Femtosecond Raman-induced Kerr effect spectroscopic study of the intermolecular dynamics in aqueous solutions of imidazolium hydrochloride, imidazole, sodium triazolide, and triazole: concentration dependence.

In this study, we employed femtosecond Raman-induced Kerr effect spectroscopy to analyze the concentration-dependent intermolecular dynamics in positively or negatively charged aromatics and their neutral analogous aromatics (imidazolium hydrochloride (ImHCl), imidazole (Im), sodium triazolide (NaTr), and triazole (Tr)) in aqueous solutions at 293 K. We also measured their liquid properties, such as density, viscosity, and surface tension, at 293 K, and compared them with their dynamic properties. Furthermore, we performed the quantum chemistry calculations of the target aromatics and some clusters to elucidate their optimized structures, interaction energies, charge populations, and Raman-active normal modes. We characterized the Kerr transients over 2 ps using a triexponential function. The results revealed that the aqueous solutions' intermediate and slow relaxation time constants were linearly proportional to the viscosities. The slopes of the time constants to the viscosity of the aqueous ImHCl solutions were steeper than those of the aqueous Im solutions, whereas the slopes of the aqueous NaTr solutions were milder than those of the aqueous Tr solutions. These findings indicated that the charge of the aromatics in the aqueous solutions affected the coupling parameter between the solute and solvent in the orientational dynamics with different ways. The first moment (M₁) of the low-frequency band (< 200 cm^-1), coming from the intermolecular vibrations, in the difference spectra between the aqueous aromatic solutions and neat water shifted to the high-frequency region as the concentration increased. The M₁ slope to the concentration for the aqueous ImHCl solutions was steeper than that for the aqueous Im solutions. Conversely, the concentration dependence of M₁ for the aqueous NaTr solutions was similar to that for the aqueous Tr solutions. We used the local structures of the target aromatics based on the quantum chemistry calculations to rationally clarify their concentration-dependent intermolecular dynamics in the aqueous solutions.