Temperature-dependent spectral spin diffusion in solid state deuteron magnetic resonance

The polarization transfer from the para to the ortho/meta deuterons was investigated in a single crystal of perdeuterated biphenyl, C12D10. It is demonstrated that this process is due to spin diffusion via dipolar coupling. We found that in the range from 290 to 315 K it is strongly dependent on the temperature. The apparent activation energy of about 75 kJ/mol is the same as that of the known 180° flips of phenyl rings about the long molecular axis. As an explanation of the observed temperature dependence of the spin diffusion we discuss an exchange broadening of the relevant zero-quantum line caused by the 180° flips.