Determination of Molecular Torsion Angles Using Nuclear Singlet Relaxation

Determination of Molecular Torsion Angles Using Nuclear Singlet Relaxatio

Recycling and Imaging of Nuclear Singlet Hyperpolarization

The strong enhancement of NMR signals achieved by hyperpolarization decays, at best, with a time constant of a few minutes. Here, we show that a combination of long-lived singlet states, molecular design, magnetic field cycling, and specific radiofrequency pulse sequences allows repeated observation of the same batch of polarized nuclei over a period of 30 min and more. We report a recycling protocol in which the enhanced nuclear polarization achieved by dissolution-DNP is observed with full intensity and then returned to singlet order. MRI experiments may be run on a portion of the available spin polarization, while the remaining is preserved and made available for a later use. An analogy is drawn with a “spin bank” or “resealable container” in which highly polarized spin order may be deposited and retrieved

Synthesis of an Isotopically Labeled Naphthalene Derivative That Supports a Long-Lived Nuclear Singlet State

The synthesis of an octa-alkoxy substituted isotopically labeled naphthalene derivative, shown to have excellent properties in singlet NMR experiments, is described. This highly substituted naphthalene system, which incorporates an adjacent ¹³C spin pair, is readily accessed from a commercially available ¹³C₂-labeled building block via sequential thermal alkynyl- and arylcyclobutenone rearrangements. The synthetic route incorporates a simple desymmetrization approach leading to a small difference in the chemical shifts of the ¹³C spin pair, a design constraint crucial for accessing nuclear singlet order

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

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)