Meier, B., et al., Long-lived nuclear spin states in methyl groups and quantum-rotor-induced polarization. J Am Chem Soc, 2013. 135(50): p. 18746-9.
Substances containing rapidly rotating methyl groups may exhibit long-lived states (LLSs) in solution, with relaxation times substantially longer than the conventional spin-lattice relaxation time T1. The states become long-lived through rapid internal rotation of the CH3 group, which imposes an approximate symmetry on the fluctuating nuclear spin interactions. In the case of very low CH3 rotational barriers, a hyperpolarized LLS is populated by thermal equilibration at liquid helium temperature. Following dissolution, cross-relaxation of the hyperpolarized LLS, induced by heteronuclear dipolar couplings, generates strongly enhanced antiphase NMR signals. This mechanism explains the NMR signal enhancements observed for (13)C-gamma-picoline (Icker, M.; Berger, S. J. Magn. Reson. 2012, 219, 1-3).