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The resonance energy of amides and their radical cations

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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.

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Authors and Affiliations

  1. Department of Chemistry and Biochemistry, University of Maryland, Baltimore County, Baltimore, MD, 21250, USA

    Joel F. Liebman

  2. Department of Chemistry, University of New Hampshire, Durham, NH, 03824, USA

    Arthur Greenberg

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  1. Joel F. Liebman
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Correspondence to Joel F. Liebman or Arthur Greenberg.

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Liebman, J.F., Greenberg, A. The resonance energy of amides and their radical cations. Struct Chem 30, 1631–1634 (2019). https://doi.org/10.1007/s11224-019-01404-y

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