Electronic and molecular structure of aminimides (1-acyl-2,2,2-trimethyl-diazan-2-ium-1-ide). 2. Substituted aminimides (RCON−N+Me3)

The electronic structure and geometries of substituted aminimides (RCON−N+(CH3)3) where R = H, F, OH, NH2, N−N+(CH3)3, NO, NO2, CN, CH3, CF3, CHC, CH2CH, C6H5, SiH3 have been examined using the B3LYP/6-311+G(d,p) model. For all aminimides (except when substituted with C6H5 and SiH3), NBO calculations locate one lone pair of the N− in the HOMO (pz) and the second lone pair in HOMO-3 (spx, x = 1.06-1.19). One lone pair of oxygen (LP1) fills the HOMO-1 (p orbital which is orthogonal to πCO). The πCO is the HOMO-2. There is a significant donor-acceptor interaction between the N− and the carbonyl group. Unlike other aminimides, when substituted with C6H5 and SiH3 the negative charge is mostly located on the carbonyl oxygen rather than on nitrogen. All RCON−N+(CH3)3 have a preference for the (Z)-conformation about the C(O)N− bond. The computed Gibbs free energy of activation (25 °C) for converting the (Z)-conformer into the (E)-conformer is in the range of 22-35 kcal/mol. The anionic nitrogen of RCON−N+(CH3)3 is more effective at charge transfer than an ordinary amide! For example, the second order perturbation stabilization as result of donor (N−)/acceptor (CO) interaction is ca. 100 kcal/mol for RCON−N+(CH3)3, while for similarly substituted amides is ca. 60 kcal/mol. The most important factors in negative hyperconjugation are the donation of the oxygen LP(2) into the adjacent antibonds σ*CO-R (the second order stabilization energy gives a good linear correlation with the calculated CO stretching vibrational values) and σ*CO-N-. Isodesmic reactions analysis at B3LYP/6-311 + G(d, p) level, G2 and G2MP2 supports the view that π donors (NH2, OH and F) are stabilizing the aminimides. The Gibbs free energy balance of the isodesmic reactions correlates linearly with the inductive and resonance substituent constants.