Solution thermochemistry of the radicals of glycine

Abstract

Using gas phase thermochemical data, the following Gibbs energies of formation in aqueous solution (in kJ mol^–1) have been estimated for radicals of glycine: H₃N⁺CH₂CO₂˙– 93, H₂N˙⁺CH₂CO₂H –163, H₃N⁺CH˙CO₂H –198, H₂NCH˙C(OH)₂⁺–268, H₃N⁺CH₂CO₂^––371, H₂NCH₂CO₂˙–95, H₂N˙⁺CH₂CO₂^––158, HN˙CH₂CO₂H –148, H₂NCH˙CO₂H –246, HN˙CH₂CO₂^––147 and H₂NCH˙CO₂^––208. The uncertainty in these values is estimated to be ±20 kJ mol^–1.

In accord with earlier EPR studies, the H₂NCH˙CO₂H and H₂NCH˙CO₂^– radicals are predicted to be the most stable. Non-equivalence of the NH protons of the latter can be rationalized by a strong internal H ⋯^–OCO bond. Formation of the H₃N⁺CH₂CO₂˙ and H₂NCH₂CO₂˙ acyloxyl species is expected to require very strong oxidants (E° > 3 V). Production of H₂N˙⁺CH₂CO₂H and H₂N˙⁺CH₂CO₂^– is proposed as a better explanation of H₂NCH₂˙ formation in SO₄˙^– oxidations. The H₂N˙⁺CH₂CO₂^– radical, which is also susceptible to loss of CO₂, would lie above H₂NCH₂CO₂˙ in the gas phase, but its Gibbs energy of formation in aqueous solution will be ca. 0.65 V less than that of H₂NCH₂CO₂˙. E°(H₂N˙⁺CH₂CO₂^–/H₂NCH₂CO₂^–) is estimated to be near 1.6 V. This is in keeping with observed one electron oxidations of H₂NCH₂CO₂^– by triplet states of organic molecules with reduction potentials in the region of 1.5–1.8 V.