School of Chemistry, University of Southampton, Southampton SO17 1BJ, United Kingdom.
Department of Chemistry, Pomona College, Claremont, California 91711, USA.
J Chem Phys. 2019 Feb 14;150(6):064315. doi: 10.1063/1.5074199.
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 [C, H]-labeled fumarate in solution.
核自旋-1/2 对的单重态受到许多常见弛豫机制的保护。单重态序，定义为核单重态和三重态之间的种群差异，通常比核磁化弛豫得更慢。然而，核单重态序仍然存在一些弛豫机制。其中一种机制称为第二类标量弛豫（SR2K），涉及与自旋-1/2 对具有标量耦合的附加核（“第三自旋”）的弛豫。这种机制需要至少一个第三自旋与自旋-1/2 对的两个成员之间的耦合差异，并且依赖于第三自旋的纵向弛豫时间。核单重态弛豫的 SR2K 机制以前曾在附加自旋的弛豫速率处于核拉莫尔频率的时间尺度的情况下进行过研究。在本文中，我们考虑了一个不同的情况，其中第三自旋的纵向弛豫与自旋对成员之间的 J 耦合处于相似的时间尺度。当自旋-1/2 对与附近的氘核具有标量耦合时，通常会遇到这种情况。我们表明，通过施加与自旋对成员或第三自旋共振的射频场，可以在该情况下抑制 SR2K 机制。这些现象进行了理论分析和数值模拟，并在溶液中[C，H]标记富马酸的二酯上进行了实验验证。
