Solvation of thiols. An infrared and nuclear magnetic resonance study of ethanethiol

Abstract

The i.r. spectra of solutions of ethanethiol in CCl₄ in the S—H fundamental (v) and first overtone (2v) regions have been measured as a function of thiol concentration. A dimerisation constant of 0.038 dm³ mol^–1 was estimated at 22 °C, the S—H stretching frequencies for free and mono-hydrogen-bonded thiol being at 2585 and 2570 cm^–1, respectively, in the fundamental, and 5056 and 4993 cm^–1 in the overtone regions. Beer's law was obeyed for the monomer in dilute solutions, giving absorbances of 9.4 and 0.53 dm² mol^–1 in the v and 2v regions. Spectra for the pure thiol in the fundamental region were analysed in terms of these two bands and a third at ca. 2559 cm^–1, assigned primarily to the central molecule of linear trimers. The results show that dimers dominate at room temperature, in contrast to results for methanol, which is highly polymerised at room temperature. Also for methanol in CCl₄ solutions, dimers never constitute a major component, the main equilibria being between monomeric and cyclic units. Thus the phenomenon of cooperativity in hydrogen bonding is far less for thiols than for alcohols. This reflects the reduced hydrogen-bond ‘basicity’ of sulphur and the reduced ‘acidity’ of the S—H protons. There is an inversion in the relative oscillator strengths for (SH)_free and (SH)_bulk oscillators on going from the fundamental (v) to the first overtone (2v). For the former, there is a marked increase as the hydrogen-bond strength in RSH-----B units increases, whereas in the 2v(SH) region, the ‘free’ band is much stronger. Both regions have been used to obtain a more complete analysis of the compositions of solutions. In basic aprotic solvents only one i.r. feature was detected, which is assigned to EtSH-----B units. We have constructed a correlation between the i.r. and the proton shift n.m.r. data, which is satisfactorily linear over the range of co-solvents from MeCN to Et₂NH. There is an abrupt discontinuity for the non-basic solvents CCl₄ and C₆H₁₂, for which the i.r. shifts are less than those predicted from the corresponding n.m.r. shifts. This is reflected in plots involving the solvent donor numbers. We estimate that the pure liquid at 22 °C contains ca. 49%(SH)_free units. This result requires that there is an average of one hydrogen bond per thiol molecule, which accords well with the results of a recent Monte Carlo calculation for thiols.