Raman Spectroscopic Study on the Solvation of p-Aminobenzonitrile in Supercritical Water and Methanol
K Osawa and T Hamamoto and T Fujisawa and M Terazima and H Sato and Y Kimura, JOURNAL OF PHYSICAL CHEMISTRY A, 113, 3143-3154 (2009).
Raman spectra of the C N stretching vibration of p-aminobenzonitrile (ABN) have been investigated in water, methanol, and cyclohexane under sub- and supercritical conditions, and in acetonitrile under subcritical condition. In all solvent fluids covering the supercritical region, the vibrational frequency of the C N stretching mode decreased with increasing solvent density from the gaseous region to the medium density region rho(r) congruent to 2, where p, is the reduced density by the critical density of the solvent. However, from the medium density region to the higher density region, the vibrational frequency turned to increase with the solvent density. The temperature-induced low frequency shift of the C N stretching Raman band was also ascertained by the measurement of the temperature dependence of Raman spectrum of ABN vapor above 543 K. The electronic absorption spectra in the UV region of ABN were also measured under the same experimental conditions. The absorption peak energies decreased with an increase of the solvent density, except in water above rho(r) = 2.8. The vibrational frequency shift in cyclohexane was explained by a sum of contributions of the repulsive interaction, the mean field attractive interaction, and the pure temperature effect probably due to the hot-band contribution. The residual frequency shift after the subtraction of the repulsive and temperature effects in water and methanol showed the low frequency shift with increasing solvent density from rho(r) congruent to 0 to 2.8. However, above p, = 2.8 in water, the residual shift showed a high frequency shift with increasing solvent density. The electronic state calculations based on the PCM model using the density functional theory (DFT) indicated that the solvent polarity change caused the low frequency shift of the C N stretching mode, which was also correlated with the shift of the electronic absorption spectrum. The results of the DFT calculations on the cluster of ABN with water molecules and the molecular dynamics simulations indicated that the high frequency shift of the C N stretching mode in water above rho(r) congruent to 2.8 could be due to the hydrogen bonding between water and ABN.
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