VIBRATIONAL ANALYSIS OF THE SPECTRA OF 1,2,5-OXADIAZOLE, 1,2,5-THIADIAZOLE AND 1,2,5-SELENADIAZOLE

Equilibrium geometries and harmonic force fields have been calculated for 1,2,5-oxadiazole, 1,2,5-thiadiazole and 1,2,5-selenadiazole at the SCF and MP2 levels of theory using the 6-31G** basis set (Binning-Curtiss for the selenium atom). Scaled quantum-mechanical (SOM) force fields have been calculated for the three molecules and some of their isotopomers using the experimental frequencies available from the literature. The IR absorption intensities were calculated using the computed dipole derivative tensors. The assignment of the experimental to the calculated frequencies was done first according to symmetry then mainly according to frequency order. The calculated vibrational spectra for the studied molecules by the scaled MP2 force fields are only slightly better than those calculated by the scaled SCF force fields. A good correlation was found between the scale factors obtained for both molecules and those obtained for 1,3,4-oxadiazole and 1,3,4-thiadiazole.