Experimental boundaries of the quantum rotor induced polarization (QRIP) in liquid state NMR

Phys. Chem. Chem. Phys.

et al. 2016

It is possible to access long-lived nuclear singlet order in monodeuterated methyl groups, in the case that a significant chemical shift difference exists between the CH 2 D protons. This occurs when the local environment is chiral, and the CH 2 D rotamers have different populations. An experimental demonstration is presented for the case of N-CH 2 D-2-methylpiperidine. The ratio of the singlet relaxation time constant T S to the longitudinal relaxation time constant T 1 is found to be equal to 3.1 ± 0.1, over a wide range of temperatures, solvents, and magnetic fields. The longest observed value of T S approaches 1 minute. The relaxation mechanisms of the long-lived state are discussed, and a modified model of the CH 2 D geometry is proposed to explain the observed ratio of T S to T 1 .

Section: Introductionmentioning

confidence: 99%

Long-lived nuclear spin states in monodeuterated methyl groups

Elliott

Brown

Phys. Chem. Chem. Phys.

et al. 2016

“…Berger and co-workers characterised the 13 C and 1 H antiphase multiplets and explored the chemical structures that give rise to this effect. 23 The effects were attributed to a quantum-rotorinduced polarization (QRIP) phenomenon. 20 In the case of a methyl group with a very low rotational barrier, equilibration at low temperature gives rise to a distribution of nuclear spin populations, which is strongly affected by the large tunnelling splitting.…”

Section: Introductionmentioning

confidence: 99%

Theory of long-lived nuclear spin states in methyl groups and quantum-rotor induced polarisation

The Journal of Chemical Physics

Håkansson

Mamone

et al. 2015

Long-lived nuclear spin states have a relaxation time much longer than the longitudinal relaxation time T1. Long-lived states extend significantly the time scales that may be probed with magnetic resonance, with possible applications to transport and binding studies, and to hyperpolarised imaging. Rapidly rotating methyl groups in solution may support a long-lived state, consisting of a population imbalance between states of different spin exchange symmetries. Here, we expand the formalism for describing the behaviour of long-lived nuclear spin states in methyl groups, with special attention to the hyperpolarisation effects observed in (13)CH3 groups upon rapidly converting a material with low-barrier methyl rotation from the cryogenic solid state to a room-temperature solution [M. Icker and S. Berger, J. Magn. Reson. 219, 1 (2012)]. We analyse the relaxation properties of methyl long-lived states using semi-classical relaxation theory. Numerical simulations are supplemented with a spherical-tensor analysis, which captures the essential properties of methyl long-lived states.

“…[19][20] QRIP has been observed for compounds such as γ-picoline (4-methylpyridine) for which the methyl rotation encounters an unusually low rotational barrier, leading to a significant tunnelling splitting of ~ 6 K between the A and E manifolds in the cryogenic solid state. For such special cases, a significant AEI may be established simply by equilibrating the sample at a temperature below 10 K, without any paramagnetic agents or microwaves.…”

mentioning

confidence: 99%

Dynamic Nuclear Polarization of Long-Lived Nuclear Spin States in Methyl Groups

Vuichoud

Mammoli

et al. 2017

J. Phys. Chem. Lett.

We have induced hyperpolarized long-lived states in compounds containing 13C-bearing methyl groups by dynamic nuclear polarization (DNP) at cryogenic temperatures, followed by dissolution with a warm solvent. The hyperpolarized methyl long-lived states give rise to enhanced antiphase 13C NMR signals in solution, which often persist for times much longer than the 13C and 1H spin–lattice relaxation times under the same conditions. The DNP-induced effects are similar to quantum-rotor-induced polarization (QRIP) but are observed in a wider range of compounds because they do not depend critically on the height of the rotational barrier. We interpret our observations with a model in which nuclear Zeeman and methyl tunnelling reservoirs adopt an approximately uniform temperature, under DNP conditions. The generation of hyperpolarized NMR signals that persist for relatively long times in a range of methyl-bearing substances may be important for applications such as investigations of metabolism, enzymatic reactions, protein–ligand binding, drug screening, and molecular imaging.