Vibrational relaxation of azide ions in liquid-to-supercritical water

Exploring the Structure and Complexation Dynamics of Azide Anion Recognition by Calix[4]pyrroles in Solution

The structure and anion recognition dynamics between calix[4]pyrroles and azide (N₃^-) anions in the form of its TBA⁺ and Na⁺ salts were investigated in dimethyl sulfoxide solutions by Fourier transform infrared (FTIR) spectroscopy and ultrafast IR spectroscopy. Vibrational energy redistribution of the N₃^- anion in the complex is accelerated through hydrogen bonding interactions with the N-H proton of the receptor. Rotational dynamics of the bound N₃^- is greatly restricted, demonstrating a distinct countercation effect. The detailed binding modes of N₃^- with the receptor were further evaluated by the density functional theoretical (DFT) calculations and nuclear magnetic resonance (NMR) spectroscopy. All of these measurements support the notion that the calix[4]pyrroles are capable of capturing the azide anion in solution. However, the calix[4]pyrroles may not necessarily undergo a conformational change to a cone-like geometry when they bind to the azide anion in the solution.

Effect of Hydrogen Bonds on the Vibrational Relaxation and Orientational Relaxation Dynamics of HN3 and N3(-) in Solutions

Hydrogen bonds (H-bonds) play an important role in determining the structures and dynamics of molecular systems. In this work, we investigated the effect of H-bonds on the vibrational population relaxation and orientational relaxation dynamics of HN3 and N3(-) in methanol (CH3OH) and N,N-dimethyl sulfoxide (DMSO) using polarization-controlled infrared pump-probe spectroscopy and quantum chemical calculations. Our detailed analysis of experimental and computational results reveals that both vibrational population relaxation and orientational relaxation dynamics of HN3 and N3(-) in CH3OH and DMSO are substantially dependent on the strength of the H-bonds between the probing solute and its surrounding solvent. Especially in the case of N3(-) in CH3OH, the vibrational population relaxation of N3(-) is found to occur by a direct intermolecular vibrational energy transfer to CH3OH due to large vibrational coupling strength. The orientational relaxation dynamics of HN3 and N3(-), which are well fit by a biexponential function, are analyzed by the wobbling-in-a-cone model and extended Debye-Stokes-Einstein equation. Depending on the intermolecular interactions, the slow overall orientational relaxation occurs under slip, stick, and superstick boundary conditions. For HN3 and N3(-) in CH3OH and DMSO, the vibrational population relaxation becomes faster but the orientational relaxation becomes slower as the H-bond strength is increased. Our current results imply that H-bonds have significant effects on the vibrational population relaxation and orientational relaxation dynamics of a small solute whose size is comparable to the size of the solvent.

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

Femtosecond two-dimensional infrared (2DIR) spectroscopy was carried out to study the dynamics of vibrational spectral diffusion of the nitrile stretching vibration of thiocyanate.

Vibrational Energy Relaxation of Thiocyanate Ions in Liquid-to-Supercritical Light and Heavy Water. A Fermi's Golden Rule Analysis

The vibrational relaxation dynamics following an ultrafast nitrile stretching (ν3) excitation of thiocyanate anions dissolved in light and heavy water have been studied over a wide temperature and density range corresponding to the aqueous liquid up to the supercritical phase. In both solvents, the relaxation of the ν3 = 1 state of the anion leads to a direct recovery of the vibrational ground state and involves the resonant transfer of the excess vibrational energy onto the solvent. In light water, the energy-accepting states are provided by the bending-librational combination band (νb + νL), while in heavy water, the relaxation is thermally assisted by virtual acceptor states derived from the stretching-librational/restricted translational hot band (νS - νL,T). The relaxation rate is found to strictly obey Fermi's Golden Rule when the density of resonant solvent states is estimated from the linear infrared spectra of the solute and the pure solvents.