Ab Initio Calculations of Thermochemical Properties of Methanol Clusters

Highly accurate ab initio calculations of binding enthalpies
and
entropies of gas phase clusters of methanol have been performed, yielding
uncertainties smaller than 1 kJ/mol per hydrogen bond in the Gibbs
free energy of reaction. This requires quantum chemical RIMP2 and
CCSD­(T) post-Hartree–Fock methods with basis sets up to aug-cc-pV5Z
for energy calculations. An analysis of topological symmetry and hindered
rotor effects proves necessary for reliable entropies. This approach
goes beyond the rigid rotor plus harmonic oscillator method implemented
in standard quantum mechanics software tools. The results demonstrate
that (1) thermochemical methanol cluster properties can nowadays be
obtained by ab initio methods with an accuracy comparable to or even
better than that of the experimental data available, especially for
larger species that cannot be studied directly by experiments and
(2) cooperativity effects and state-dependent cluster distributions
cause a strongly varying average enthalpy and entropy per bond as
a function of temperature and density for methanol.