Spectroscopic Studies of Ionic Solvation
Acknowledgment. The authors gratefully acknowledge partial support of this work by research grant from the National Scisnce Foundation.
References a n d Notes (1) V. Gutmann, "Coordination Chemistry in Nonaqueous Solvents," Springer-Verlag, Vienna, 1968. (2) R. H. Erlich and A. I . Popov, J. Amer. Chem. Soc.. 93, 5620 (1971). (3) R. H. Erlich, M. S. Greenberg, and A. I . Popov, Spectrochim. Acta. P a r t A , 29, 543 (1973). (4) H. H. Szmant in "Dimethyl Sulfoxide," S. W. Jacob, E. E. Rosenbaum, and D. C. Wood, Ed., Marcel Dekker, New York, N. Y., 1971, pp 1-98. (5) S. J. Gaurner, Ph.D. Thesis, "A Brillouin Spectrophotometer," Department of Chemistry, Michigan State University, 1973. (6) H. L. Schlafer and W. Schaffernicht, Angew. Chem., 72, 618 (1960). (7) W. S. MacGregor, Ann. N. Y. Acad. Sci., 141, 3 (1967). (8) J. J. Lindberg, J. Kenttamaa, and A. Nissema, Suom. Kernistiiehti 6, 34, 156 (1961).
2449 (9) J. J. Llndberg, J. Kenttamaa. and A. Nissema. Suom. Kemistiiehti 6, 34, 98 (1961). (10) R . L', Amey,J. Phys. Chem., 72, 3358 (1968). (1 1) J. D. Ramshaw, J. Chem. Phys., 57,2684 ( 1972). (12) R . Figueroa, E. Roig, and H. H . Szmant, Spectrochim. Acta, Part A, 22, 587 (1966). (13) L. Brillouin,Ann. Phys. (Paris), 17, 88 (1922). (14) G. A. Miller and C, S. Lee, J. Phys. Chem.. 72,4644 (1968). (15) R. D. Mountain and J. M . Deutch, J. Chem. Phys., 50, 1103 (1969), (16) L. Fishman and R . D. Mountain, J. Phys. Chem., 74, 2178 (1970). (17) I. L. Fabelinskii, "Molecular Scattering of Light," translated by R. T. Beyer, Plenum Press, New York, N. Y . , 1968. (18) R . S. Krishnan, "The Raman Effect," A. Anderson, Ed., Marcel Dekker, New York, N. Y., 1971, Chapter 6. (19) K . F. Herzfeid and T. A. Litovitz, "Absorption and Dispersion of Ultrasonic Waves," Academic Press, New York, N. Y., 1959. (20) E. G. Richardson, "Ultrasonic Physics," Elsevier, Amsterdam, 1952. (21) G. W. Marks,J. Acoust. SOC.Amer.. 41, 103 (1967). (22) A. E. Lutskii and V. N. Solon'ko, Russ. J . Phys. Chem.. 38, 217 (1964). (23) A. E. Lutskii and V. N. Solon'ko, Russ. J. Phys. Chem.. 39, 414 (1965). (24) V . Volterra, J. A . Bucaro, and T. A. Litovitz, Der. Runsenges. Phys. Chem.. 75, 309 (1971).
Spectroscopic Studies of Ionic Solvation. XIV. A Sodium-23 Nuclear Magnetic Resonance and Electrical Conductance Study of Contact Ion Pairs in Nonaqueous Solvents Mark S. Greenberg, Richard L. Bodner, and Alexander I. Popov* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824
/Received May 17, 1973)
Publication costs assisted bv the National Science Foundation
Sodium-23 nuclear magnetic resonance measurements have been carried out on several sodium salts in 1,1,3,3-tetramethylurea, 1,1,3,3-tetramethylguanidine,sulfolane, tetrahydrofuran, dimethylformamide, formamide, ethanol, methanol, pyridine, and ethyl acetate. Chemical shifts were measured relative to aqueous 3.0 M sodium chloride solution. The direction, magnitude, and concentration dependence of the chemical shifts are strongly influenced by the "donicity" (or solvating ability) of the solvents. Formation of contact ion pairs depends not only on the dielectric constants of the solvents but also on their solvating abilities. Electrical conductance studies of NaI solutions in pyridine and tetramethylguanidine yield ion pair dissociation constants of 3.91 x 10C4 and 6.2 x respectively. The interpretations obtained from the 23Na chemical shifts correlate well with the data obtained from electrical conductance measurements.
Introduction It is becoming increasingly obvious that the mechanisms and rates of most reactions in solutions are strongly dependent on the nature and physicochemical properties of the solvent. In order to elucidate the role of the solvent in chemical reactions it is necessary to have a sound knowledge of solvent-solute, solvent-solvent and solutesolute interactions in the given medium. Yet such data are seldom available and even the knowledge of the ionic species and the equilibria in solutions of simple salts in water or in nonaqueous solvents is in a very rudimentary state.
In recent years it has been shown that the alkali metal nmr, and particularly sodium-23 nmr, is a very sensitive probe of the immediate chemical environment of alkali metal i ~ n s . l The - ~ magnitude and the direction of 23Na chemical shifts in various solvents have been related either to the Lewis basicity* of the solvents or to their donor (or solvating) abi1ities.l" The purpose of this investigation is to extend our earlier studies of ion-ion and ion-solvent interactions in nonaqueous solutions of various sodium salts by sodium-23 nmr.la The solvents were selected so as t o vary as much as possible their dielectric constants and solvating abilities. The Journal of Physicai Chemistry, Vol. 77, No. 20, 1973
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Experimental Section Chemicals. Sodium salts used in this study were of reagent grade quality and were not further purified before use except for drying. Solvents, 1,1,3,3-tetramethylurea (Aldrich) and 1,1,3,3-tetramethylguanidine(Eastman), were purified by refluxing over granulated barium oxide for 24 hr followed by fractional distillation under vacuum. Sulfolane (Shell) was purified by fractional freezing followed by vacuum distillation over sodium hydroxide pellets. Tetrahydrofuran (Matheson Coleman and Bell) was fractionally distilled over calcium hydride. Dimethylformamide (Fisher) was vacuum distilled over phosphorus pentoxide. Commercially available absolute ethanol and reagent grade ethyl acetate (Baker) were used without further purification. Methanol (Baker) was fractionally distilled over calcium sulfate. Formamide (Fisher) was purified by fractional freezing. Pyridine (Fisher) was refluxed over barium oxide for 24 hr and fractionally distilled. Dimethyl sulfoxide (Baker) was dried over molecular sieves and vacuum distilled. Purified solvents were stored over Linde 4A molecular seives. Stock solutions of sodium salts (0.500 M) were prepared by weighing out the desired amount of a salt into a 5-ml volumetric flask and diluting to the mark with solvent. The remaining solutions were prepared by appropriate dilutions of the stock solutions. Measurements. Nmr. Sodium-23 nuclear magnetic measurements were made a t ambient temperature on a modified NMRS MP-1000 spectrometer a t 60 MHz (53.3 kG). The experimental details are described in a previous pub1ication.ld The Kontes K-897155, 5-mm 0.d. polished nmr sample tube was fitted with a Wilmad precision coaxial 520-2 nmr tube for the reference solution. The reference for 23Na measurements was 3.0 M aqueous sodium chloride solution. When the chemical shifts were so small that the sample resonance was masked by the reference, a secondary reference of 2.5 M sodium perchlorate in methanol was used. In the latter case. the shifts were corrected so as to apply to the same sodium chloride reference solution. Magnetic Susceptibility Corrections. Bulk diamagnetic susceptibility measurements of the solutions were made on a Gouy balance employing the Alpha Magnetic Susceptibility System Model 4520. It should be noted that when the applied field is transverse to the long axis of the cylindrical sample, the correction to the observed chemical shift due to different bulk diamagnetic susceptibilities of the sample and reference is given by
Live and Chan6 have shown that for high-field nmr spectrometers with superconducting solenoids, where the applied field is parallel the long axis of the cylindrical sample, the correction to the observed chemical shift is
All of the data presented in this paper have been corrected according to eq 2. In general the corrections were of the order of
