AM1 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-Ď] [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-l,3,4thiadiazole (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. © 1990 The Chemical Society of Japan.