Abstract
The singlet state of nuclear spin-1/2 pairs is protected against many common relaxation mechanisms. Singlet order, which is defined as the population difference between the nuclear singlet and triplet states, usually decays more slowly than the nuclear magnetization. Nevertheless, some decay mechanisms for nuclear singlet order persist. One such mechanism is called scalar relaxation of the second kind (SR2K) and involves the relaxation of additional nuclei ("third spins") which have scalar couplings to the spin-1/2 pair. This mechanism requires a difference between the couplings of at least one third spin with the two members of the spin-1/2 pair, and depends on the longitudinal relaxation time of the third spin. The SR2K mechanism of nuclear singlet relaxation has previously been examined in the case where the relaxation rate of the additional spins is on the time scale of the nuclear Larmor frequency. In this paper, we consider a different regime, in which the longitudinal relaxation of the third spins is on a similar time scale to the J-coupling between the members of the spin pair. This regime is often encountered when the spin-1/2 pair has scalar couplings to nearby deuterium nuclei. We show that the SR2K mechanism may be suppressed in this regime by applying a radiofrequency field which is resonant either with the members of the spin pair, or with the third spins. These phenomena are analyzed theoretically and by numerical simulations, and demonstrated experimentally on a diester of [13C2, 2H2]-labeled fumarate in solution.
Highlights
Clusters of spin-1/2 nuclei may form magnetically “silent” configurations which are sheltered from NMR relaxation and decay with extended lifetimes.[1–46] These long-lived states (LLS) may have decay rate constants which greatly exceed that of longitudinal magnetization
We show that radio-frequency irradiation which is resonant with either of the two spin species (“spin locking”) can lead to a significantly reduced singlet relaxation contribution from Singlet relaxation by SR2K (S-SR2K), and returns the decay of nuclear singlet order to a single exponential form
The rapid quenching of nuclear singlet order and the biexponential decay support a strong role for SR2K as a singlet relaxation mechanism in this system
Summary
Clusters of spin-1/2 nuclei may form magnetically “silent” configurations which are sheltered from NMR relaxation and decay with extended lifetimes.[1–46] These long-lived states (LLS) may have decay rate constants which greatly exceed that of longitudinal magnetization. A long-lived state lifetime has surpassed the relaxation time constant T1 by a factor of 50 in one case,[27,28] and a lifetime exceeding 1 h was observed for a naphthalene derivative in solution.[33]. Experiments have been performed in which LLS phenomena are combined with hyperpolarization methodology in order to generate nuclear spin systems which are far from equilibrium, and to maintain the non-equilibrium state for extended time intervals.[8,19,20,43,47].
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