Ab initio molecular dynamics study of proton mobility in liquid methanol

Abstract

The transport of protons through aqueous, partially aqueous, or nonaqueous hydrogen-bonded media is a fundamental process in many biologically and technologically important systems. Liquid methanol is an example of a hydrogen-bonded system that, like water, supports anomalously fast proton transport. Using the methodology of ab initio molecular dynamics, in which internuclear forces are computed directly from electronic structure calculations as the simulation proceeds, we have investigated the microscopic mechanism of the proton transport process in liquid methanol at 300 K. It is found that the defect structure associated with an excess proton in liquid methanol is a hydrogen-bonded cationic chain whose length generally exceeds the average chain length in pure liquid methanol. Hydrogen bonds in the first and second solvation shells of the excess proton are considerably shorter and stronger than ordinary methanol–methanol hydrogen bonds. Along this chain, proton transfer reactions occur in an essentially random manner described by Poisson statistics. Structural diffusion of the defect structure is possible if the proton migrates toward an end of the defect chain, which causes a weakening of the hydrogen bonds at the opposite end. The latter can, therefore, be easily ruptured by ordinary thermal fluctuations. At the end of the chain where the proton resides, new hydrogen bonds are likely to form due to the strong associative nature of the excess proton. It is through this “snake-like” mechanism that the defect structure is able to diffuse through the hydrogen-bond network of the liquid. The estimated activation enthalpy of this proposed mechanism is found to be in reasonable agreement with the experimentally determined activation enthalpy.

Long-range specific ion-ion interactions in hydrogen-bonded liquid films

“…The failure of AN to propagate LR-SIE confirms our conjecture, but the experiments in IPA appear to contradict it. However, we consider that the latter result represents a false negative because IPA, in contrast with H 2 O and MeOH, [49][50][51][52][53][54][55][56] lacks an extensive H-bond network. Alkyl groups restrict the number of configurations amenable to hydrogen bonding, and this effect increases with the number and size of the alkyl groups surrounding O-H moieties.…”

Calculation of the Solvation Free Energy of the Proton in Methanol

“…+ distance is calculated to be 1.20 Å, which is similar to the previous result from MP2/6-311+g(2d,2p) calculation yielding a symmetric hydrogen bond of length 2.38 Å with the proton located 1.19 Å from each oxygen. 43 In H + (CH 3 OH) 3 , the excess proton was located on the central methanol. In contrast to the neutral methanol cluster, the linear chain was the most stable conformation.…”

Liquid methanol Monte Carlo simulations with a refined potential which includes polarizability, nonadditivity, and intramolecular relaxation

“…16,17 It has therefore been thought that something similar could be occurring in the liquid phase and be responsible for its peculiar behavior. Several numerical simulations [18][19][20][21][22][23] and neutron diffraction experiments [24][25][26] have produced data that support this view. On the other hand, experimental results of neutron diffraction, 27,28 x-ray scattering, 29,30 and x-ray emission spectroscopy 31 have been taken to support the existence of cyclic clusters of methanol in the liquid phase.…”

“…Kashtanov et al 31 have suggested that numerical simulations could be using potentials that are not able to reproduce the hydrogen bonding network in ring structures. There is a substantial number of numerical simulations of methanol [18][19][20][21][22][23][32][33][34] and some of them which include polarizability [35][36][37][38] have shown the need for refined potentials.…”