Gas and solution phase thermochemistry and transition energies of NH2• and NH3•+, and their aquo complexes: an ab initio study

Ab initio calculations were performed on several aquo complexes of NH2•, and NH3•+, and on monomeric parent species. The geometries were optimized at the HF/6-31 + G* level and the vibrational frequencies were calculated. The total energies and the binding energies of complexes were 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 properties of the radicals was facilitated by study of the structures and transition energies of aquo complexes. H-bonding interaction energies decreased in the order [Formula: see text] but were generally stronger than σ–σ* interactions involving the unpaired electron. From calculations with the CIS method, the weak absorption observed at 520 nm for aqueous NH2• is confirmed as a 2B1 → 2A1 transition, 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 free energies and related E0 values for NH2• and NH3•+ are in agreement with those of Stanbury. The ab initio structure studies show that water protons are bound to N, and proton transfer from solvent in reaction [18], NH2• + e− + H2O → NH3 + OH−, is likely to be the dominant redox reaction of NH2• in alkaline solution. The free energy of solution of NH3•+ is shown to be larger than that of [Formula: see text].