Temperature and solvent dependence of vibrational relaxation of tri-iodide: A simulation study

摘要

Simulations of a flexible model of the tri-iodide ion have been used to study the effects of temperature and solvent on the vibrational energy relaxation rate and the pure dephasing rate of the two stretching modes. These rates were determined in ethanol and 2-methyltetrahydrofuran at 100 K, 200 K, and 300 K, covering the range between glass and mobile liquid states for both solvents. In agreement with experiment the temperature dependence is small. In ethanol the symmetry of the tri-iodide ion is broken at the three temperatures studied, while in methyltetrahydrofuran symmetry is retained except at the lowest temperature. In order to examine different contributions to the relaxation processes, perturbation theory was applied to simulations with rigid ions. Although the large amplitude of the solvent-induced geometry changes suggests that perturbation theory is at best qualitative, reasonable agreement was obtained between the results from the flexible simulation and perturbation theory in 2-methyltetrahydrofuran. Both solvent-induced curvature and solvent force terms were found to contribute to the solvent frequency shifts and the pure dephasing rates. In ethanolic solutions the time correlation functions of the forces decay rapidly, but the curvature term probes a slow dynamical process. At least in our model, the lines would not be completely motionally narrowed in solution in ethanol. Comparisons are made with experiment and previous work.