The resonance energy of amides and their radical cations
- 24 Downloads
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.
KeywordsAmides Radical cations Resonance energy Enthalpies of formation Ionization potentials ESR
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.
- 1.Zabicky J (ed) (1970) The chemistry of amides in the Chemistry of Functional Groups (Patai S (ed) Chemistry of functional groups. Wiley New YorkGoogle Scholar
- 2.Greenberg A, Breneman CM, Liebman JF (eds) (2000) The amide linkage: structural significance in chemistry, biochemistry and materials science. Wiley New YorkGoogle Scholar
- 6.Abboud JLM, Jiménez P, Roux MV, Turrión C, Lopez-Mardomingo C, Podosenin A, Rogers DW, Liebman JF (1995) Interrelations of the energetics of amides and olefins: the enthalpies of formation of N,N-dimethyl derivatives of pivalamide, 1-adamantylcarboxamide and benzamide and of styrene and its α-, Trans-β- and β,β-methylated derivatives. J Phys Org Chem 8:15–25CrossRefGoogle Scholar
- 8.Roux MV, Jiménez P, Martin-Luengo MA, Dávalos JZ, Sun Z, Hosmane RS, Liebman JF (1997) The elusive antiaromaticity of maleimides and maleic anhydride: enthalpies of formation of N-methylmaleimide, N-methylsuccinimide, N-methylphthalimide and N-benzoyl-N-methylbenzamide. J Organomet Chem 62:2732–2737CrossRefGoogle Scholar
- 10.Greenberg A (2000) The amide linkage as a ligand: its properties and the role of distortion. In Greenberg A, Breneman CM, Liebman JF (eds) The amide linkage: structural significance in chemistry, biochemistry and materials science. Wiley New York, pp 47-83Google Scholar
- 11.Liebman JF, Afeefy HY, Slayden SW (2000), The thermochemistry of amides. In The amide linkage: structural significance in chemistry, biochemistry and materials science, Wiley New York, pp 115–136Google Scholar
- 32.Rademacher P (2000) Photoelectron spectra of amides and lactams. In: Greenberg A, Breneman CM, Liebman JF (eds) The amide linkage: structural significance in chemistry. Biochemistry and Materials Science, Wiley New York pp, pp 247–289Google Scholar
- 33.Cassady CJ (2000) Gas-phase ion chemistry of amides, peptides, and proteins. In: Greenberg A, Breneman CM, Liebman JF (eds) The amide linkage: structural significance in chemistry. Biochemistry and Materials Science, Wiley New York, pp 463–493Google Scholar
- 38.Pedley JB (1994) Thermochemical data and structures of organic compounds. TRC data series, vol 1. TRC, College StationGoogle Scholar
- 39.Lias SG, Bartmess JE Gas phase ion thermochemistry In NIST Chemistry WebBook, SRD 69 NIST Stand. Ref. Data Base, (Ed. P.J. Linstrom, accessed May 22, 2019)Google Scholar
- 43.Eastland GW, Rao DNR, Symons MCR (1966) Radiation generation of radical cations of amides. J Chem Soc Faraday Trans 1(82):2833–2842Google Scholar