School of Chemistry, University of Southampton , SO17 1BJ Southampton, U.K.
J Am Chem Soc. 2013 Dec 18;135(50):18746-9. doi: 10.1021/ja410432f. Epub 2013 Dec 2.
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-γ-picoline (Icker, M.; Berger, S. J. Magn. Reson. 2012, 219, 1-3).
含有快速旋转甲基基团的物质在溶液中可能表现出长寿命状态（LLS），其弛豫时间远远长于传统的自旋晶格弛豫时间 T1。这些状态通过 CH3 基团的快速内旋转而变得长寿命，这对内旋转核自旋相互作用施加了近似的对称性。在 CH3 旋转势垒非常低的情况下，通过液氦温度下的热平衡来填充超极化 LLS。溶解后，异核偶极耦合诱导的超极化 LLS 的交叉弛豫产生了强烈增强的反相 NMR 信号。这种机制解释了（13）C-γ-吡啶（Icker，M.；Berger，S. J. Magn. Reson. 2012, 219, 1-3）观察到的 NMR 信号增强。
