Nitroxide radical pairs in the plastic crystal camphene

摘要

Electron spin resonance is employed to study pairs of nitroxide radicals in the plastic phase ofdlminus;camphene. Both Dgr;m= 1 and Dgr;m= 2 transitions are observed, and an adiabatic theory is presented for the dependence of the absorption intensity, secondminus;order shift, and linewidth of the latter transitions upon radical pair distributions and motions. Results are interpreted by a model in which radicals are nearly randomly distributed over lattice sites, lattice vacancies are thermally generated, and radical diffusion occurs by a vacancy mechanism. The enthalpies for vacancy formation and radical diffusion are, respectively, 4.6 and 11 kcal/mole, while at the melting pointDquest; 10minus;7cm2/sec.