Theoretical investigation on the conformational preferences of sulfinimines

The potential energy surfaces of sulfinimine, H2C=NS(O)H, 1, and methylsulfinimine, MeHC=NS(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 degrees, 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 n(N) --> sigma(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.