Intermolecular Dynamics of Positively and Negatively Charged Aromatics and Their Isoelectronic Neutral Analogs in Aqueous Solutions

Abstract

In this study, we investigated the temperature dependence of intermolecular vibrations and orientational dynamics in the aqueous solutions of imidazole hydrochloride, imidazole, sodium triazolide, and triazole using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and steady-state Raman spectroscopy. The difference low-frequency Raman spectra under 250 cm–1 of the aqueous solutions relative to the neat water showed that the spectral shoulder in the high-frequency region at 60–100 cm–1, assigned to the libration of an aromatic ring, was higher in frequency for the imidazolium cation but lower for the triazolide anion than those of the respective neutral aromatics. The results of the ab initio quantum chemistry calculations of the clusters of the aromatics and water molecule(s) were consistent with the experimental spectra of the aqueous solutions. Further, the results of the temperature-dependent experiments showed that the signal intensity in the low-frequency region below 50 cm–1 increased for all solutions with an increase in temperature. In contrast, the spectral density in the high-frequency region above 80 cm–1 exhibited almost no shift for the 1.0 M solutions, while a significant red shift was observed for the 5.0 M solutions. In addition, the temperature-dependent densities, viscosities, and surface tensions were characterized for the aqueous aromatic solutions from 293 to 353 K.

Physical Properties and Low-Frequency Polarizability Anisotropy and Dipole Responses of Phosphonium Bis(fluorosulfonyl)amide Ionic Liquids with Pentyl, Ethoxyethyl, or 2-(Ethylthio)ethyl Group

“…In addition, the peak shifted to the low-frequency side with increasing temperature. This type of temperature-dependent spectral feature was commonly observed in nonaromatic cation-based ILs, including phosphonium-based ILs. ,− Notably, each liquid/solution including IL shows a unique line shape in the difference spectrum between the low-frequency spectra at 293 K and a higher temperature, and the line shape of the difference spectrum is different from that of the Bose–Einstein thermal occupation factor. ,, …”

“…Figure 6 shows that the line shapes of the three ILs were rectangular triangles with a peak at ∼20 cm shows a unique line shape in the difference spectrum between the low-frequency spectra at 293 K and a higher temperature, and the line shape of the difference spectrum is different from that of the Bose−Einstein thermal occupation factor. 99,100,105 The peak frequencies, ν p , of the spectra for the three ILs are plotted as a function of temperature to illustrate the temperature dependence of the low-frequency spectrum semiquantitatively (Figure 8). The details of the analysis procedure used in this study have been previously reported elsewhere.…”