Structure and magnetic properties of glycine radical in aqueous solution at different pH values

A recently developed quantum mechanical approach devoted to the study of unstable species in solution was applied to the radicals resulting from the homolytic breaking of the C alpha-H alpha bond of glycine in aqueous solution at different pH values.The computational protocol includes density functional calculations, simulation of the solvent by a mixed discrete-continuum approach, and averaging of spectroscopic properties over the most important vibrational motions. In vacuo computations provide reliable results for the zwitterionic form when using hybrid Hartree-Fock/density-functional methods with purposely tailored basis sets. Under the same conditions, disappointing results are obtained for the magnetic properties of neutral and, especially, anionic forms. Although the modifications of the structure and the magnetic properties of these species induced by the solvent are well reproduced by either a continuum model or a supermolecule approach, quantitative results can be obtained only by a mixed discrete-continuum model. Vibrational averaging effects further improve the results, leading to remarkable agreement between computed and experimental hyperfine coupling constants. Together with its numerical accuracy, the interest of the proposed approach is that it can be routinely applied to large systems also by nonspecialists and that it allows a straightforward interpretation of the results in terms of different intrinsic and environmental effects.