Rotational Energy Surfaces of 3-methyl-2-benzothiazolylidenaminodichlorophosphine: A Semi-empirical Study

Semi-empirical PM3 calculations have been performed with MOPAC 6.0 programme to study conformational analysis of different stereoisomer of 3-methyl-2-benzothiazolylidenaminodichlorophosphine (I) and their rotational energy surfaces to find the most stable conformation. The geometry optimization of four possible conformers of I gives rise to two minimum energy structures of IA and IB having syn orientation of lone pair of electrons on nitrogen and phosphorus. These structures were confirmed to be global minima by the force calculations. In terms of heat of formation, the geometrical isomer IA is more stable than IB by ~0.6 kcal/mol. Force calculation were carried out to locate transition state and global minima. The minimum energy structure on potential energy surface represents rotamer IA, (Planar orientation of lone pair of electrons on N10 & P11) confirms this to be most stable conformer. The maxima on the graph correspond to the stereoisomer IB; which is least least stable conformer. The rotational barrier is found to be 1.5 kcal/mol. The increased stability of the rotamer IA, representing minima can be attributed to the presence of negative hyperconjugation effect.