Dynamics of solitary water in benzene and hexafluorobenzene: An infrared and Raman study

Solutions of water very diluted (x(w) similar to 10(-3) mf) in benzene and hexafluorobenzene have been investigated using vibrational spectroscopy. In these two solvents, it was found that water is essentially in its monomeric form. The band-shape analysis of the infrared and Raman profiles associated with the symmetric upsilon(1) and antisymmetric upsilon(3) stretching vibrations of the water molecule in benzenic compounds has been performed and compared with the spectral results obtained for water diluted in liquid CCl4, considered here as the standard "inert" solvent. It is found that the reorientational motions of water around its different axes are always more hindered in benzene and hexafluorobenzene than in liquid CCl4. Moreover, the reorientational motion of the main symmetry axis (z) of water is more hindered than that of the y-axis (in the molecular plane). Although the rotational behavior of water appears similar in both aromatic solvents, the shifts in the vibrational frequencies and the change in the vibrational intensities suggest that the solute-solvent interactions in these two solutions are in fact quite different. In benzene, a weak H-bonding type of interaction with water is likely to exist in the liquid state in agreement with previous works. In contrast, in liquid perfluorinated benzene, a Lewis acid-base interaction is more likely to be involved. This conclusion is consistent with our previous ab initio calculations [Y. Danten , J. Phys. Chem. 103, 3530 (1999)]. (C) 2000 American Institute of Physics. [S0021-9606(00)50933-7].