Aj. Horsewill et al., HYDROGEN-BOND DYNAMICS IN BENZOIC-ACID DIMERS AS A FUNCTION OF HYDROSTATIC-PRESSURE MEASURED BY NUCLEAR-MAGNETIC-RESONANCE, The Journal of chemical physics, 100(3), 1994, pp. 1889-1894
The dynamics of hydrogen atoms in the hydrogen bonds of benzoic acid d
imers have been studied as a function of hydrostatic pressure to press
ures in excess of 4 kbar. This paper is primarily concerned with resul
ts up to 3.3 kbar. The temperature dependence of the correlation time
for the motion at a series of pressures has been investigated using me
asurements of the proton spin-lattice relaxation time. Strong non-Arrh
enius behavior is exhibited and the data are in good agreement with a
model which invokes phonon assisted tunneling at low temperature and t
hermally activated Arrhenius dynamics at high temperature. The paramet
ers 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 tunn
eling is observed to increase exponentially with increasing pressure a
nd 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 incre
ases. We also observe a decrease in the asymmetry of the potential wit
h increasing pressure which is attributable to modifications to the in
terdimer contributions to the potential. There is evidence in the nucl
ear magnetic resonance (NMR) data for two phase transitions below 4 kb
ar and one of these may be correlated with the reduction in asymmetry
of the potential.