Crystal structure and conformational analysis of 2-thioamide derivatives of thiazole

Molecular and crystal structures of two thioamide derivatives of thiazole w ere analysed based on X-ray diffraction data and results obtained from quan tum chemistry (QC) and molecular mechanics (MM) calculations. It was found that within-the thiazole rings, the S1-C2 bond appeared to be more affected by the action of two-substituents rather than the C2-N3 bonds. The ab init io calculations showed that the C2-C6 bond connecting the thioamide group w ith thiazole is shortened when both the systems are coplanar, but a conjuga tion is relatively weak and does not enforce such conformation. The QC and MM calculations indicated that the particular parts of the thioamide group may exist in two conformations. However, relatively high energy barrier exc luded the possibility of change of one conformation into the other by simpl e rotation around the C2-C6 or C6-N8 bonds. Moreover, the MM calculations e xplained that steric hindrance is not the main reason of energy barrier for suggested rotations. The observed cis-trans conformation is stabilised by intramolecular hydrogen bonds. (C) 1999 Elsevier Science B.V. All rights re served.