Vibrational energy relaxation (VER) of molecules in condensed phase provides crucial information about the intermolecular and intramolecular energy exchange phenomena. For systems like cyanide ion in aqueous solution, the strong forces generated by the interactions between the ionic species and the polar solvent molecules, coupled with the quantum effects arising from the high vibrational energy of the solute (2080 cm-1), result in a very fast vibrational relaxation. An in-depth computational analysis is essential in order to develop a molecular level understanding of the fast relaxation processes observed in systems like these. We have calculated the VER rate constant for three isotopomers of cyanide ion in water and D2O at 300K using Landau Teller formula where the quantum-mechanical bath force autocorrelation function is calculated using a linearized semiclassical (LSC) approximation. The calculations are repeated using classical molecular dynamics simulations, wherein the quantum effect on the relaxation is accounted for by using quantum correction factors (QCFs). Furthermore, using two solvent models we elucidated the plausible relaxation pathways: (1) A pathway involving the translational and rotational modes of water as accepting modes (studied using a rigid water model), (2) A pathway involving the bending modes of water as accepting modes (studied using a flexible water model). Our results confirm the experimentally observed trend of relaxation rate constants for the range of cyanide ion isotopomers. We find that although classical mechanics predicts that pathway 2 is the dominant one, the stronger quantum enhancement of the VER rate in pathway 1 compensates for it, thereby making both pathways viable.

Vibrational energy relaxation of cyanide ion in aqueous solution: Computational study of condensed phase dynamics

246th National Meeting (2013)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.