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 13C-γ-picoline (Icker, M.; Berger, S. J. Magn. Reson. 2012, 219, 1-3).
|Number of pages||4|
|Journal||Journal of the American Chemical Society|
|Early online date||20 Nov 2013|
|Publication status||Published - 18 Dec 2013|