Institute of Child Health, University College London, 30 Guilford Street, London WC1N 1EH, UK.
Phys Chem Chem Phys. 2012 Apr 28;14(16):5397-402. doi: 10.1039/c2cp23536f. Epub 2012 Mar 12.
Over the years, several strategies have been developed for generating highly polarized nuclear spin systems, including dynamic nuclear polarization, optical pumping, and methods exploiting parahydrogen. Here, we present an alternative strategy, using an enhanced 'brute-force' approach (i.e. exposure to low temperatures and high applied magnetic fields). The main problem with this approach is that it may take an excessively long time for the nuclear polarization to approach thermal equilibrium at low temperatures, since nuclear relaxation becomes exceedingly slow due to the loss of molecular motion. We show that low-field thermal mixing can alleviate the problem by increasing the rate at which slowly-relaxing nuclei reach equilibrium. More specifically, we show that polarization can be transferred from a relatively rapidly relaxing (1)H reservoir to more slowly relaxing (13)C and (31)P nuclei. The effects are particularly dramatic for the (31)P nuclei, which in experiments at a temperature of 4.2 K and a field of 2 T show a 75-fold enhancement in their effective rate of approach to equilibrium, and an even greater (150-fold) enhancement in the presence of a relaxation agent. The mixing step is also very effective in terms of the amount of polarization transferred-70-90% of the maximum theoretical value in the experiments reported here. These findings have important implications for brute-force polarization, for the problem becomes one of how to relax the solvent protons rather than individual more slowly-relaxing nuclei of interest. This should be a much more tractable proposition, and offers the additional attraction that a wide range of nuclear species can be polarized simultaneously. We further show that the (1)H reservoir can be tapped repeatedly through a number of consecutive thermal mixing steps, and that this could provide additional sensitivity enhancement in solid-state NMR.
多年来，人们已经开发出多种产生高度极化核自旋系统的策略，包括动态核极化、光学泵浦以及利用仲氢的方法。在这里，我们提出了一种替代策略，采用增强的“暴力”方法（即暴露于低温和强磁场下）。这种方法的主要问题是，在低温下，核极化达到热平衡可能需要过长的时间，因为由于分子运动的损失，核弛豫变得非常缓慢。我们表明，低场热混合可以通过增加缓慢弛豫核达到平衡的速率来缓解该问题。更具体地说，我们表明极化可以从相对快速弛豫的（1）H储层转移到更缓慢弛豫的（13）C和（31）P核。对于（31）P核，这种效应特别明显，在 4.2 K 温度和 2 T 场的实验中，其有效接近平衡的速率提高了 75 倍，在存在弛豫剂的情况下，提高了 150 倍。混合步骤在转移的极化量方面也非常有效-在本文报道的实验中，达到了最大理论值的 70-90％。这些发现对强力极化具有重要意义，因为问题变成了如何弛豫溶剂质子，而不是个体更缓慢弛豫的感兴趣核。这应该是一个更容易处理的问题，并且具有同时极化多种核物种的额外吸引力。我们进一步表明，可以通过多个连续的热混合步骤重复利用（1）H储层，这可以在固态 NMR 中提供额外的灵敏度增强。
