GAS AND SOLUTION-PHASE THERMOCHEMISTRY AND TRANSITION ENERGIES OF NH2-CENTER-DOT AND NH3-CENTER-DOT- AN AB-INITIO STUDY(, AND THEIR AQUO COMPLEXES )

Ab initio calculations were performed on several aquo complexes of NH2 ., and NH3.+, and on monomeric parent species. The geometries were opt imized at the HF/6-31+G level and the vibrational frequencies were ca lculated. The total energies and the binding energies of complexes wer e evaluated at the MP2/6-31+G + ZPE level of theory. Gas and aqueous solution phase thermodynamic properites of NH2. and NH3.+ and several other species were calculated. The examination of solution phase prope rties of the radicals was facilitated by study of the structures and t ransition energies of aquo complexes. H- bonding interaction energies decreased in the order OH ... N > OH ... O > NH ... O, but were genera lly stronger than sigma-sigma interactions involving the unpaired ele ctron. From calculations with the CIS method, the weak absorption obse rved at 520 nm for aqueous NH2. is confirmed as a B-2(1)-->(2)A(1) tra nsition, while the stronger NH2. absorption occurring below 250 nm and the absorption of NH3.+, which rises monotonically below 370 nm, are attributed to solvent-to-solute charge transfer bands. The solution fr ee energies and related E(0) values for NH2. and NH3.+ are in agreemen t with those of Stanbury. The ab initio structure studies show that wa ter protons are bound to N, and proton transfer from solvent in reacti on [18], NH2. + e(-) + H2O --> NH3 + OH-, is likely to be the dominant redox reaction of NH2. in alkaline solution. The free energy of solut ion of NH3.+ is shown to be larger than that of NB4+.