Complete basis set calculations (CBS-QB3) were used to compute the CN rotation barriers for acetamide and eight related compounds, including acetamide enolate and O-protonated acetamide. Natural resonance theory analysis was employed to quantify the "amide resonance" contribution to ground-state electronic structures. A range of rotation barriers, spanning nearly 50 kcal/mol, correlates well to the ground-state resonance weights without the need to account for transition-state effects. Use of appropriate model compounds is crucial to gain an understanding of the structural and electronic changes taking place during rotation of the CN bond in acetamide. The disparate changes in bond length (DeltarCO << DeltarCN) are found to be consonant with the resonance model. Similarly, charge differences are consistent with donation from the nitrogen lone pair electrons into the carbonyl pi* orbital. Despite recent attacks on the resonance model, these findings demonstrate it to be a sophisticated and highly predictive tool in the chemist's arsenal.

High level ab initio calculations find that nitrenes are more stable than carbenes, as indicated by the computed enthalpy differences of 25−26 kcal/mol between triplet phenylnitrene and the isomeric triplet pyridylcarbenes. More generally, the greater thermodynamic stability of nitrenes manifests itself in the finding that the N−H bond dissociation energies (BDEs) of aminyl radicals are approximately 20 kcal/mol lower than the C−H BDEs of analogous alkyl radicals. The greater thermodynamic stability of nitrenes, relative to carbenes, is attributed to the large amount of 2s character in the orbital that is occupied by the lone pair of electrons in nitrenes.

12/11)CASSCF and (12/11)CASPT2 ab initio electronic structure calculations with both the cc-pVDZ and cc-pVTZ basis sets find that there is a barrier to the very exothermic hydrogen shift that converts singlet methylnitrene, CH 3 N, to methyleneimine, H 2 CdNH. These two energy minima are connected by a transition structure of C s symmetry, which is computed to lie 3.8 kcal/mol above the reactant at the (12/ 11)CASPT2/cc-pVTZ//(12/11)CASSCF/cc-pVTZ level of theory. The (12/11)CASSCF/cc-pVTZ value for the lowest frequency vibration in the transition structure is 854 cm -1 , and CASPT2 calculations concur that this a′′ vibration does indeed have a positive force constant. Thus, there is no evidence that this geometry is actually a mountain top, rather than a transition structure, on the global potential energy surface or that a C 1 pathway of lower energy connects the reactant to the product. Therefore, our computational results indicate that the bands seen for singlet methylnitrene in the negative ion photoelectron spectrum of CH 3 Nare due to singlet methylnitrene being an energy minimum, rather than a transition state. Our results also lead us to predict that, at least in principle, singlet methylnitrene should be an observable intermediate in the formation of methyleneimine.