Polarizability relaxation in protic ionic liquids: a comprehensive approach

A molecular dynamic simulation has been performed to investigate the anisotropic polarizability relaxation for a series of alkylammonium nitrate protic ionic liquids. The main structural features along these systems were reviewed and compared with available data in literature. A new CPE parameter set for a series of alkylammonium ions has been proposed to calculate the atomic partial charges as well as the polarizability tensor in condensed phases. Polarizability relaxation takes place in the ~ 10 ps range due to the expected slow structural/reorientational relaxation in ionic liquids. From the set of polarizability TCFs we observe: i - a gradual depletion of reorientational motions of the ammonium cations with the increase cations size, ii – the total polarizability relaxation is dominated by the slow reorientational motions of the "N" "O" _"3" ^"-" . In frequency domain, there is a noticeable presence of a progression of librational bands in the light MeN denoting that in this system we find a dynamic characterized by fast and strong short-range pulses between H-bonded ionic species. The increase of the cations causes the depletion of the librational dynamics, and the H-bonding stretching mode emerges as a clear band composed by the reorientational motions of the cations and the interaction induced- mechanism in qualitative agreement with the corresponding experimental data. Beyond the traditional assignment of the bands arising from the cation-anion interaction we observe a compelling theoretical suggestion of pronounced librational dynamic in the low-frequency Raman spectra of the monomethylammonium nitrate.