Precise analysis of small wavenumber shift of pyridine on dilution with H2O and D2O using RDS technique

Abstract

The wavenumber difference (Δv) of the ring breathing mode (v 1), ring deformation mode (v12) and C-H stretching mode (v3) of pyridine (Py) dissolved in both H2Oand D 2O at equal mole fractions of the solute (Py) and the solvent (H 2O/D2O) has been determined precisely by using the technique of Raman difference spectroscopy (RDS) in order to analyze the relative shift caused by these two solvents. The spectra of the two systems, for which the difference spectrum is to be generated, were recorded simultaneously and the wavenumber shifts up to almost one hundredth part of the linewidth of a band could be determined precisely. The values of Δv for the three modes as a function of mole fraction are compared with the Δv obtained by the solvation of Py molecule in the media of dielectric constants equal to that of the mixture of Py and H2O/D2O at the experimental mole fractions using DFT and MP2 methods. The variation in Δv with mole fraction seems to be the result of difference of dielectric constants of Py + H2Oand Py + D2Oat equal mole fraction. The absolute peak position and linewidth of the three modes as a function of mole fraction have been discussed in terms of relevant models, which show the effective role of diffusion and concentration fluctuation in the mixture. © by Oldenbourg Wissenschaftsverlag, München.