Kinetic solvation and electrical conductance of proton in infinitely diluted solutions of hydrogen halides in primary alcohols and in water: influence of temperature and solvent

Abstract

Until now, there is no comprehensive quantitative description of the forms of existence of the solvated proton in protic solvents. The research of the mechanism of its conductance in a such solvents also is far from completion. In the present paper we attempt to estimate two contributions into limiting molar electrical conductance (LMEC) of proton λi0(H+)=λi0(L+)+λi0(Z+) in n-alcohols and water at 278.15–328.15K. Our estimations are based on our own conductometric data on hydrogen halides solutions and literature data on some 1-1 electrolytes in water. The first contribution corresponds to the lyonium ion L+ (or MH+, where M is solvent molecule), and the second one – to the so-called Zundel ion Z+ (or M2H+). We also have divided each of these contributions into two parts in accordance with known proton transfer mechanisms – hydrodynamic and prototropic (relay), so that the total conductance of proton can be represented as the sum of four terms: λi0(H+)=λi0(L+)hydr+λi0(L+)rel+λi0(Z+)hydr+λi0(Z+)rel, and we have evaluated each of them. Then the friction coefficients of ions (ζat(L+) and ζat(Z+)) have been calculated from values of the hydrodynamic terms. This allows to distinguish the negative solvation from positive one by the ζat sign. In result, Z+ has found to be solvated negatively in all the solvents mentioned, but L+ has such solvation only in water, in contrast to its weak positive solvation in n-alcohols. The alternating sign of ζat suggests this coefficient as a sum of two parts: a negative part ζatMM, and positive one ζatIM, which also have been calculated by us in accordance with Samoilov's kinetic solvation theory. Moreover, analyzing ζat(L+) dependence on the solvent and temperature, we became able to confirm the alternation effect in the homologous series of n-alcohols and explain it. Generally, this phenomenon can be detected experimentally only by highly sensitive methods of spectroscopy. In conclusion, the details of the mechanism of proton transfer in protic solvents have been discussed considering ab initio optimized geometry (B3LYP/6-31+G(d,p), Gaussian’03) of proton-contained clusters.