Charge Effects of Aromatic Side Group on Intermolecular Dynamics in Aqueous Solutions of Anilinium Chloride, Sodium...

In this study, we have investigated the concentration-dependent intermolecular dynamics of aqueous solutions of aniline hydrochloride, sodium phenoxide, and 4-methylpyridine using femtosecond Raman-induced Kerr effect spectroscopy at 298 K. The densities, viscosities, and surface tensions of the aqueous solutions were also measured at 298 K. Quantum chemistry calculations of the target aromatics and their clusters with water molecule(s) or a counterion were performed to obtain their optimized structures and cluster interaction energies. In the difference low-frequency Kerr spectra (<250 cm^-1) of the aqueous aromatic solutions and neat water, the first moment (M₁) of the intermolecular vibrational band, which mainly originated from the aromatic ring, showed that the librations of the anilinium cation and phenoxide anion were higher in frequency than that of 4-methylpydine. Furthermore, the libration of the phenoxide anion was also higher in frequency than that of the anilinium cation. Quantum chemistry calculations indicated that the strong hydrogen bonding and compact hydration structure resulting from the negatively charged aromatic ring led to higher-frequency libration of the phenoxide anion than the anilinium cation. In addition, the M₁ increased with increasing concentration. The concentration sensitivities were stronger in aqueous solutions of aniline hydrochloride and sodium phenoxide than in aqueous solutions of 4-methylpyridine. Based on the quantum chemistry calculation results, we conclude that strong aromatic-water and aromatic-counterion interactions lead to a higher-frequency libration of aromatics with charged side groups. The collective orientational relaxation times of the aqueous aromatic solutions showed the fractional Stokes-Einstein-Debye behavior.