JPC Abstract
Abstract
The concentration dependence of the anomalous proton transport mechanism
in aqueous KOD solution is studied using ab initio molecular dynamics.
A high concentration of 13 M is chosen because of the availability of
Raman and infrared spectroscopic data at this concentration.
Differences in certain features
of these spectra have been interpreted in terms of the so called ``proton hole''
picture of the proton transport mechanism in basic solutions. The
proton hole mechanism asserts that the charged defect transport in
basic solutions follows the same mechanism as in acidic solutions
(where the charged defect is
H3O+) with all of the hydrogen-bond
polarities reversed. By computing the infrared spectrum directly from
an ab initio molecular dynamics simulations, we are able to
validate our ab initio approach against the experimental data.
However, the mechanism of charged defect transport
that emerges from the simulation is considerably different from
the proton hole mechanism and follows that recently reported by
Tuckerman, et al. [M. E. Tuckerman,
D. Marx and M. Parrinello,
Nature 2002, 417, 925]. For comparison, a lower concentration, 1.5 M,
is also simulated and the transport mechanism compared to the
high concentration case. It is found that the mechanisms are similar,
however, the mobility of both K+ and
OD- is slower at high
concentration, a finding that is in keeping with the fact that
the molar conductivity of electrolytes decreases with increasing
concentration. Other similarities and differences between the
two concentrations are highlighted, and a new interpretation of
the spectral data is proposed.