Near infrared spectroscopic study of the structures of water in proton acceptor solvents.

Abstract

The structures of water diluted in solvents, which are varied from weak to strong proton acceptors, are discussed from the infrared spectra between 1200 and 7500 cm−1. Three water species are defined without reference to eventual local structures of solvent molecules: in species S0, S1, and S2, the subscript indicates the number of hydrogen bonded OH groups of water. The assignment of the species in every solvent is based on the study of ternary mixtures. It turns out that the fraction of non-bonded water is negligible in all solvents, and that the lifetime of this structure is much shorter than the hydrogen bond residence time at room temperature. The wavenumbers of the non-bonded OH group in species S1 are mainly determined by the local environment and not by the dielectric constant. The ratio K2 = [S2]/[S1] strongly depends on the bulkiness of the base. A relation is found between lnK2 and the number of CH2 or CH3 groups per solvent molecule: it is discussed in terms of entropy and enthalpy of complexation. From the species assignment, the spectral features of species S2 according to the hydrogen bond strength are emphasized. They can be used, with the previously published pattern for species S1, as a guide to determine the state of water in a sample. The contribution of thermal fluctuations to the absorption profiles is considered. It is pointed out that the resolution of absorption envelopes into components of arbitrary shapes, such as Gaussian, should not be related to species without physical argument.