,4 and even higher fluorines may be appreciable when the intervening atoms are carbon. The couplings may be even greater if nitrogen is part of the chain. The nearzero coupling has been explained as due either to an averaging of nonzero couplings over three stable configurations or a cancelling of mutual interactions of the same order of magnitude but opposite sign. Reasonable arguments may be raised against these VOL. 36, NO. 5, APRIL 1964
e
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explanations and Petrakis and Sederholm propose an across-space interaction so that geometrical factors enter into the presence or absence of coupling (217, 218). Work by Rogers and Graham with perfluoroalkyl derivatives of SF6 lends support to the idea that acrossspace interaction may be important (234). This view has not met with unanimous acceptance (138, 204). The study of C13-F19coupling constants has been a rich field for theoretical speculation. The splittings in a series of seven simple fluoroaliphatic derivatives have been interpreted as a function of the hybridization of the carbon molecular orbitals (265). Muller and Carr have treated a highly varied group of 52 compounds and found that in general the coupling constant increases as the chemical shift moves to a lower field (201). This suggests that the controlling factor is the estent of double bond character. Increasing C@-Fepolarity would have the same inverse effect, but the authors do not regard this as important. Increasing s-character in the C-F bond would also decrease J. Harris has presented a simple additive relationship for the value of the C13-F19 coupling constant as a function of the other substituents on the carbon (133, 134). Correlation with the position in the periodic table is suggested. h qualitative hypothesis is discussed. The elucidation of the structure of boron compounds is often hindered by the complications introduced into the spectra by coupling with bonded hydrogens. Lipscomb and Kaczmarczyk (179) report that the addition of paramagnetic ions in the S-states-e.g., Fef3-simplifies the spectra by decoupling the protons without greatly broadening the BI1 peaks. Reasons are suggested for the preferential action on the protons and for the probable formation of specific compleses. NaarColin and Heying have reported on the B11 spectra of decaborane and some of its derivatives (206). The B" spectrum of U2H6in ethers has been interpreted as arising from an ether-induced proton evchange with an activation energy of ca. 4.8 kcal. (113). It is probable that security regulations are largely responsible for the poor showing of boron spectroscopy in the literature of the immediate past. Two comprehensive reviews (160, 196) cover the use of N M R in the study of phosphorus compounds. It seems fair to say that, between these reviews and those which have appeared earlier, the XMR behavior of phosphorus is better summarized than that of any other nucleus. The paper of Groenweghe and coworkers (123) covers the P31 shifts of compounds with organic ligands quadruply bound to the phosphorus. Empirical rules are stated 272 R
ANALYTICAL CHEMISTRY
which govern the effect on the shift of successive substitutions. The phosphorus shifts and splittings of a series of compounds of the type (CF&=P-X and closely related series have been given by Packer (211). The phosphorus shifts are consistent with the dominant role of the paramagnetic contribution of nuclei having p- and/or d-electrons associated with them. So far as the writer knows, no work has appeared in which C13 resonance has been used as a means of structure proof, although it is a matter of common knowledge among resonance spectroscopists that i t is being so employed on a limited scale in various laboratories. Reasons for the restricted use of this resonance are not hard to find. Low isotopic abundance, unfavorable resonance frequency for good sensitivity, and ease of saturation are ever present to give trouble. I n all but the most favorable cases one will encounter a plurality of carbon nuclei giving closely spaced spectra with complicated overlapping coupling patterns. Spin decoupling and the use of summing techniques, as with the CAT, may greatly enhance the utility of C13 resonance in structure analysis. The work of Friedel and Retcofsky (110), emphasizing olefins, clearly indicates the potential. There is considerable activity in the use of C13 resonance as a theoretical tool. Here the CI3coupling constants are usually the important parameters. These constants may be obtained either directly from C13spectra or in many cases from the C13 satellites of some other resonant nucleus. Indeed, when one sees a reference to C13 work, more often than not he will find that esperimentally it was the proton resonance satellites arising from C13 that were actually observed. I n a series of compounds of the types CHa-X and CH3CH2-X the effect of both the electronegativity and the magnetic anisotropy of -X on the chemical shift of the C13 of both the CH3- and -CH2has been studied and it is believed that these two factors account for most of the relative shlFt. The large contribution of the magnetic anisotropy to the C13 resonance is opposite to the effect on the proton shift (250). Mavel presents evidence for a coherent increase of the C13-H spin coupling with increasing electronegativity of other substituents on the carbon (192). Muller, on the basis of the study of a series of formyl compounds, concludes that spin coupling is largely independent of the polarity of the bond and provides the best available measure of the hybridization of the carbon orbital (299). An additive relationship for the C13-H splitting in simple compounds with s p 3 hybridization has been proposed by Malinowski, 4
whereby the coupling is calculated as the sum of constants characteristic of the substituents on the carbon. A table of constants has been given. The relationship implies that the splitting depends on the character of the first atom from the carbon (184). The same author has proposed a similar relationship for sp2 hybridization (185). Gutowsky and Juan have given a n interpretation of this type of correlation based upon the s-character of the carbon orbital involved in the bond. The treatment developed for methanes has been estended to substituted ethylenes and to Si29-H coupling in silanes (129, 163). Muller and Rose have not been convinced of the validity of these simple relationships and have called attention to exceptions, suggesting reasons for the deviations (202). I n addition to the support, both theoretical and esperimental, for a relationship between C13-H coupling and bond order there is to be fbund evidence for the truth of a similar relationship for C13-C-H splittings where calculations based on the dominance of the Fermi contact term give good agreement with experiment. The constants range from 4 to 10.6 cps. (167). The correlation breaks down when another bond is added. Calculations based on similar assumptions as for C13-C-H predict couplings of the order of 2 to 3 cps. for C13-C-C-H while in fact they are greater than for the (2'3-C-H case (165). Correlation of coupling between nonbonded carbon13 and protons with bond order alone is made difficult by the results obtained with cis and trans isomers. For instance, with cis- and trans-l12-dichloroethylene the respective couplings are 15.7 and 0.0 cps. but the bond orders are the same (200). Spiescke and Schneider have investigated the chemical shifts of the ring C13 resonance in monosubstituted benzenes. The ortho position seems to be principally affected by the magnetic anisotropy of the substituent. The meta position shift is small and uniform, indicating little inductive influence from the substituents. The para position shifts correlate with the Hammett u-parameter (251). I n methylnitrobenzenes steric inhibition of conjugation leads to marked increases in the para carbon shielding in the o-methyl compounds. The observed changes are in good agreement with those calculated from earlier studies of such inhibition and the dependence of para-shielding on the r-electron densities (174). A detailed study of the influence of methyl substituents on the Cl3 spectra of aniline derivatives has been reported and estimates of the changes in charge distribution have been derived. The N-methyl carbon shielding seems to be related to the amine base
strength (17 5 ) . I n some symmetrically substihted benzenes those substituents which act as electron acceptors in a n inductive manner are Eound to increase the C*3-H coupling constants by as much as 20 cps. (160). Carbon-carbon c o q l i n g constants have been measured in enriched samples characterized by a variety of types of C-C bonds of different hybridizations. The coupling constants are proportional to the products of the .$-character of the carbon at'oms forming the bond. The enhancement of the ('onstants by unsaturation is diminished somewhat by conjugation (109). Goldstein and coworkers have used CI3-H couplings to assist in analyzing the spectra of pyrimidine, imidazole, and their monomethyl derivatives (629) and furan (227). BROAD-LINE NMR
Broad-line nuclear magnetic resonance spectroscopy has been employed principally ir the study of polymers and crystalline solids. Both subjects have been the, object of rather extensive reviews ( 9 , 249). The article on polymers contains some material on high revolution spectroscopy. Fortunately for most potential readers, both these Russian articles have been published in E n g h h translations. Taking due regard for the existence of these two reviews, altention is given here only to a selection of recent papers illustrating the field and the method of its application. Close agreement between the spectra of crystalline and vitrsous boron oxide has been observed. It is inferred that the similarity of the spectra arises from a corresponding similarity of the boron coordination in the two states (258). The line shape of the proton resonance spectrum of sodium tetraborate pentahydrate can be accountJed for by writing the molecular formula 8,s NazB40c(OH)z. 4H20. For lower hydrates an amorphous structure is proposed. The B1l resonance suggests that two types of boron exist in these borates-one with trigonal and the ot.her with tetrahedral coordination (75). The proton magnetic resonance spectra of single crystals of ?vlgS04.7 H 2 0 and ZnS04.7H20 have been studied to determine the positions of the protons. The average distance between osygens connwted by a hydrogen bond was found to be 2.83 A. and the proton-proton distsnce was 1.56 A. The average deviatior. was 10" (193). The splitting in the spectrum of Fe3(P04)2.8H20 was studied a t low temperatures. Two type:; of temperature depmdcnce were found, corresponding to two syst,ems of Fe+* occupying diffcrcnt kinds of lattice sites (180). X small but significant change in the line width of the pro';on resonance of polycrystalline NaH3(Eie03)2is detected
near the Curie point. The occurrence of the transition a t this point is regarded as indicating that the ferroelectric transition is associated with a rearrangement of proton sites (38). Studies along similar lines have been carried out on ferroelectric K p F e ( C N B . 3Ha0 (37). From the change in line shape with temperature of the Fl9 spectrum of polytetrafluoroethylene, the effect of molecular magnetic anisotropy was recognized and measured for the first time (280). Second moments and spinlattice relayation times were measured for a series of polypropylenes over a range of temperatures. Results show that the predominant process by which nuclei lose energy to the lattice is methyl group rotation (181). Determination of second moments as a function of temperature for a series of polystyrenes has shown the elistence of three regionswith characteri-tic motions, CH3- rotation from liquid nitrogen temperature u p to 300' to 310" K. when angular motion of the phenyl groups sets in, and finally the onset of segmental motion a t ca. 360" to 380" K. (126). Filipovich has sounded a note of warning in pointing out that the NMR curves of some polypeptides may have been subject to overinterpretation in the past and that a narron component of the curves, ascribed to CY- and pforms, may actually represent adsorhed water and some oily contaminants (92). Solution-grown specimens of solid polyethylene have been studied by Slichter (247). Restricted motion attributable to defect regions develops as the temperature is raised. The number of protons in motion changes reversibly with temperature. The technique has been employed to gain an insight into the molecular processes taking place during polymerization of vinyl compounds such as styrene, methyl methacrylate, and ~ i n y l acetate (40). During polymerization the spin-lattice relaxation time decreases slowly while the viscosity increases rapidly, indicating that the motion responsible for relaxation is largely rotational. The NMR signals from polyamide and epoxy resin specimens with water contents between 0.09 and 8.00% have been found to be directly proportional to the water content and independent of the resin type. The water may be considered as in a state between ice and free liquid (118). The proton iignal for water in partially dried tendon shows a dependence on the angle between the fiber direction and the magnetic field that is most convincingly interpreted as due to effective orientation interaction in the fiber direction (35). The water adsorbed on various clays is apparently not so tightly bound as is sometimes thought. This is indicated by the narrowness of the proton res-
onance line and is believed to be due to rapid and unrestricted movement of the water molecules ( 7 7 ) . Two interesting articles on the N M R of ion exchange resin systems have appeared. The first dealing with water-swollen sulfonated cross-linked polystyrene (251) gives more experimental data and is informative for the use of the technique. The second (119), while considerably less extensive in details, is very suggestive of the possibilities of using NMR to study a variety of phenomena important in the chemistry of exchangers, such as detection of heterogeneity, determination of molality of counter ions, measurement of distribution and selectivity coefficients, and estimation of restriction in the gel of the freedom of the counter ion and sorbed liquid. Results of wide-line studies of adducts of urea and hydrocarbons show the enclosed component to have considerable freedom of motion. Estimates of the energy barrier indicate that interaction between the urea cage and the adducted hydrocarbon is slight (116).
Aicrivos has pointed out the advantages and extensively treated the method of directly observing the absorption spectrum and eliminating the broadening due to finite modulation which may arise in derivative presentation of broad-line spectra (6). Ai device has been described which enables S M R spectra to be taken in the double coil system under axial stresses in excess of 10,000 p.s.i. or hydrostatic pressures u p t o 10,000p.s.i. (270). The analysis of broad-line spectra depends very heavily on line shape and contributions bearing on line-shape analysis are always of significance. Several have appeared in the last few years. h formal mathematical analysis has been used to clarify the difficulties associated with the theoretical and experimental determination of moments together with several examples of its application (257). Hughes and MacDonald give particular attention to determining how the properties of multiply broadened lines depend on the properties of the individual lines ( f 4 9 ) . The use of Fourier transforms for the analysis of spectra generated by two broadening mechanisms has been discussed by Svanson. Examples are given for spectra perturbed by quadrupole interaction and groups of interacting nuclei (256). Explicit expressions have been derived for correcting line shapes which are distorted by finite frequency and field modulations of various forms and by the use of circuits with finite time constants ( 3 7 ) . ELECTRON SPIN RESONANCE
X number of reviews of some length have appeared on the subject of electron spin resonance (ESR). Of these three VOL. 36, NO. 5, APRIL 1964
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may be mentioned particularly. The chapter of Elion and Shapiro (83) covers the whole field rather than being confined to a single area such as that of free radicals The section on instrumentation is of especial interest, as it contains a comparison of radio-frequency and microwave methods. The chapter by Symons (261) on the identification of organic free radicals is broader than its title would indicate and discusses organic free radicals rather generally. There is no treatment of instrumentation. The author makes the interesting statement that, owing to the restricted origin of electron resonance-namely, an unpaired electron-it “will never be of much importance to the analytical chemist.’’ At the same time the point is made that because of this restricted origin the technique may have an especially important, if limited, application in a given situation. The article of Carrington (52) covers much the same ground as the foregoing but in less detail. A recent entry into the list of suppliers of ESR spectrometers is Alpha Scientific Laboratories, Berkeley 1, Calif. These suppliers have three instrument models. One is a modest piece of equipment operating in the radio-frequency range and intended primarily for instructional and demonstration purposes. The other two models are intended for more sophisticated applications. The first of these operates in the radio-frequency range. A later development is an X-band instrument which uses a traveling wave helix in place of a resonant cavity. This acts as a microwave delay line, reducing the velocity and thereby shortening the apparent wave length. This in effect compresses the microwaves and increases the energy density in the helix. In an extensive study of the tetracinium ion, Hyde and Brown discuss a t length the experimental techniques which. are useful for investigating very narrow lines (161). Hausser also considers the significance for ESR technique of the study of the 17-milligauss lines in 1,3-bisdiphenyleneallyl. Additional couplings appear and the broadening effect of dissolved impurities such as oxygen becomes very important (142). For the quantitative evaluation of an electron resonance signal an integrator has been described, though not in detail (240). Quartz inserts placed in the cavity of an ESR spectrometer may be used to pull the frequency by an amount sufficient to tell whether broadening or separation betlveen lines is due to hyperfine interaction or other causes (36). Electrolytic methods for the generation of free radicals within the cavity are being increasingly used (219, 232). The use of ultraviolet radiation to produce free radicals from nitro and carbonyl com274 R
ANALYTICAL CHEMISTRY
pounds gives clean, narrow lines and also provides for control in kinetic studies by adjusting the light intensity (20). A number of valuable accessories, which for the most part have been reported previously in the literature, have become commercially available from Varian Associates, Palo Alto, Calif. These components either improve spectrometer performance or facilitate the carrying out of special experiments. In the first category are flux stabilizer and field homogeneity control systems which enable one to achieve and hold a degree of magnetic field homogeneity of the same order as that which has been used in nuclear resonance. This control makes it practical to use the now available slow-sweep unit, also brought over from nuclear resonance, to sweep slowly and precisely through narrow ranges. A dual sample cavity makes possible a more exact comparison of peak areas for quantitative work and facilitates precise determination of gfactors and hyperfine splittings by the simultaneous observation of the spectrum of an unknown and a standard. Specialized experimentation can be carried out with a mixing-chamber accessory, an electrolytic cell, a verticalaxis rotating cavity, and compensated pole caps with holes to permit radiation along the axial coordinate. That electron spin resonance spectra are subject to solvent effects has been known for several years. I t is true that some early findings attributed to solvents were subsequently related to other influences such as dissolved oxygen in the solvent. However, when these factors are recognized and steps taken to eliminate them, very real effects attributable only to the solvent remain. It is found that the nitrogen coupling constant for radicals generated electrolytically in acetonitrile-water solutions from aromatic nitro compounds increases with increasing water content. Aliphatic nitro compounds do not show this effect (219, 220). .5. paper on electrolytically generated nitrile radicals includes, among other subjects, a treatment of solvent effects in a very comprehensive discussion section (232). Line narrowing with solvents has been attributed to the formation of a solvation shell which isolates the molecule and prevents exchange broadening (162). This means that the line broadening or narrowing effect, as the case might be, would not be a constant property of a given solvent but would depend upon its associative action with a given system. Gendell, Freed, and Fraenkel have developed a simple model to account for the solvent dependence of hyperfine splitting in organic free radicals. The effects are regarded as arising entirely from redistribution of the r-electron spin
density which is affected by localized complexes between the solvent and polar substituents or heteroatoms in the radical ( 1 1 4 ) . A method of calculating simple ESR spectra with hyperfine structure has been presented which covers both Gaussian and Lorentzian line shapes. Nomograms for deriving true line parameters from observed ones and methods of analyzing actual spectra are given (176). Zhidomirov and coworkers have worked out means of determining the width of individual lines from an unresolved doublet under limited circumstances such as a Gaussian line (283). The analysis of asymmetric ESR lines where there is a small axial anisotropy in the g-factor and splitting constants is treated by Lebedev (176). Nonlinear statistical methods are used to make a leastsquares fit between an observed spectrum and that calculated from a particular model and thereby verify a free radical site (187). Weil and Hecht assert that for randomly oriented fixed molecules the total first derivative spectral envelope frequently has the exact shape of the individual lines, thus making available immediately many spectral parameters (271), A theory of ESR coupling constants has been proposed and applied to anion radicals of nitro compounds. The theory indicates a twofold barrier to rotation which may account for the difference in coupling constants of radicals derived from nitroethane and nitropropane radicals (262). I n some alkyl radicals in solution there are more lines than can be accounted for by simple first-order theory, but the additional spectral detail is predicted if the usual treatment is extended t o second-order in the coupling constants (91). For the case of aromatic free radicals the simple linear relationship between hyperfine coupling constants and spin density has been modified by the addition of a term for the excess charge density on the carbons (61). With careful work it is possible in many cases to separate out the Cl3 hyperfine splitting a t the natural abundance level in ESR spectra. In experiments with semiquinones and cyclooctatetraene anion radical this splitting has been found to be a sensitive measure of r-electron spin density (264). h quantitative theory which permits the prediction of the magnitude and signs of the hyperfine structure in the ESR spectra of T-radicals has been presented (169). This result of the theory provides a useful criterion for checking approsimate wave functions. The broadening of the resonance peak due to shortened lifetime in the interaction of paramagnetic cations and anions can be used to measure rates of formation of ion pairs in cases where
interaction is strong (213). Electron spin resonance has b e m used to study the kinetics of the reaction of 2,2diphenylpicrylhydrazyl and a variety of phenols (147) and the ]-ate was found to be first-order with respect to both components. The resonance-stabilized hydroxycyclohexadien3.1 radical has been shown to be an intermediate in the formation of phenol and biphenyl from benzene, hydrogen peroxide, and water ( 7 5 ) . Electron resonance studies of the oxidation of alcohols have given information about the structure of the radicals produced and hence about the course of the reaction (74). One of the most studied of free radicals is the Wurster’s Blue cation and anyone working with elertron re-onance is likely to deal with it or one of its close relatives at some time. I n spite of this eltensive past tittention, Bolton and coworkers have shown the previous interpretations of the spectrum to be wrong (39). The misinterpretation has arisen from use of a n erroneous nitrogen coupling constant and f iilure to examine the wings of the spe-trum with sufficient care. Perhaps earlier workers should not be taken to 1 ask too severely, since good values of t i e coupling constant were not always available and only recently has instrumentation become good enough to make the spectral wings meaningful. The presence of free radicals of aromal ic hydrocarbons in Friedel-Crafts systems has been demonstrated ( 7 ) . Th,s does not necessarily mean, of course, that the reaction is free radical ralher than ionic in nature. The free radicals may be quite incidental. T h e ESR spectra of ,he negative ions of a number of nitrolqen heterocycles have been studied and the proton and nitrogen coupling const i n t s determined. The reqults indicated that the J N is proportional to the unpaired electron density on the nitrogen atom, with Q N having a value of 25 4 2 gauss but with no dependence on the electron density on the adjacent carbon atom (53). t\n interesting alternation of line width in the hyperfine components of the spectra of a variety of presumably pure free radicals has been reported by several authors. Various esplanations have been offered, such as modulation of the spectrum by changing tonformations in the free radical moleculp (51, 100, 101). A theoretical treatment has been given (54). Commoner and Ternberg have reported the results of a series of investigations of ESR signalii in surviving tissues from laboratory animals and from man (62). Free radicals are found to be present znd their concentrations are related to the biological origin of the tissue :nd vary with physiological and pal hological conditions. The free radicrtl properties are
consistent with those previously obtained in functional oxidation-reduction enzyme syptems and are regarded as associated with the mitochondria present. There is good evidence that the ESR lines previously observed in nucleic acid preparations are not due to special magnetic properties of the preparations themselves but rather to the presence of ferromagnetic impurities (8,221). \ . 5 7 ~and ~ l silk may be modified by rertain oxidizing agents to produce free radicals which, in the case of wool, may initiate addition polymerization. The ESR spectra of these radicals are very similar to those produced by 7-radiation of the natural fibers (60). Sicholas and coworkers have given some examples of the use of ESR spectroscopy in the study of the function of transition metals in microbial metabolism which indicate the potential usefulness of the technique in providing a neu parameter in this field (208). Low concentrations of osygen in solutions have been measured by monitoring with electron resonance the concentration of a free radical with which the oq-gen readily reacts. The method is quantitative in the range of 10-5 to N (153). Less than mole of certain hydrocarbons has been detected and estimated by ESR after their quantitative conversion to free radicals on the surface of a strongly dehydrated silica-alumina catalyst (95). Somewhat similarly, the possibility of using ESR for the quantitative determination of nitrogen after conversion to a stable free radical has been investiExperimental comgated (17 7 ) . plications make this look like an unpromising attack on nitrogen analysis. *in interesting linking of the ancient and the modern was the demonstration of the identity of “Tyrian Purple” and 6,6’-dibromoindigo. Bruin, Heineken, and Bruin (47) followed directions of Pliny and Aristotle to prepare samples from the same species of mollusks as those whose shells are found in the ancient Mediterranean heaps where the dye was prepared. The ESR spectrum was then compared with that of authentic 6,6’-dibromoindigo and found to be identical. Electron resonance studies of the oxide products formed in the thermal decomposition of chromic anhydride confirm an earlier assumption that the intermediate osides are chromium chromates containing triand quadrivalent chromium (233). Such results may be the basis for judgments about the active components of chromium catalysts. The free radicals produced in materials by irradiation are most frequently studied by electron resonance, and many papers have been published on this subject in the past few years. Frequently single crystals of known pure compounds are involved in
which i t is possible to speculate intelligently about the nature of the free radicals to be espected. Electron resonance enters the picture in many cases simply to supply confirmation of a reasonable conjecture. From an analysis of the hyperfine interaction constants the free radical produced by the 7-radiation of N-carbamylglycine was found to be H2NCOkHCHCOOH (225). It has been concluded that the radical (C02H)CH2CH2CH(COOH) is trapped in ?-irradiated glutaric acid in two conformatjons (148). Evidence of anisotropy in the spectrum indicates that the free radical (COOH)CH(CH2)s(COOH) in irradiated adipic acid has almost the same orientation as the undamaged molecule, although the plane of the free radical carbon is twisted approsimately 10’ from the original plane of the three terminal carbon atoms ( f 97). Xirradiation of glutaconic acid produces an oriented radical (HOOC)C(l)H= C(2)-H--C(3)H(COOH) and a detailed analysis shows the spin density on carbon 2 to be opposite in sign to those on carbons 1 and 3 (144). The situation with irradiated polymers is somewhat more complicated than that with single crystals of known simple pure compounds. The prediction of the free radicals to be expected is accordingly less sure, even without considering the additional cornplications arising from steric distribution in the polymer chain. I n stretched specimens of irradiated polyethylene the spectrum of the free radical -CH2CHCH = CH-CH2was observed a t 142’ C. As the temperature was lowered to - 180’ C., the spectrum changed continuously in a complicated and unexplained fashion. Below - 180’ there was no change. The phenomenon was reversible with temperature (209). Changes in spectral shape with temperature were also noted for the radical -CF2CFCF2from irradiated polytetrafluoroethylene. The changes were compared with those undergone by the NMR and mechanical properties over the same temperature range and were ascribed to rotational motion of chains in the crystalline region which averages the angular dependence of the 8fluorine relative to the p-orbital of the radical carbon atom ($62). Internal motion effects have also been invoked to explain the changes with temperature in the ESR spectrum of irradiated polyisobutylene (283). If mixtures of natural rubber with sulfur are 7-irradiated a t - 196’ C. and then warmed up in stepwise fashion, new short-lived radicals with g-factors characteristic of sulfur radicals are observed. This is regarded as evidence for the reaction between polymer molecules and sulfur molecules (263). VOL. 36, NO. 5 , APRIL 1964
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