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

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, amf and bxv, were considered, while the molar volumes of both the liquid and the vapor were obtained from experiment. Model 1, in which amf and bxv 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.