4362
JOSEPH
Vol . s3
BUFALINIAND KURTH. STERN
[COSTRILlUTIOS FROM THE DEPARThlEST O F CHEXISTRY, U C I V E R S I T Y 02.’ .IRKASSA5, 17AUETTEVILLE, lIRKATSAS]
Interactions in Ionic Solutions. The Effect of Electrolytes on the Infrared Spectra of Some Hydrogen-Bonded Compounds BY JOSEPH BUFALINI~ AND KURTH . STERN^ RECEIVED MARCH 10, 1961 Tlie effect of several electrolytes on the vibrational spectra of some hydrogen-boiided coiripouiids-tiiethanol, 1-butanol, t-butanol, N-methylacetamide-iri dilute benzene solution is examined. The results :ire interpreted in terms of hydrogen bond formation t o the anion of the electrolyte ion pair. This “solvation” increases with the anion charge density and is absent for picrate ion. T h e frequency of the absorption maximum is interpreted in terms of t.he 0-H group as a n harmonic oscillator. The addition of Bu&Br t o 1-butanol, t-butanol and S-methylacetamidc reduces dimerization in these compounds while simultaneously anion solvation increases. X detailed spectral study of the BuJ?;Br-MeOH-CeHo system, combined with previously obtained dielectric data ( J . Am. Cizewz. Soc., 82, 1296 (19130): yields ?z = 1 for the ion pair solvation number, the equilibrium constant for ion pair solvation and A H o = -6.7 k c d . and A S o = - 13.6 e.u. for h>-drogen bond formation in agreement with commonly accepted values for tnis process. Some quadrupole solvation also occurs.
Introduction Evidence is steadily accumulating, that, although many properties of electrolytic solutions can be interpreted in terms of a continuum model for the solvent, others cannot. I n the former category lie the success of the Debye-Huckel theory and its recent extensions by Fuoss and Onsager, the application of this theory to ionic association4 and the Denison-Ransey5 treatment of ion pairing which correctly predicts the dependence of the ion pair association constant on solvent dielectric constant. -4ttempts to account quantitatively for the molecular features of the solvent have been most vigorous as applied to aqueous solutions. We refer here only to the “iceberg” theory of Frank and Evans,Gionic number theories’ and X-ray structure determinations of ionic solutions.8 Quantitative studies of ion-molecule interactions in low dielectric solvents have been rare, although i t is of course recognized that these occur. For example, Gilkerson’sg treatment of ion pairing includes the ionic solvation energy, but the theory has not been widely applied, partly because of the difficulties in assigning numerical values to some of the parameters. The study of molecule-molecule interactions in solution is considerably further advanced, thanks in large part to the availability of infrared spectrometers, but this tool seems not to have been applied extensively to the electrolyte problem. A naive first look a t the problem would lead one t o argue that if a molecule becomes attached to an ion, thereby losing some of its rotational degrees of freedom, this should be detectable as a decrease in intensity of the corresponding rotational line. Because of collisional processes in solution, sharp lines are not observed (in contrast to the gas phase) in the far infrared. Moreover, the problems (1) Extracted from t h e P h . D . Dissertation of J. Bufalini. (2) Electrochemistry Section, h-ational Bureau of Standards, Washington 2 3 , D. C. (3) R. M. Fuoss a n d L. Onsager, J . P h y s . Chem., 61, 6G8 (1957). (4) Cf.R. AI. Fuoss and F. Accascina, ”Electrolytic Conductarce,” Interscience Publishers, Inc., New York, N. Y . , 1959. (5) J. T . Denison and J . B. Ramsey, J . Am. Clzem. Soc., 77, 2Gl.5 (1955). (6) H. S. F r a n k ar.d & f . W.E v a n s , J . Chem. Phys., 13, 5 O i (1045). (7) Cf.G. K o r t u m a n d J. O’M. Bockris, “Textbook of Electrochemi s t r y , ” Elsevier, Houston, Texas, 1951, C h a p t e r 3. ( 8 ) G. E. B r a d y and J. T. Krause, J . Phys. Ckei;z., 27, 304 (19:i); 28, 4 0 4 (1958). j)!( K’, R Gilkerson, J . Cheiii. Phrs., 25, 1199 (195G).
of absorption of the weak radiation by atmospheric water vapor are very severe in this region. One is thus restricted to the study of vibrational lines in the near infrared. The hydrogen bond in alcohols has been well studied and is also of interest for electrolytes since a number of studies have dealt with systems containing alcohol^.^^-'^ In this work we report on the efiect of a number of electrolytes on the infrared spectra of some hydrogen-bonded compounds-mainly alcohols-when both are dissolved in dilute solution, and an application of the technique to the study of solvation equilibria in the system Bu&Br-MeOH-C6H6 which had previously been studied by a dielectric method. Some preliminary results have been published elsewhere.l5 The line found most useful is the sharp fundamental for the OH stretching vibration. This mas first noted in alcohols by Freymann.16 Errera and Mollet17 also observed the sharp OH band in the 3600 cm-’ region. This band is nonlinear with concentration and was assigned to the OH stretching mode of the monomer. Some lower bands in the 3300 C I I ~ . - ~region also were noted. These bands increase in intensity with increasing concentration and consequently were assigned to the OH stretching vibrations of H-bonded molecules. Subsequently, Fox and lZartin,’? Liddel and IYulf’“ and Pauling?”recognized that the results could be explained in terms of chemical equilibria and that the infrared spectrum can facilitate the study of inolecular complexes involving hydrogel1 bonds. Tsuboi?’ and Tsubomura?? have shown that an equilibrium between a proton cloiior and a proton acceptor in an “inert” solvent can be studied by observing the OH frequency. For such a system as 0-H . . . X, the energy of the hydrogen bond was found to be between 1 to 8 kcal./mole depending on the riature of X. The best and most (10) 13. Sadek and In.Soc., 7 2 , 301 (10.70). (11) R. C. 3Iiller a n d 14. >I, Fuoss, ibid., 7 5 , 3076 (1953j. (12) F. AI, S w h s and R . >I. Fuoss, i b i d . , 75, 5172 (1933). (13) H. Sadek a n d R . XI. Fuoss, ibid., 76, 5902 (1954). (14) E. 4 . Richardson and K. 13. Stern, i b i d . , 82, 12!3(i (1
