Slow-motional ESR spectra for vanadyl complexes and their model dependence

Abstract

The slow-motional ESR analysis appropriate for vanadyl(1V) ions is developed with an accurate treatment of the nonsecular contributions. This results in good agreement between theory and experiments on VO(acac2(pm)) in toluene with axially symmetric magnetic parameters over the whole motional range when a Brownian motion model is used. It is also found that a slightly modified motional narrowing theory based upon the stochastic Liouville equation leads to improved agreement between theory and experiment for VO(a~ac)~ in toluene which also obeys a Brownian motion model. It is shown that vanadyl slow-tumbling spectra are very sensitive to model (even more so than nitroxides). In particular, experiments on VO(H20),2+ in aqueous solution are approximately fit by a model of moderate jump, while those on VO(NCS)t- could only be crudely fit by temperature-dependent variations in model. The theory given here is appropriate for any S = 1/2 radical with a single nuclear spin I provided only the high-field approximation is valid, so that nonsecular terms may be treated by a perturbation approach. The effects of (1) angular-dependent transition probabilities and (2) field- vs. frequency-swept spectra upon the slow-motional theory are also discussed.