Ab initio molecular dynamics investigation of the concentration dependence of charged defect transport in basic solutions via calculation of the infrared spectrum

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 simulation, 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. (Tuckerman, M.E.; Marx, D.; Parrinello, M. 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.