Small energy differences in molecular crystals: A first principles study of tautomerism and dynamics in benzoic acid derivatives

Abstract

The carboxylic acid dimer unit (COOH)₂ is a common hydrogen bonding motif that frequently exhibits tautomerism-driven proton disorder in the solid state. This study considers such disorder in the representative compounds benzoic acid, 4-chlorobenzoic acid and terephthalic acid, in which the energy scales for tautomerism have been determined experimentally to lie in the range 0.5–2 kJ mol⁻¹. Periodic plane-wave PBE functional calculations are shown to yield accurate relative energies for such tautomers, whereas LDA calculations underestimate the energy differences. The implications of these findings are discussed within the wider context of polymorphism. The optimised structures obtained from gas phase and solid-state calculations are compared, and the contributions of various contacts (including weak C–H⋯O bonding) to cohesive energies are determined within a supercell approach. The anharmonic vibrational spectra of benzoic and terephthalic acid are obtained from finite temperature molecular dynamics, and compared with previous experimental and theoretical results.