Long-Lived Nuclear Spin States in Methyl Groups and Quantum-Rotor-Induced Polarization

Abstract

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 <i>T</i>₁. 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. <i>J. Magn. Reson.</i> <b>2012</b>, <i>219</i>, 1–3)