HYDROGEN-BOND DYNAMICS IN BENZOIC-ACID DIMERS AS A FUNCTION OF HYDROSTATIC-PRESSURE MEASURED BY NUCLEAR-MAGNETIC-RESONANCE

The dynamics of hydrogen atoms in the hydrogen bonds of benzoic acid dimers have been studied as a function of hydrostatic pressure to pressures in excess of 4 kbar. This paper is primarily concerned with results up to 3.3 kbar. The temperature dependence of the correlation time for the motion at a series of pressures has been investigated using measurements of the proton spin-lattice relaxation time. Strong non-Arrhenius behavior is exhibited and the data are in good agreement with a model which invokes phonon assisted tunneling at low temperature and thermally activated Arrhenius dynamics at high temperature. The parameters in the model include the asymmetry of the double minimum potential experienced by the hydrogen atoms and dynamical variables relating to the tunneling and hopping processes. The rate of phonon assisted tunneling is observed to increase exponentially with increasing pressure and this is attributed to the increase in the tunneling matrix element which occurs as the distance between the potential wells is decreased and the overlap of the localized eigenstates beneath the barrier increases. We also observe a decrease in the asymmetry of the potential with increasing pressure which is attributable to modifications to the interdimer contributions to the potential. There is evidence in the nuclear magnetic resonance (NMR) data for two phase transitions below 4 kbar and one of these may be correlated with the reduction in asymmetry of the potential.