SOLUTION THERMOCHEMISTRY OF THE RADICALS OF GLYCINE

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+CHCO2H)-C-.-198, (H2NCHC)-C-.(OH)(2)(+)-268, H3N+CH2CO2- -371, H2NCH2CO2(.) -95, (H2NCH2CO2-)-C-.+ -158, (HNCH2CO2H)-C-. -148, (H2NCHCO2H)-C-. -246, (HNCH2CO2)-C-.(-) -147 and (H2NCHCO2-)-C-. -208. The uncertainty in these values is estimated to be +/- 20 kJ mol(-1). In accord with earlier EPR Studies, the (H2NCHCO2H)-C-. and (H2NCHCO2-)-C-. 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)-O-- bond. Formation of the H3N+CH2CO2. and H2NCH2CO2. acyloxyl species is expected to require very strong oxidants (E degrees > 3 V). Production of (H2NCH2CO2H)-C-.+ and (H2NCH2CO2-)-C-.+ is proposed as a better explanation of H2NCH2. formation in SO4.- oxidations. The (H2NCH2CO2-)-C-.+ 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 degrees(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.