Static zero field splitting effects on the electronic relaxation of paramagnetic metal ion complexes in solution

A general theory of the electronic relaxation of an S state complexed param agnetic metal ion (Mn2+, Gd3+) in solution is developed. Contrarily to the usual assumption, the electronic relaxation is not only due to the effects of the transient zero field splitting, but is strongly influenced by the st atic crystal field effect which is modulated by the random Brownian rotatio n of the complex. The electron paramagnetic resonance measured linewidths o f three Gd3+ (S=7/2) complexes at various temperatures and fields [D. H. Po well J. Am. Chem. Soc. 118, 9333 (1996)] are well interpreted in the framew ork of this model and show the contributions of both mechanisms. The fitted crystal field parameters, the correlation times, and the activation energi es are in good agreement with their expected values from the underlying pro cesses. Moreover, our interpretation does not require the addition of any f ield independent contribution to the linewidth like the spin-rotation mecha nism. The longitudinal relaxation function is well approximated using a sin gle relaxation time, whereas the transverse relaxation function is a superp osition of four decreasing exponentials. (C) 2000 American Institute of Phy sics. [S0021-9606(00)50537-6].