Femtosecond 2DIR spectroscopy of the nitrile stretching vibration of thiocyanate anions in liquid-to-supercritical heavy water. Spectral diffusion and libration-induced hydrogen-bond dynamics

Abstract

Femtosecond two-dimensional infrared (2DIR) spectroscopy was carried out to study the dynamics of vibrational spectral diffusion of the nitrile stretching vibration of thiocyanate anions (S–CN⁻) dissolved in liquid-to-supercritical heavy water (D₂O). The 2DIR line shapes were used to extract through a nodal slope analysis quantitative information about the correlation function for temporal fluctuations of the CN-stretching frequency. The inverse nodal slope could be fitted phenomenologically by a simple double-exponential decay whose predominant component had a time constant ranging between 300 fs and 1 ps depending on the temperature. The temperature dependence is interpreted in terms of solvent structural fluctuations that are driven by the librational motions of the D₂O molecules located in the first solvation shell of the anion. Complementary molecular dynamics simulations of the SCN⁻/D₂O system indicate that the breaking and making of hydrogen-bonds between the terminal N-atom of the anion and the D₂O molecules are induced by the same solvent-shell librational degrees of freedom that drive the vibrational line broadening dynamics seen in the 2DIR experiment.