| Abstract | The [small alpha]-amino C-H bond dissociation enthalpies of glycine anhydride and alanine anhydride and the C-O bond dissociation enthalpies of the corresponding anhydride peroxyl radicals have been measured by photoacoustic calorimetry in aqueous solution. Furthermore, the one-electron oxidation potentials of the C-centered radicals formed upon hydrogen abstraction from glycine anhydride and alanine anhydride have been measured by photomodulation voltammetry in aqueous solution. In addition, the C-H bond dissociation enthalpies of glycine anhydride and alanine anhydride and one-electron reduction potentials of the corresponding radicals have been calculated by ab initio methods [at the B3LYP/6-31G(D) level]. The resulting experimental gas-phase C-H bond dissociation enthalpies are 340 and 325+/-15 kJ mol-1, the C-O bond dissociation enthalpies are 56 and 64+/-15 kJ mol-1 and the one-electron oxidation potentials are 0.175 and 0.086 V vs. NHE for the glycine anhydride and alanine anhydride related species, respectively. The calculated C-H bond dissociation enthalpies are 351.1, 334.7 and 332.7+/-10 kJ mol-1 for glycine anhydride, L-alanine anhydride and D,L-alanine anhydride, respectively. The one-electron reduction potentials of the corresponding radicals are 1.19, 1.00 and 0.99 V vs. NHE. The thermochemical properties of amino acid anhydrides and the corresponding C-centered radicals and peroxyl radicals determined in this work are consistent with each other and with previously published observations on the radical chemistry of amino acid anhydrides and related species. |
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