Compression effect on structure and proton transfer in crystalline benzoic acid

Geometry optimizations for an isolated dimer and a crystal of benzoic acid were performed in order to evaluate the equilibrium geometries and the energy difference between the dimers in isolated and crystalline states using model potentials. The optimization in the crystal field results in a shortening of the O⋯O distance in comparison with that in an isolated dimer. The magnitude of the shortening agrees well with the difference between the observed values of the O⋯O distance in the gaseous (2.703 Å) and crystalline (2.64 Å) states. The energy increase due to this shortening is estimated to be about 0.24–0.40 kcal mol⁻¹ and is found to be one of the causes of the discrepancy between the barrier height of 1–2 kcal mol⁻¹ measured by NMR for crystalline carboxylic acids and that of 7.1–9.1 kcal mol⁻¹ calculated by the ab initio method for the isolated dimer.