Electronic and molecular structure of aminimides (1-acyl-2,2,2-trimethyldiazan-2-ium-1-ide). 1. Formaminimide (HCON- N+ Me3).

The electronic structure and geometries of (Z)- and (E)-H-CON-N+(CH3)3 have been examined at two levels of theory:  B3LYP (basis sets 6-311+G(d,p), 6-311++G(d,p), and 6-311G(3df,3pd)) and MP2(full)/6-311++G(d,p). The (Z) conformation about the C(O)−N- bond is thermodynamically preferred over the (E) configuration. Natural bond orbital calculation locates one lone pair of the N- in the HOMO, which is the pz natural hybrid orbital (perpendicular to the OCN-N+ plane). The second lone pair (of lower energy) of N- occupies the HOMO-3, which is the natural hybrid orbital sp1.12 (sp1.01 for the (E) conformation, sp1.74 in the rotational transition state). The carbonyl π bond is the HOMO-2. The charge-transfer ability of the negative nitrogen in H-CON-N+(CH3)3 is more powerful than that of the neutral amidic nitrogen in dimethylformamide. The following facts convincingly sustain this view:  (1) the higher rotational barrier (stronger C−N- bond) in the case of H−CON-N+(CH3)3, (2) natural resonance theory analysis ...