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 inc ludes density functional calculations, simulation of the solvent by a mixed discrete-continuum approach, and averaging of spectroscopic prop erties over the most important vibrational motions. In vacuo computati ons provide reliable results for the zwitterionic form when using hybr id Hartree-Fock/density-functional methods with purposely tailored bas is 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 propert ies of these species induced by the solvent are well reproduced by eit her a continuum model or a supermolecule approach, quantitative result s can be obtained only by a mixed discrete-continuum model. Vibrationa l averaging effects further improve the results, leading to remarkable agreement between computed and experimental hyperfine coupling consta nts. Together with its numerical accuracy, the interest of the propose d approach is that it can be routinely applied to large systems also b y nonspecialists and that it allows a straightforward interpretation o f the results in terms of different intrinsic and environmental effect s.