The dynamics of hydrogens in double well potentials: The transition of the jump rate from the low temperature quantum-mechanical to the high temperature activated regime

Abstract

The temperature dependence of the deuteron spin lattice relaxation timeT 1 in four carboxylic acids, which were deuterated in the carboxyl groups, is presented. These data allow determination of the rate Γ of the hydrogen transfer between the two minima of the double well potential in which the hydrogens move along the hydrogen bonds. The temperature dependence of Γ has clearly distinct low temperature quantum‐mechanical and high temperature thermally activated regimes. Evidence is presented that the transfer of hydrogens in dimers of carboxylic acids is strongly influenced by a high‐frequency mode which is strongly coupled to the hydrogens. This supports the conclusions recently put forward by the ETH group [J. Chem. Phys. 9 3, 1502 (1990)]. A model based on the spin–Boson–Hamiltonian with a bath consisting of acoustic phonons plus a unique high frequency mode explains fully the jump rate in the quantum mechanical regime of all four compounds studied. The way in which the unique bath mode influences the hydrogen transfer is clarified.