J. Chem. Phys. Abstract
Abstract
The transport of protons through aqueous, partially aqueous, or non-aqueous
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 wth 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.