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-C≡N(-)) 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.

A 100 kHz Pulse Shaping 2D-IR Spectrometer Based on Dual Yb:KGW Amplifiers

“…Figure c,d shows an early and a late-time 2D-IR spectrum of room temperature NaSCN dissolved in H 2 O. The center-line slope derived FFCC is shown in Figure e and is based on 23 2D-IR spectra, each averaged for 10 s, though the earlier times could have easily been acquired in <1 s. The slope exponential decay is 0.8 ps and in excellent agreement with earlier studies . Likewise, at higher temperatures, we see the observed CLS decay time constant decrease, following Arrhenius-like behavior.…”

“…The center-line slope derived FFCC is shown in Figure 3e and is based on 23 2D-IR spectra, each averaged for 10 s, though the earlier times could have easily been acquired in <1 s. The slope exponential decay is 0.8 ps and in excellent agreement with earlier studies. 37 Likewise, at higher temperatures, we see the observed CLS decay time constant decrease, following Arrhenius-like behavior. We did not explore the limit at which the CLS decay becomes obscured by the instrument response; however, at 365 K, CLS exponential decay fits with a decay time of 0.4 ps were reliably returned, still closely following Arrhenius kinetics.…”

Direct observation of dynamic crossover in fragile molecular glass formers with 2D IR vibrational echo spectroscopy

“…This model has had great success in fitting 2D IR data for a variety of systems including organic liquids, water, proteins, liquid crystals, and room-temperature ionic liquids. [55][56][57][58][59][71][72][73][74][75] The multiexponential model was chosen as our starting point, as above T m , these glass forming liquids are simple organic liquids. At these higher temperatures, the CLS(T w ) decay curves for all three samples fit well to biexponential decays.…”

Two-Dimensional Infrared Spectroscopy From the Gas to Liquid Phase: Density Dependent J-Scrambling, Vibrational Relaxation, and the Onset of Liquid Character

“…These include some vibrationally resonant ultrafast pump-probe experiments. [22][23][24][25][26] Very dramatic effects near the critical point are generally not observed in these lifetime and transient absorption data, and most pump-probe responses follow the density dependence of trends both below and above the critical density (rc). However, a faster lifetime (T1) was observed for an IR active mode of W(CO)6 in supercritical CO2, C2H6, and CHF3 at densities just below rc than expected by simple phenomenological trends, and T1 remained essentially constant for a range of densities around the critical point, ~0.6rc to ~1.4rc, when the temperature was near Tc.…”