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 data. From the computed structures, together with the available experimental data, new parameters for the NO moiety have been derived for two of the most commonly used force fields, namely, the MM+ and the universal force field. This provides the background for computations of reliable structures and spectromagnetic observables of large nitroxide systems. As examples of applications we report the following: (a) the structure of 2,2,5,5-tetramethyl-3-hydroxypyrrolidine-1-oxyl radical (PROXYL) optimized in the gas phase and in the crystalline solid; (b) the calculation of the magnetic exchange coupling constant of the large organic biradical 1,4-bis(4',4'-dimethyloxazolidine-N-oxyl)cyclohexane (OXYL) using geometries optimized by the new force field; (c) solvent effects 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 structures in good agreement with the experiment both in vacuo and in condensed phases (solution and crystalline solid). Using these structures, remarkably accurate spectromagnetic observables are obtained by the combined use of the B3LYP method and of the polarizable continuum model for the description of bulk solvent effects.