No abstract

The effects of applied electric fields on the magnetic parameters of a nitroxide spin label have been calculated using an intermediate level of Rayleigh–Schrödinger perturbation theory based on unrestricted Hartree–Fock ab initio calculations. The theory is tested for selected simple model organic radicals and gives electronic g-tensors that compare well with previous calculations and experimental values. The method is then applied to calculate effects 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 gx and gy of TMDP, but the predicted shifts in these quantities with field are in excellent agreement with recent experimental observations using high-frequency (220 GHz) electron paramagnetic resonance. Results are expressed in terms of the phenomenological formalism given earlier by Mims to characterize linear electric field effects in metal centers.