JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Regislered in U.S. Paten1 Ofice.
@ Copyright 1973 by
the American Chemical Society
APRIL18, 1973
VOLUME95, NUMBER 8
Solvent Effects on the Five Shielding Constants in Tetramethylsilane and Cyclohexane’” Mark R. Bacon and Gary E. Maciel*Ib
Contribution from the Departments of Chemistry, Colorado State University, Fort Collins, Colorado 80521, and the University of California, Davis, California 95616.lC Received October 4, 1971 Abstract: Intrinsic solvent shifts of the four 1H and 3Cresonances of tetramethylsilane (TMS) and cyclohexane have been determined in 38 aromatic and nonaromatic solvents, and compared with *9Si shifts for 15 of these solvents. Apparent 13Cand hydrogen solvent shifts in 54 additional solvents have been measured. The maximum ranges of intrinsic solvent shifts are 0.6,4.0, and 0.7 ppm for ‘H, I3C, and 29Si, respectively, of TMS; and 0.6 and 1.5 ppm for lH and *3C,respectively, of cyclohexane. Trends in the observed TMS and cyclohexane solvent effects are compared and contrasted, The intrinsic solvent shifts have been subjected to factor analysis calculations. The results indicate that for most of the solvents in which intrinsic shifts were determined, these shifts are described by just two statistically significant solvent influences. Attempts to identify these influenceswith particular “physically significant” solute-solvent interactions are described.
T
he classic paper by Buckingham, Schaefer, and Schneider (referred to hereafter as BSS) delineated a set of postulated contributions to the resonance shift imparted by a solvent on a solute (“solvent shift”). Since then, much effort has been expended toward gaining further insight into the various postulated contribut i o n ~ . * - ~The BSS approach is summarized in eq 1,
+ + +
A U = ‘Jb Ua ‘Jw UE (1) where Au is the observed solvent shift, U b is the overall bulk susceptibility effect, ua arises from anisotropy in the magnetic susceptibility of the solvent molecules, uw reflects van der Waals interactions between solute and solvent molecules, and UE stems from permanent or induced “reaction field” dipoles in neighboring solvent molecules which polarize the solute and hence alter its electronic environment. Concurrently, Schaefer and Schneider5 focussed on an additional effect, the specific association property which arises from the formation of weak complexes between a solvent and solute. Hydrogen bonding is an example. The “polar effect,” a relatively small contribution for nonpolar solutes, aris-
(1) (a) Supported by National Science Foundation Grants GP-8119 and GP-27577. (b) Colorado State University. (c) The experimental work was performed at the Department of Chemistry, University of California, Davis, Calif. 95616. (2) A. D. Buckingham, T. Schaefer, and W. G. Schneider, J . Chem. Phys., 32, 1227 (1960). (3) P. Laszlo, Progr. Nucl. Magn. Resonance Spectrosc., 3, 231 (1967). (4) J. Ronayne and D. H. Williams, Ann. Reo. N M R (Nucl. Magn. Resonance) Specrrosc., 2, 83 (1969). ( 5 ) T. Schaefer and W. G. Schneider, J. Chem. Phys., 32, 1218, 1224 (1960).
ing from dipole-induced dipole interactions between a polar solvent and the solute, was included in UE by BSS, although other have chosen to consider this contribution to be part of the van der Waals effect uw, which, in the BSS view, primarily encompasses the generally stronger dispersion interaction between solute and solvent. We shall consider uw to represent only dispersion interactions, and UE to embrace only polar effects, including those attributed to the reaction field. In spite of the attention given to the last three terms of eq 1, both through empirical correlations and postulated models, the detailed nature of these interactions remains obscure and shrouded in substantial conNevertheless, recent experimental troversy. work2l3+-l7has clearly demonstrated that the hydro3,4b8,9
(6) B. Linder, ibid., 33, 668 (1960). (7) B. B. Howard, B. Linder, and M. T. Emerson, ibid., 36, 485 (1962). (8) M. A. Raza and W. T. Ravnes. Mol. Phvs.. 19. 199 (1970). (9) P. H. Weiner, E. R. Malinowslii, and A. R.’Levinstone, J . Phys. Chem., 14, 4537 (1970). (10) P. Laszlo, A. Speert, R. Ottinger, and J. Reisse, J . Chem. Phys., 48, 1732 (1968). (11) J . Homer, Tetrahedron, 23, 4065 (1967). (12) W. T. Ravnes and M. A. Raza. Mol. Phys., 17. 157(1969). (13) J. Homer; E. J. Hartland, and C. J. Jackson, i. Chem. SOC.A , 931 (1970). (14) E. R. Malinowski and P. H. Weiner, J . Amer. Chem. SOC.,92, 4193 (1970). (15) P. H. Weiner and E. R. Malinowski, J . Phys. Chem., 75, 1207, 3160, 2971 (1971). (16) J. K. Becconsall, G. D. Daves, Jr., and W. R. Anderson, Jr., J . Amer. Chem. Soc., 92, 430 (1970); J. Homer and P. M. Whitney, J . Chem. SOC., Chem. Commun., 153 (1972). (17) M. Bacon, G. E. M a d , W. K. Musker, and R. Scholl, ibid., 93, 2537 (1971).
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gens of nonpolar solutes, for which the reaction fi.eld effect is usually considered negligible,2 are subject to solvent shifts of several tenths of a part per million. Most of this work has focussed on the ubiquitous hydrogen internal reference, tetramethylsilane (TMS).18 Some of the results have been interpreted as evidence’O~’~that TMS forms weak complexes with aromatic solvents, while ostensibly less polarizable nonpolar hydrocarbons, such as neopentane and cyclohexane, do not. This interpretation is based on the finding that TMS experiences a decreased shielding, relative to hydrocarbon solutes, in ternary solutions in aromatic solvents,’” compared to solutions in nonaromatic solvents. Other findings were rationalized in terms of “solute exposure” to the surrounding solvent molecules.8~’2 For example, the observation of large solvent shifts experienced by the peripheral methyl hydrogens of tetraethylsilane, relative to the inner methylene h j drogens, supports the idea of closer contact with the solvent of the methyl hydrogens. This is one of the few pieces of direct experimental evidence that different regions of a solute molecule experience different degrees of interaction with the solvent. Other evidence is offered in conjunction with the present results (vide injia). Solvent effects investigations in nonaqueous media of nuclides other than hydrogen have been rather sparse. Most of what has been done centers on the resonances of 13C and 19F.3 Spiesecke and Schneiderlg and Becconsall and HampsonZodisclosed large (several parts per million) solvent effects on the 13C resonances of methyl iodide.19’20Maciel and coworkers21,22uncovered similarly large solvent effects on polar carbonyl groups, and more recently a paper has appeared that demonstrates solvent shifts of several ppm for the 13Cresonance of c h l ~ r o f o r m . ~ ~ In a preliminary report of the present work, it was pointed out that the lZCof TMS exhibits a range of solvent shifts about three times that of the hydrogen resonance.” The silicon resonance of TMS is influenced by solvent to about the same degree as is the hydrogen. This preliminary study is the only one thus far reported on 13Csolvent effects on a nonpolar solute and demonstrates that such solutes are also subject to solvent shifts of >1 ppm. This study centers on the tetrahedral TMS and on the more oblate cyclohexane as solutes. The primary aspect of this investigation is a study of “intrinsic” (Le., not susceptibility effect) solvent shifts” on the ‘H and I3C signals of these solutes, dissolved in a series of monohalocyclohexanes, in a series of mono- and polyhalo- and methylbenzenes, and in some noncyclic, organic solvents. In addition, we consider here corresponding data on the 29Siresonance of TMS in 15 of these solvents. 2 4 These “intrinsic” shifts hinge on hydrogen shifts obtained on these solutes by a previously described methodl6,I7(see the Experimental Section) (18) G. V. D. Tiers, J . Phys. Chem., 6 2 , 1151 (1958). (19) H. Spiesecke and W. G. Scheider, J . Chem. Phys., 35,722 (1961). (20) J. I
