Proton-transfer dynamics in the hydrogen bond. Inelastic neutron scattering spectra of sodium, rubidium and caesium hydrogen carbonates at low temperature

Abstract The inelastic neutron scattering spectra of NaHCO 3 , RbHCO 3 and CsHCO 3 crystal powders at 20 K are compared to the spectrum of KHCO 3 . The relative intensities in the internal and lattice mode regions reveal that the proton dynamics are not coupled to the heavy atoms of the CO 3 entities. The proton-transfer dynamics are very similar for those salts forming centrosymmetric dimers (namely KHCO 3 , RbHCO 3 and CsHCO 3 ). They show “tunnelling” transitions consistent with quasisymmetric double-minimum potentials. In contrast to this, NaHCO 3 , which forms infinite chains, shows no evidence of proton transfer. The importance of short-range versus long-range interactions determining the potential shape is discussed. It appears that proton—proton dynamical correlation is a key factor in the transfer mechanism.