Pravdivtsev Andrey N, Ivanov Konstantin L, Yurkovskaya Alexandra V, Petrov Pavel A, Limbach Hans-Heinrich, Kaptein Robert, Vieth Hans-Martin
International Tomography Center SB RAS, Institutskaya 3a, Novosibirsk 630090, Russia; Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia.
International Tomography Center SB RAS, Institutskaya 3a, Novosibirsk 630090, Russia; Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia.
J Magn Reson. 2015 Dec;261:73-82. doi: 10.1016/j.jmr.2015.10.006. Epub 2015 Oct 31.
We have investigated the magnetic field dependence of Signal Amplification By Reversible Exchange (SABRE) arising from binding of para-hydrogen (p-H2) and a substrate to a suitable transition metal complex. The magnetic field dependence of the amplification of the (1)H Nuclear Magnetic Resonance (NMR) signals of the released substrates and dihydrogen, and the transient transition metal dihydride species shows characteristic patterns, which is explained using the theory presented here. The generation of SABRE is most efficient at low magnetic fields due to coherent spin mixing at nuclear spin Level Anti-Crossings (LACs) in the SABRE complexes. We studied two Ir-complexes and have shown that the presence of a (31)P atom in the SABRE complex doubles the number of LACs and, consequently, the number of peaks in the SABRE field dependence. Interestingly, the polarization of SABRE substrates is always accompanied by the para-to-ortho conversion in dihydride species that results in enhancement of the NMR signal of free (H2) and catalyst-bound H2 (Ir-HH). The field dependences of hyperpolarized H2 and Ir-HH by means of SABRE are studied here, for the first time, in detail. The field dependences depend on the chemical shifts and coupling constants of Ir-HH, in which the polarization transfer takes place. A negative coupling constant of -7Hz between the two chemically equivalent but magnetically inequivalent hydride nuclei is determined, which indicates that Ir-HH is a dihydride with an HH distance larger than 2Å. Finally, the field dependence of SABRE at high fields as found earlier has been investigated and attributed to polarization transfer to the substrate by cross-relaxation. The present study provides further evidence for the key role of LACs in the formation of SABRE-derived polarization. Understanding the spin dynamics behind the SABRE method opens the way to optimizing its performance and overcoming the main limitation of NMR, its notoriously low sensitivity.
我们研究了由对氢（p-H₂）和底物与合适的过渡金属配合物结合引起的可逆交换信号放大（SABRE）的磁场依赖性。释放的底物和二氢以及瞬态过渡金属二氢化物物种的¹H核磁共振（NMR）信号放大的磁场依赖性呈现出特征模式，本文提出的理论对此进行了解释。由于SABRE配合物中核自旋能级反交叉（LACs）处的相干自旋混合，SABRE的产生在低磁场下最为有效。我们研究了两种铱配合物，并表明SABRE配合物中³¹P原子的存在使LACs的数量增加了一倍，因此也使SABRE磁场依赖性中的峰数增加了一倍。有趣的是，SABRE底物的极化总是伴随着二氢化物物种中对映体向邻位体的转化，这导致游离H₂和催化剂结合的H₂（Ir-HH）的NMR信号增强。本文首次详细研究了通过SABRE实现的超极化H₂和Ir-HH的磁场依赖性。磁场依赖性取决于发生极化转移的Ir-HH的化学位移和耦合常数。确定了两个化学等价但磁不等价的氢化物核之间的负耦合常数为-7Hz，这表明Ir-HH是一种HH距离大于2Å的二氢化物。最后，对先前发现的高场下SABRE的磁场依赖性进行了研究，并将其归因于通过交叉弛豫向底物的极化转移。本研究为LACs在SABRE衍生极化形成中的关键作用提供了进一步证据。理解SABRE方法背后的自旋动力学为优化其性能和克服NMR的主要局限性（其极低的灵敏度）开辟了道路。
