VIBRATIONAL ANALYSIS OF THE SPECTRA OF 1,3,4-OXADIAZOLE AND 1,3,4-THIADIAZOLE

Ab initio harmonic force fields were calculated for 1,3,4-oxadiazole and 1,3,4-thiadiazole at the Moller-Plesset (MP2) level of theory using the 6-31G** basis set at the corresponding optimized geometries assuming a C-2 nu symmetry. Scaled quantum mechanical (SQM) force fields for both molecules were calculated using the experimental vibrational frequencies available from the literature. Under the C-2 nu symmetry constraint, the assignment of the experimental to the calculated frequencies for both molecules and their -2,5-d(2) and -2-d(1) (C-s symmetry) isotopomers was performed. Vibrational absorption intensities were calculated using the MP2 dipole derivative tensors. The calculations showed possible misassignment for 1,3,4-oxadiazole and its -2,5-d(2) and -2-d(1) isotopomers and indicated misassignments for 1,3,4-thiadizole and its -2,5-d and -2-d(1) isotopomers. Good agreement between the experimental and calculated frequencies and absorption intensities was found.