Hyperpolarized long-lived states in monodeuterated methyl groups & singlet-scalar relaxation in the regime of slow quadrupolar relaxation

Abstract

Nuclear magnetic resonance (NMR) experiments are time-limited by relaxation dynamics. Observing non-equilibrium magnetization is restricted to timescales governed by the longitudinal relaxation time T1. The use of long-lived states (LLS) offers a promising means to transcend this limitation. LLS are configurations of nuclear spins that are protected against the in pair dipole-dipole relaxation mechanism, with other sources of relaxation significantly attenuated. In systems of spin-1/2 pairs, the LLS is called singlet order and the decay time constant is denoted TS.
The field of LLS NMR is now flourishing, LLS lifetimes exceeding T1 by a factor of 50 have been observed, with a lifetime TS > 1 hour observed in room-temperature solution in one case. LLS have even been observed in the 3-spin-1/2 systems of rapidly rotating methyl groups in solution.
The work presented in this thesis builds on previous efforts from the LLS community.Most notably, prior attempts at methyl LLS are restricted to just a single case.Through my work, I have extended the family of molecules in which methyl LLS are accessible, achieved with high conversion efficiencies in suitable cases. The use of monodeuterated methyl groups as coherently accessibly reservoirs for nuclear singlet order has lead to the longest observed methyl LLS. The relaxation dynamics of two motionally different cases are examined and geometrical models are presented to explain the experimental results. Hyperpolarization results for these systems are also presented.
My work has lead to the investigation of more curious phenomena such as the singlet-scalar relaxation of the second kind (S-SR2K) mechanism. In the regime of slow quadrupolar relaxation, where T1 is significantly slower than the timescale of the nuclear Larmor period, this relaxation mechanism dramatically shortens singlet lifetimes. An experimental demonstration is provided for the case of a 13C labelled, deuterated fumarate derivative. This study differs from previous work on this subject, which examines the limit where the T1 of the third spin is on the timescale of the nuclear Larmor frequency, rarely the case for deuterium nuclei. I provide rate expressions and numerical simulations for the LLS decay in the S-SR2K regime of slow quadrupolar relaxation.

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Identifiers

ORCID for Malcolm H. Levitt: orcid.org/0000-0001-9878-1180

ORCID for Marcel Utz: orcid.org/0000-0003-2274-9672

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