Concentration dependent wavenumber shifts and linewidth changes of some prominent vibrational modes of C4H8O investigated in a binary system (C4H8O+H2O) by polarized Raman study and ab initio calculations

Raman spectra of neat tetrahydrofuran (hereinafter, frequently referred to as THF; C4H8O) and its binary mixtures in aqueous (H2O) media were investigated using a rather sensitive scanning multichannel detection scheme. The spectra were recorded in the region, 800-1000 cm−1 for the neat liquid, C4H8O, and mixtures with varying mole fractions of C4H8O from 0.1 to 0.9. The spectra were, however, analyzed in two different regions, 800-1000 and 975-1100 cm−1. The spectra in the former region show an asymmetric peak also in neat liquid and were deconvoluted to two Raman line profiles having peaks at ∼910 and ∼914 cm−1, corresponding to ν(C-O) and ν(C-C) stretchings, respectively. At other concentrations, the spectra were analyzed for three Raman line profiles and the concentration dependence of wavenumber position and the linewidth (FWHM) for an additional peak at ∼896 cm−1 (at mole fraction of C4H8O, C=0.9) exhibited a peculiar trend, which has been explained by using the indirect dephasing model. The spectra in the latter region show two Raman peaks, an intense one at ∼1030 cm−1 and a relatively less intense at ∼1071 cm−1 in neat liquid. A striking feature is that their separation goes on decreasing with dilution. The optimized geometries and vibrational wavenumbers for various normal modes for neat THF as well as hydrogen bonded complexes were calculated using ab initio theory at the MP2 level and the results have been used to understand the changes in the spectral features with the varying concentration of the two components in the binary mixture (C4H8O+H2O).