The resonance energy of amides and their radical cations

Abstract

The resonance energy of an amide can be calculated through comparison with a model amine and a model ketone (or aldehyde) with subtraction of the “residual” fragments. This classical approach, employing gas-phase experimental enthalpies of formation, corresponds to ca. 70 kJ mol−1 (≈ 16 kcal mol−1) for N,N-dimethylacetamide (DMA), a value close to that of the C(O)-N rotational barrier. Using a similar approach to explore the resonance energy of the DMA radical cation leads to the question of whether the amine radical cation or the ketone/aldehyde radical cation is the most appropriate model. An additional complication is whether the amide radical cation loses a π electron or an nO electron. These issues are analyzed. For DMA radical cation, the π orbital loses the electron and spin is mostly localized at nitrogen. A negative resonance energy of ca. 22 kJ mol−1 (5 kcal mol−1) is obtained. This somewhat surprising result can be attributed to reduced stabilization due to delocalization of a single electron relative to an electron pair and its significant localization on nitrogen which is exceeded by coulombic destabilization of the carbonyl carbon by the adjacent nitrogen.