THEORETICAL COMPARISON AND EXPERIMENTAL TEST OF THE SECULAR AND NONPERTURBATIVE APPROACHES ON THE ESR LINESHAPES OF RANDOMLY ORIENTED, ANISOTROPIC SYSTEMS UNDERGOING INTERNAL MOTION

The nuclear Zeeman and the electronic nonsecular parts of the spin Hamiltonian complicate the ESR lineshape of exchanging anisotropic spin systems by introducing, at high field, "forbidden" transitions and, at low field, additional shift and splitting. We compare the nonperturbative with the secular approach for such systems. The exchange is treated within the Kaplan-Alexander limit and both A and g tensors are included, resulting in spectrum asymmetry, in contrast to previous separate treatments. The two approaches are then used to simulate the powder spectrum of OCH2COO- and compare the results to experimental spectra of an irradiated powder of ZnAc. The powder X-band spectra simulations using the secular approach appear to be accurate. For both the low-field (20 to 200 G) and the high-field (Q-band) regions, however, the nonsecular part of the electronic term and the nuclear Zeeman term, respectively, cannot be neglected. On the other hand, the approximate approach is much faster and consequently more appropriate for treating large, multisite exchanging systems. © 1991.