RADICALS AND IONS OF GLYCINE - AN AB-INITIO STUDY OF THE STRUCTURES AND GAS-PHASE THERMOCHEMISTRY

The structures of glycine and the related radicals and ions were optimized at both HF/6-31G(D) and NP2/6-31G(D) levels. The vibrational frequencies were calculated at the HF/6-31G(D) or MP2/6-31G(D) levels and the total energies of these molecules were evaluated at the G2(MP2) level. Proton affinities (PAs) and bond dissociation energies (BDEs) were found. The PAs and BDEs demonstrate that there is a strong captodative stabilizing effect for the C-centered radicals. Thus for H2NCH2COOH the C-H BDE is relatively low, 331 kJ mol(-1). Using the G2(MP2) energies, the glycine ionization potential (894 kJ mol(-1)) and the acyloxyl radical electron affinity (358 kJ mol(-1)) were also derived. Values of C-p(o), S-o, H-o - H-0(o), Delta(i)H(o); and Delta(f)G(o) at 298 K were calculated. The recommended values of Delta(f)H(298)(o) (in kJ mol(-1)) are as follows: H3NCH2COOH+ (249), H3NCHCOOH.+ (454), H-2-NCH2COOH.+ (502), H3NCH2COO.+ (547), H2NCH2(OH)(2)(.+) (399), H2NCH2COOH (-392), H2NCHCOOH. (-279), HNCH2COOH. (-181), H2NCH2COO. (-138), H2NCH2COO- (-491), H2NCHCOO.(-) (-346), and HNCH2COO.- (-303).