A single-crystal neutron diffraction study of the temperature dependence of hydrogen-atom disorder in benzoic acid dimers

Hydrogen-atom transfer across the hydrogen bonds of benzoic acid dimers has been studied previously by spectroscopic and X-ray methods, but no high-resolution neutron single crystal structures have ever been reported. The system is of interest because the hydrogen-atom dynamics change from classical Arrhenius behaviour to phonon-assisted quantum tunnelling as a function of decreasing temperature. Here we present a series of neutron single-crystal structure refinements at five temperatures ranging from 175 K, where Arrhenius behaviour holds, down to 20 K, where translational tunnelling is dominant. The refinements yield accurate atomic coordinates, thermal parameters and site occupancy factors for all atoms, and show that conversion of the B dimer configuration to the lower energy A dimer configuration in the range 175 to 50 K is primarily enthalpy driven, in good agreement with spectroscopic data. Site occupancies for the hydrogen-bonded hydrogen atom at 20 K do not fit the trend in occupancies observed at the four higher temperatures, and a further series of structure refinements at temperatures below 50 K is planned to investigate this.