Theoretical study of the effect of torsional anharmonicity on the thermodynamic properties of methanol

A study is presented of the effect of torsional anharmonicity on the structure and thermodynamic properties of methanol. The data are obtained from ab initio calculations at the MP2/aug-cc-pTVZ level. It is observed that the torsion significantly affects the methyl group. Incorporating the experimental data into a one-dimensional model, the barrier to rotation is estimated to be 377.94 cm−1. Thermodynamic properties are calculated using a harmonic model and an anharmonic model where the methyl torsion is considered as a large-amplitude motion. The effect of torsional anharmonicity is especially important for the entropy and Gibbs energy as a consequence of the large variation in the molecular partition function.