Weinhold's QCE model - A modified parameter fit. Model study of liquid methanol based on MP2 cluster geometries

摘要

The quantum cluster equilibrium (QCE) theory was applied for liquid methanol using MP2(fc)/6-31+G(d,p) cluster geometries and MP2/6-311++G(d,p) energies. Three approaches for the determination of the empirical parameters inherent in the QCE model, a_(mf) and b_(xv), were considered, while the molar volumes of both the liquid and the vapor were obtained from experiment. Model 1, in which a_(mf) and b_(xv) were optimized for the vapor and liquid phase, respectively, gave the best results for thermochemical parameters (constant pressure heat capacity C and entropy S) of methanol, especially for the liquid state. In line with Pauling's suggestions concerning the structure of liquid methanol, cyclic hexamers cyclo-(MeOH)_6 are the dominant species, even near the boiling point (～50%) increasing to ～90% at the freezing temperature.