DFT and TD-DFT study of isomeric 5-(pyridyl)-1,3,4-oxadiazol-2-thiones and 2-methylthio-5-(pyridyl)-1,3,4-oxadiazoles.

Internal rotations of pyridine fragment around single bond of isomeric 5-(2', 3' and 4'-pyridyl)-1,3,4-oxadiazol-2-thiones and 2-methylthio-5-(2', 3' and 4'-pyridyl)-1,3,4-oxadiazoles have been performed by DFT (B3LYP) and MP2 methods with RHF/6-31G(d,p) basis set. It was found that the MP2 barrier height lies a few below than DFT barrier heights for all studied compounds. In the molecule of 2-methylthio-5-(2'-pyridyl)-1,3,4-oxadiazole an anomeric effect detected due to a lone pair-lone pair interaction of pyridine and N4 oxadiazole nitrogen atoms. But it not found in the molecule of 5-(2'-pyridyl)-1,3,4-oxadiazol-2-thione. Furthermore, theoretical UV spectra of the compounds were studied using TD-DFT/6-31G(d,p) method. Theoretical transitions located about 10nm in the long-wavelength region relatively to the experimental bands field. Whereas, the theoretical transitions of 2-methylthio-5-(2', 3' and 4'-pyridyl)-1,3,4-oxadiazoles have very good agreement with the position of experimental bands. The longest wavelength band in both of experimental and theoretical spectra of the investigated compounds have been observed for 5-(4'-pyridyl)-1,3,4-oxadiazole-2-thione and 2-methylthio-5-(2'-pyridyl)-1,3,4-oxadiazole. Furthermore, the longest-wavelength experimental band's position are in good agreement with (r=0.95) the HOMO-LUMO energetic gaps of studied compounds.