Concilio Maria Grazia, Soundararajan Murari, Frydman Lucio, Kuprov Ilya
Department of Chemical and Biological Physics, Weizmann Institute of Science, Rehovot, Israel; School of Chemistry, University of Southampton, Southampton, UK.
National High Magnetic Field Laboratory, Tallahassee, FL, USA.
J Magn Reson. 2021 May;326:106940. doi: 10.1016/j.jmr.2021.106940. Epub 2021 Feb 18.
At the magnetic fields of common NMR instruments, electron Zeeman frequencies are too high for efficient electron-nuclear dipolar cross-relaxation to occur in solution. The rate of that process fades with the electron Zeeman frequency as ω - in the absence of isotropic hyperfine couplings, liquid state dynamic nuclear polarisation (DNP) in high-field magnets is therefore impractical. However, contact coupling and dipolar cross-relaxation are not the only mechanisms that can move electron magnetisation to nuclei in liquids: multiple cross-correlated (CC) relaxation processes also exist, involving various combinations of interaction tensor anisotropies. The rates of some of those processes have more favourable high-field behaviour than dipolar cross-relaxation, but due to the difficulty of their numerical - and particularly analytical - treatment, they remain largely uncharted. In this communication, we report analytical evaluation of every rotationally driven relaxation process in liquid state for 1e1n and 2e1n spin systems, as well as numerical optimisations of the steady-state DNP with respect to spin Hamiltonian parameters. A previously unreported cross-correlated DNP (CCDNP) mechanism was identified for the 2e1n system, involving multiple relaxation interference effects and inter-electron exchange coupling. Using simulations, we found realistic spin Hamiltonian parameters that yield stronger nuclear polarisation at high magnetic fields than dipolar cross-relaxation.
在常见核磁共振仪器的磁场下，电子塞曼频率过高，以至于在溶液中无法有效发生电子 - 核偶极交叉弛豫。在没有各向同性超精细耦合的情况下，该过程的速率会随着电子塞曼频率以ω的形式衰减，因此在高场磁体中进行液态动态核极化（DNP）是不切实际的。然而，接触耦合和偶极交叉弛豫并不是液体中能将电子磁化转移到原子核的唯一机制：还存在多种交叉相关（CC）弛豫过程，涉及相互作用张量各向异性的各种组合。其中一些过程的速率在高场下的表现比偶极交叉弛豫更有利，但由于其数值处理（尤其是解析处理）难度较大，它们在很大程度上仍未被充分研究。在本通讯中，我们报告了对1e1n和2e1n自旋系统液态中每个旋转驱动弛豫过程的解析评估，以及关于自旋哈密顿量参数的稳态DNP数值优化。我们为2e1n系统确定了一种先前未报道的交叉相关DNP（CCDNP）机制，该机制涉及多种弛豫干涉效应和电子间交换耦合。通过模拟，我们找到了实际的自旋哈密顿量参数，这些参数在高磁场下能产生比偶极交叉弛豫更强的核极化。
