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The resonance energy of amides and their radical cations

  • Joel F. LiebmanEmail author
  • Arthur GreenbergEmail author
Original Research
  • 6 Downloads

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.

Keywords

Amides Radical cations Resonance energy Enthalpies of formation Ionization potentials ESR 

Notes

Compliance with ethical standards

We did not perform any experiments when preparing this review article, so neither ethics review nor informed consent was necessary.

Conflict of interest

The authors declare that they have no conflicts of interest.

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© Springer Science+Business Media, LLC, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of Chemistry and BiochemistryUniversity of MarylandBaltimoreUSA
  2. 2.Department of ChemistryUniversity of New HampshireDurhamUSA

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