ELECTRON-SPIN RELAXATION STUDIES OF LA-AT-C-82 IN THE SOLID-STATE BY EPR

The temperature dependence of EPR spectrum of La@C-82 in the powder of empty C-60 and C-70 mixed crystals was studied by EPR spectroscopy em ploying X- and Q-band microwave frequencies. The rigid limit spectra ( at 4.2 K for the X-band and at 132 K for the Q-band) could be analyzed by static spectral simulation which yielded the EPR parameters, g(par allel to) = 2.0021, g(perpendicular to) = 1.9970, (La)A(perpendicular to) = 7.8 MHZ, (La)A(parallel to) similar to 0 MHZ and an isotropic C- 13 coupling value of about 3 MHz. For higher temperatures an appreciab le motional averaging effect was observed and the spectra were analyze d by using dynamic spectral simulation based on the stochastic Liouvil le equation, where we assumed an isotropic rotational motion with the Brownian diffusion. The calculated spectra reproduced the dominant fea ture of the temperature dependence of the spectra almost satisfactoril y for both the X- and Q-band frequencies with the appropriate rotation al correlation times. The Arrhenius plots of the correlation time gave two activation energies of 0.9 kcal/mol and 2.9-3.8 kcal/mol for the temperatures below and above 200 K, respectively.