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 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).

Symmetry constraints on spin dynamics: Application to hyperpolarized NMR

“…The transformation of total spin magnetization into singlet order, and back again, is an important manipulation in the NMR of long-lived states [13,14,15,16,17,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44] and in parahydrogen-enhanced NMR [18,19,20,21,22,45,46,47,48,49].…”

“…If only corresponding pairs of heteronuclear J-couplings are equal (J 13 = J 24 = J 23 = J 14 ), the pairs of I-spins and S-spins are chemically equivalent but magnetically inequivalent. The magnetic inequivalence group G 12,34 MIE is appropriate in this case (equation 21).…”

Long-lived nuclear spin states in rapidly rotating CH 2 D groups

“…We also assume that the overall correlation time τ C is short enough relative to the nuclear Larmor period to invoke the extreme narrowing approximation [40]. A model of this kind was used to treat methyl long-lived states and quantum-rotor-induced polarization in γ-picoline [41,42].…”

Singlet NMR methodology in two-spin-1/2 systems

“…Since its discovery in the Levitt group [24,25,26] a number of research groups have investigated the field of singlet NMR over the past 12 years with the purpose of defining and characterising singlet order relaxation properties [3,27,28,29,30], and the conditions for its existence [31,32,33,34], and of developing the methodology to access singlet order in different magnetic regimes [24,26,35,36,37,38,39,9,40,11,41,42,43] and to demonstrate the existence of long-lived singlet order in multiple-spin systems [44,45,46,47,48,49] and in heteronuclear systems [50]. Significant studies on the chemical requirements for obtaining long-lived spin order have been also conducted [17,51,52,53,54] with a record lifetime of 70 min for a 13 C-pair [54], 26 min for a 15 N-pair [51], and 10 min for a 1 H-pair [17] demonstrated.…”