ASSESSMENT OF A COMBINED QM MM APPROACH FOR THE STUDY OF LARGE NITROXIDE SYSTEMS IN-VACUO AND IN CONDENSED PHASES/

The structures and spectromagnetic properties of some model nitroxides were studied by a self-consistent hybrid of Hartree-Fock and density functional methods (B3LYP) obtaining results close to experimental dat a. From the computed structures, together with the available experimen tal data, new parameters for the NO moiety have been derived for two o f the most commonly used force fields, namely, the MM+ and the univers al force field. This provides the background for computations of relia ble structures and spectromagnetic observables of large nitroxide syst ems. As examples of applications we report the following: (a) the stru cture of 2,2,5,5-tetramethyl-3-hydroxypyrrolidine-1-oxyl radical (PROX YL) optimized in the gas phase and in the crystalline solid; (b) the c alculation of the magnetic exchange coupling constant of the large org anic biradical 4-bis(4',4'-dimethyloxazolidine-N-oxyl)cyclohexane (OXY L) using geometries optimized by the new force field; (c) solvent effe cts on the hyperfine properties of representative nitroxides computed by a discrete-continuum model using structural parameters optimized by the new force field for the solute and for its first solvation shell. Geometry optimizations with the new force fields always give structur es in good agreement with the experiment both in vacuo and in condense d phases (solution and crystalline solid). Using these structures, rem arkably accurate spectromagnetic observables are obtained by the combi ned use of the B3LYP method and of the polarizable continuum model for the description of bulk solvent effects.