Vibrational study of thiophene and its solvation in two polar solvents, DMSO and methanol by Raman spectroscopy combined with ab initio and DFT calculations

Abstract

Raman spectra of neat thiophene (THP) and its highly diluted (<20% THP) solution with two polar solvents, dimethyl sulfoxide (DMSO) and methanol were recorded in order to explore the influence of molecular interaction on spectral features corresponding to some selected vibrational bands of THP in the range, 400–1600 cm −1. Only few vibrational bands of THP show a slight change in their peak position in going from neat THP to (THP + DMSO/methanol) solution. Ab initio and density functional calculations were performed employing HF, MP2 and B3LYP methods and high level basis set, 6-311++G(d,p) to obtain the ground state geometry of neat THP and its hydrogen bonded complexes with DMSO and methanol in gas phase. In order to account for the solvent effect on vibrational features and changes in optimized structural parameters of THP, polarizable continuum model (PCM) calculations at HF, MP2 and B3LYP level were performed to simulate the vibrational spectra of THP in both the solvents, DMSO and methanol. The calculated wavenumbers of neat thiophene were compared with the reported experimental data on wavenumbers of 21 normal modes of thiophene and for a detailed vibrational assignment of the various normal modes, potential energy distribution (PED) calculation was also performed. No significant variation was found in the Raman spectra as a function of medium polarity. However, the ground state energy of THP decreases in going from gas phase to solution in both the cases. The dipole-dipole interaction between THP and the solvent molecules, DMSO and methanol was also calculated at different levels of theory.