ENDOR Enhancements and relaxation properties of the −OOCĊHCOO − radical in a KH malonate single crystal

Abstract

ENDOR enhancements and linewidths of the a proton in the −OOCĊHCOO − radical included in a potassium hydrogen malonate single crystal were determined in the three principal planes of the hyperfine tensor. In two planes the ENDOR intensities are very anisotropic, whereas in the third a nearly constant intensity is found. Very strong ENDOR signals (up to 15% of the EPR intensity) are detected near the crossing points of the “ββ” and “βα” energy levels. ENDOR enhancements were calculated by the method developed by Freed and co-workers, taking into account the anisotropies of transition moments and spin-lattice transition probabilities. Calculated intensities are in good agreement with the experimental ones. The dependence of ENDOR intensities on transition moments is very small, whereas they are strongly dependent on spin-lattice transition probabilities. Orientationally dependent W N values must be introduced in the calculations. A strong contribution to W N by the modulation of the dipolar hyperfine field due to the electron relaxation must be assumed. The electron spin-lattice relaxation mechanism is isotropic, being the phonon modulation of the spin-orbit coupling. The W E values giving the best agreement between experimental and calculated enhancements are in the range 10 2 to 5 × 10 2 s −1.