Spin-lattice relaxation of deuterated methyl groups: Implications of the Pauli principle

The high-field spin-lattice relaxation of deuterated methyl groups undergoi ng relational tunneling is investigated theoretically. It is found that for systems showing a tunneling frequency comparable to accessible Larmor freq uencies the relaxation to equilibrium of the Zeeman energy does not follow a simple exponential time dependence even in powdered samples due to a fini te coupling to the relaxation of the tunneling system. This finding contras ts to the high-temperature behavior of reorienting methyl groups which unde rgo simple exponential relaxation. The nonexponentiality has its origin in the statistical coupling of the three deuteron spins due to the Pauli princ iple.