Proton transfer in the KHCO3 and benzoic acid crystals: A quantum view

A quantum rationale is formulated for proton dynamics in crystals composed of hydrogen bonded centrosymmetric dimers. The purpose is to account for experimental data from various techniques: diffraction, solid-state NMR, quasi-elastic neutron scattering, and vibrational spectroscopy. Spatially coherent distributions, adiabatic separation, nonlocal dynamics, and quantum interferences, are opposed to statistical disorder, semiclassical dynamics, local double-well potentials and stochastic jumps. The tunnelling-level schemes, determined from spectroscopy data, evidence different interconversion mechanisms: adiabatic two-stepwise for KHCO3; coherent phonon-assisted tunnelling for benzoic acid. This latter can be further decomposed into single-step pairwise, single-step single-particle, and single-step two-particles processes. Calculated proton distributions and fluctuation rates at thermal equilibrium are in rather good agreement with measurements.