ENDOR AMPLITUDES OF TRIPLET-STATE MOLECULES - II - ORIENTATIONAL DEPENDENCE OF THE NU(P) FREQUENCY LINE AND S-T-0 MIXING

The specific ENDOR line at the free Larmor frequency nu(p) in the low temperature spectra of triplet state molecules is caused by degenerate NMR transitions within the M-S = 0 zero-level (ZL) electron spin mani fold. This ZL line was found to be orientationally dependent for the d iradical complex Zn(3,6-di-tert-butyl-o-semiquinone)(2)Zn(DBSQ)(2): th e ZL line dominates the ENDOR spectrum if it is detected at the perpen dicular canonical components of the EPR spectrum, and vanishes if the complex is oriented with its ZFS z-axis parallel to the direction of t he magnetic field, i.e., if detected at the parallel canonical EPR com ponents. This effect is shown to result from the interaction between n uclear spin substates of the S and T-0 manifolds, their levels being c lose to each other for the Zn(DBSQ)(2) complex. Such an interaction mi xes the states and shifts energy levels. Consequently, it cancels the degeneracy of the nuclear substates within the ZL manifold and reduces the rate of nuclear flip-flop relaxation. This specific relaxation me chanism has been shown to substantially affect the amplitude of the ZL line (Doubinskii A.A., Lebedev Ya.S., Mobius K.: Appl. Magn. Reson. 1 3, 439 (1997)). The nuclear flip-flop relaxation effect is expected to be orientationally dependent since the S-T-0 separation depends upon the orientation of the diradical with respect to the external magnetic field.