Dynamic nuclear polarisation by the cooling of tunnelling methyl groups

The energy associated with the tunnelling motion of methyl groups in solids at low temperatures relaxes extremely slowly to the lattice bath. Paramagnetic centres speed up this relaxation enormously when an external magnetic field is chosen so that the methyl tunnel frequency and the electronic Larmor frequency are nearly equal. The difference between the two energies is made up by nuclear spin flips. The consequent nuclear dynamic polarisation provides a sensitive way of following the cooling of the methyl groups by the paramagnetic centres, with the associated diffusion of tunnelling energy to the vicinity of paramagnetic ions. The characteristic signature of the process is an anomaly in the field dependence of the nuclear magnetism having the approximate form of the derivative of a bell-shaped curve. Three examples of this are reported and a theoretical explanation is given.