Ab initio molecular dynamics study of polyfluoride anions

The structure and dynamical behavior of liquid HF with dissolved KF have been investigated using the Car–Parrinello ab initio molecular dynamics scheme. Specifically, a system with stoichiometry KF⋅2HF was studied at temperatures of 400 and 1000 K. This system, which was started from a phase separated mixture, rapidly formed into solvated potassium ions and HnFn+1− polyfluoride anions with n=1, 2, 3, and 4. The resulting polyfluoride anions were classified, and their structures and dynamical behavior were compared with the known structures and spectra of crystalline compounds KF⋅xHF and with theoretical predictions of isolated gas phase species. In the present study we reveal dramatic frequency shifts in the H atom vibrational modes with variation in the HF coordination number of the polyfluoride anion. In particular, the FH wagging motion red shifts while the FH stretch blue shifts as n increases. The present calculations suggest that FH stretching motion infrared modes previously assigned as combination bands should likely be reinterpreted as fundamental modes of the polyfluoride anions.