Five-membered heterocycles. Part II. Crystal structures and HOMA index calculations for selected 1,3,4-thiadiazole derivatives

Crystal structures of four bisubstituted 1,3,4-thiadiazole derivatives were determined. Detailed description of the five-membered heteroring geometry is enabled thorough analysis of ring aromaticity. For this purpose the HOMA index was used as a quantitative measure of aromaticity. The calculated HOMA indices evidenced the role of substituents. In particular, increase in the substituent electrophilicity brings about increase in aromaticity. (C) 2000 Elsevier Science B.V. All rights reserved.