Theoretical study of the infrared spectrum of 5-phenyl-1,3,4-oxadiazole-2-thiol by using DFT calculations

We have carried out a structural and vibrational study for 5-phenyl-1,3,4-oxadiazole-2-thiol by using the infrared (IR) spectrum and theoretical calculations. For a complete assignment of the compound IR spectrum, density functional theory calculations were combined with Pulay's scaled quantum mechanical force field methodology in order to fit the theoretical wavenumber values to the experimental ones. An agreement between theoretical and available experimental results was found. The theoretical vibrational calculations allowed us to obtain a set of scaled force constants fitting the observed wavenumbers. The results were then used to predict the Raman spectra, for which there are no experimental data. The nature of the benzyl and oxadiazole rings was studied by means of natural bond order and atoms in molecules theory calculations. In addition, the frontier molecular (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) orbitals were analysed and compared with those calculated for the oxadiazole molecule.