AMI and CNDO/S Study of 2,5-Dimercapto-1,3,4-thiadiazole, the Mesoionic 1,3,4-Thiadiazolo[2,3-b][1,3]thiazine-2-thione and Its Dimerization Derivative

The molecular geometries of the mesoionic 1,3,4-thiadiazolo[2,3-b][1,3]-thiazine-2-thione (II) and its dimerization product, 1,10-dithia[4.4](3,5)-1,3,4-thiadiazolinophane-6,15-dithione (III) have been optimized by means of the AMI method. Their electronic spectra, as well as those of the parent 2,5-dimercapto-1,3,4-thiadiazole (I), have been calculated at CNDO/S level. The analysis of the calculated electronic transitions and of the experimental UV absorption bands confirms the mixed thiolic–thionic structure of I as the most stable one, in agreement with 4-31G* prediction. The electronic transitions of compound III are practically coincident with those found for I; on the contrary, a band in the visible and a band in the UV region are predicted for compound II, so accounting for the yellow color of II and the lack of color of III.