Theoretical investigation on the conformational preferences of sulfinimines

The potential energy surfaces of sulfinimine, H2CNS(O)H, 1, and methylsulfinimine, MeHCNS(O)H, 2, have been searched, using the ab initio MO and Density Functional Methods, to study the conformational preferences. Complete optimizations at HF/6-31G*, HF/6-31+G*, MP2/6-31+G* and B3LYP/6-31+G* levels on 1 showed that there are three minima on the path of rotation around N–S bond in 1. A conformer with synperiplanar arrangement, with the C–N–S–O torsional angle close to 13°, has been found to be the most preferred. Repulsions between the lone pairs of electrons present on N, S and O atoms are responsible for the observed conformational preferences of 1. The N–S bond rotational barrier in 1 is 9.16 kcal mol−1 at the B3LYP/6-31+G*(+ZPE) level. This high energy barrier can be attributed to the nN → σ*SO negative hyperconjugation and to the repulsive interactions between the lone pairs of electrons. The planar N-inversion barrier in 2Z is 18.72 kcal mol−1 at the B3LYP/6-31+G* (+ZPE) level, comparable to experimental values.