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

Ab initio MO calculations of sulfonium, selenonium and telluronium imides ( R2X)-X-1-->NR2 (X = S, Se, Te) have been performed to obtain their pyramida l inversion (vertex inversion) barriers and rotation barriers around the ch alcogen-nitrogen bonds. The activation energies for the pyramidal inversion reaction, which is an important pathway for racemization of the chiral cha lcogen-onium imides when the two R-1 groups are different, depend on the su bstituents, R-1 and R-2, on the chalcogen and nitrogen atoms. The computati ons indicated that the telluronium imides require larger activation energie s for the pyramidal inversion reaction than those of corresponding sulfoniu m and selenonium imides. The rotation barriers around the chalcogen-nitroge n bonds, which is the conformational exchange pathway between two local min ima of the chalcogen-onium imides, showed the opposite trend.