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 paramagnetic 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 static crystal field effect which is modulated by the random Brownian rotation of the complex. The electron paramagnetic resonance measured linewidths of three Gd3+ (S=7/2) complexes at various temperatures and fields [D. H. Powell J. Am. Chem. Soc. 118, 9333 (1996)] are well interpreted in the framework of this model and show the contributions of both mechanisms. The fitted crystal field parameters, the correlation times, and the activation energies are in good agreement with their expected values from the underlying processes. Moreover, our interpretation does not require the addition of any field independent contribution to the linewidth like the spin-rotation mechanism. The longitudinal relaxation function is well approximated using a single relaxation time, whereas the transverse relaxation function is a superposition of four decreasing exponentials. (C) 2000 American Institute of Physics. [S0021-9606(00)50537-6].