Temperature dependence of contact and dipolar NMR chemical shifts in paramagnetic molecules
Temperature dependence of contact and dipolar NMR chemical shifts in paramagnetic molecules
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DOI：
10.1063/1.4906318
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发表时间：
2015-02-07
影响因子：
4.4
通讯作者：
Autschbach, Jochen
中科院分区：
文献类型：
--
作者：
Martin, Bob;Autschbach, Jochen
使用最近提出的方程对具有不成对电子的分子进行 NMR 核磁屏蔽 [A. Soncini 和 W. Van den Heuvel，J. Chem。物理。 138, 021103 (2013)]，根据电子顺磁共振自旋哈密顿参数，导出了有效自旋 S 等于 1/2、1、3/2、2 和 5/2 的多重态的温度 (T) 相关各向同性屏蔽方程，然后以 1/T 的幂进行展开。使用的一种简化假设是，从零场分裂 (ZFS) 张量导出的矩阵和塞曼耦合矩阵（g 张量）共享相同的主轴系统。仅在 S = 1 时研究菱形 ZFS 参数 E 的影响。分别考虑顺磁接触屏蔽（来自超精细耦合矩阵的各向同性部分）和赝接触或偶极屏蔽（来自超精细耦合矩阵的各向异性部分）的表达式。领先顺序始终为 1/T。接触屏蔽的温度依赖性为 1/T，偶极屏蔽的温度依赖性为 1/T-2（有时在顺磁化学位移的分配中假设）仅当 S >= 1 且零场分裂明显时，并且仅当塞曼耦合矩阵接近各向同性（Delta g = 0）时，才会出现。在这种情况下，接触与偶极位移的分配可能仅基于测量的变温化学位移与 1/T 的线性和二次拟合。提供了二茂镍 (S = 1) 的数值数据。即使在 Delta g = 0 的假设下，对于 S = 3/2，也不会获得 1/T 幂中的接触和偶极位移的不同主导顺序。当 Delta g 不是很小时，偶极位移和接触位移在所有情况下都依赖于 1/T 的前导阶，并且在 1/T-n 阶中贡献相当大，其中 n = 2 及更高。 (C) 2015 AIP 出版有限责任公司。
Using a recently proposed equation for NMR nuclear magnetic shielding for molecules with unpaired electrons [A. Soncini and W. Van den Heuvel, J. Chem. Phys. 138, 021103 (2013)], equations for the temperature (T) dependent isotropic shielding for multiplets with an effective spin S equal to 1/2, 1, 3/2, 2, and 5/2 in terms of electron paramagnetic resonance spin Hamiltonian parameters are derived and then expanded in powers of 1/T. One simplifying assumption used is that a matrix derived from the zero-field splitting (ZFS) tensor and the Zeeman coupling matrix (g-tensor) share the same principal axis system. The influence of the rhombic ZFS parameter E is only investigated for S = 1. Expressions for paramagnetic contact shielding (from the isotropic part of the hyperfine coupling matrix) and pseudo-contact or dipolar shielding (from the anisotropic part of the hyperfine coupling matrix) are considered separately. The leading order is always 1/T. A temperature dependence of the contact shielding as 1/T and of the dipolar shielding as 1/T-2, which is sometimes assumed in the assignment of paramagnetic chemical shifts, is shown to arise only if S >= 1 and zero-field splitting is appreciable, and only if the Zeeman coupling matrix is nearly isotropic (Delta g = 0). In such situations, an assignment of contact versus dipolar shifts may be possible based only on linear and quadratic fits of measured variable-temperature chemical shifts versus 1/T. Numerical data are provided for nickelocene (S = 1). Even under the assumption of Delta g = 0, a different leading order of contact and dipolar shifts in powers of 1/T is not obtained for S = 3/2. When Delta g is not very small, dipolar and contact shifts both depend in leading order in 1/T in all cases, with sizable contributions in order 1/T-n with n = 2 and higher. (C) 2015 AIP Publishing LLC.