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.