Ab initio calculations of electric field effects on the g-tensor of a nitroxide radical

The effects of applied electric fields on the magnetic parameters of a nitr oxide spin label have been calculated using an intermediate level of Raylei gh-Schrodinger perturbation theory based on unrestricted Hartree-Fock ab in itio calculations. The theory is tested for selected simple model organic r adicals and gives electronic g-tensors that compare well with previous calc ulations and experimental values. The method is then applied to calculate e ffects of a local electrostatic field on the larger nitroxide radical, 2,2, 5,5-tetramethyl-3,4-dehydro pyrrolidine-1-oxyl (TMDP) using a finite field approach. The method slightly underestimates the g(x) and g(y) of TMDP, but the predicted shifts in these quantities with field are in excellent agree ment with recent experimental observations using high-frequency (220 GHz) e lectron paramagnetic resonance. Results are expressed in terms of the pheno menological formalism given earlier by Mims to characterize linear electric field effects in metal centers. (C) 2001 American Institute of Physics.