Results Molecular Mechanics Calculations. Molecular mechanics calculations carried out with PCModel (Serena software) show that the most stable structure for trans-MTHS is one in which the benzene ring and the two double bonds are coplanar. Similar results were reported earlie~~ a In cis-MTHS, the barrier to rotation about CS-Cl'is about 1 kcal/mol. Hence rotation about this bond is relatively unhindered. However, substitution at the ortho ...