Pyramidal inversion energies and conformational analysis of chalcogen-onium imides based on ab initio MO calculations†

Abstract

Ab initio MO calculations of sulfonium, selenonium and telluronium imides R¹₂X→NR² (X = S, Se, Te) have been performed to obtain their pyramidal inversion (vertex inversion) barriers and rotation barriers around the chalcogen–nitrogen bonds. The activation energies for the pyramidal inversion reaction, which is an important pathway for racemization of the chiral chalcogen-onium imides when the two R¹ groups are different, depend on the substituents, R¹ and R², on the chalcogen and nitrogen atoms. The computations indicated that the telluronium imides require larger activation energies for the pyramidal inversion reaction than those of corresponding sulfonium and selenonium imides. The rotation barriers around the chalcogen–nitrogen bonds, which is the conformational exchange pathway between two local minima of the chalcogen-onium imides, showed the opposite trend.