Effective modeling of intrinsic and environmental effects on the structureand electron plaramagnetic resonance parameters of nitroxides by an integrated quantum mechanical/molecular mechanics/polarizable continuum model approach

Experimental and density functional theory geometries have been used to ext end the AMBER force field to nitroxides, An optimum set of transferable ato mic charges for the calculation of electrostatic interactions both in vacuo and in aqueous solution has been obtained by averaging the charles obtaine d by a restrained electrostatic potential fitting of representative compoun ds. Besides reliable structural data, our implementation allows the computa tion of accurate spectromagnetic properties by single-point B3LYP computati ons on geometries optimized at the AMBER level. Solvent shifts in aqueous s olution can be reproduced quantitatively by a mixed model in which specific solvent effects are described by two water molecules strongly coordinated to the nitroxide oxygen, while bulk effects are described by the polarizabl e continuum model.