Slow spectral diffusion of the NO stretching mode of [RuCl₅(NO)]^2- in D₂O studied by 2D-IR spectroscopy and molecular dynamics simulations

The vibrational dynamics of the NO stretching mode of [RuCl₅(NO)]^2- in D₂O were investigated by nonlinear infrared (IR) spectroscopy. We performed IR pump-probe measurements to obtain the vibrational lifetime of this molecule. The lifetime is 31 ps, which is sufficiently long enough to study the vibrational frequency fluctuation on a slower time scale with high precision. By two-dimensional IR spectroscopy, the frequency-frequency time correlation function (FFTCF) of the NO stretching mode was characterized with a delta function plus a double-exponential function. The time constant of the slower component was ∼10 ps. We also found that the time constant does not strongly depend on temperature. In order to investigate the microscopic origin of this component, we performed classical molecular dynamics simulations. It was found that the hydration structure around the NO group was influenced by the negatively charged Cl ligands. To calculate the FFTCF decay, we employed an approximate theoretical model based on the vibrational solvatochromism theory. It was demonstrated that water fluctuations around the Cl ligands projected on the NO group correspond to the 10 ps decay component in the FFTCF. The fluctuation is related to the orientational dynamics of the water molecules attracted by the Cl ligands. By comparing the FFTCF parameters of the present solute with those of previously reported metal complexes and SCN^- in D₂O, we conclude that the presence of different electrostatic environments around the vibrational probe and the other interaction sites of the solute is important for understanding the slow decay component in the FFTCFs.