Rouf Syed Awais, Mareš Jiří, Vaara Juha
NMR Research Unit, University of Oulu , P.O. Box 3000, Oulu FIN-90014, Finland.
J Chem Theory Comput. 2017 Aug 8;13(8):3731-3745. doi: 10.1021/acs.jctc.7b00168. Epub 2017 Jul 12.
We apply approximate relativistic methods to calculate the magnetic property tensors, i.e., the g-tensor, zero-field splitting (ZFS) tensor (D), and hyperfine coupling (HFC) tensors, for the purpose of constructing paramagnetic nuclear magnetic resonance (pNMR) shielding tensors. The chemical shift and shielding anisotropy are calculated by applying a modern implementation of the classic Kurland-McGarvey theory ( J. Magn. Reson. 1970 , 2 , 286 ), which formulates the shielding tensor in terms of the g- and HFC tensors obtained for the ground multiplet, in the case of higher than doublet multiplicity defined by the ZFS interaction. The g- and ZFS tensors are calculated by ab initio complete active space self-consistent field and N-electron valence-state perturbation theory methods with spin-orbit (SO) effects treated via quasidegenerate perturbation theory. Results obtained with the scalar relativistic (SR) Douglas-Kroll-Hess Hamiltonian used for the g- and ZFS tensor calculations are compared with nonrelativistically based computations. The HFC tensors computed using the fully relativistic four-component matrix Dirac-Kohn-Sham approach are contrasted against perturbationally SO-corrected nonrelativistic results in the density functional theory framework. These approximations are applied on paramagnetic metallocenes (MCp) (M = Ni, Cr, V, Mn, Co, Rh, Ir), a Co(II) pyrazolylborate complex, and a Cr(III) complex. SR effects are found to be small for g and D in these systems. The HFCs are found to be more influenced by relativistic effects for the 3d systems. However, for some of the 3d complexes, nonrelativistic calculations give a reasonable agreement with the experimental chemical shift and shielding anisotropy. The influence of scalar relativity is strong for the 5d IrCp system. This mixed ab initio/DFT technique, with a fully relativistic method used for the critical HFC tensor, should be useful for the treatment of both electron correlation and relativistic effects at a reasonable computational cost to compute the pNMR shielding tensors in transition metal systems.
我们应用近似相对论方法来计算磁性质张量，即g张量、零场分裂（ZFS）张量（D）和超精细耦合（HFC）张量，目的是构建顺磁核磁共振（pNMR）屏蔽张量。化学位移和屏蔽各向异性通过应用经典的Kurland-McGarvey理论（《磁共振杂志》，1970年，第2卷，第286页）的现代实现方式来计算，该理论在由ZFS相互作用定义的高于双重态多重性的情况下，根据基态多重态的g张量和HFC张量来表述屏蔽张量。g张量和ZFS张量通过从头算完全活性空间自洽场和N电子价态微扰理论方法计算，自旋轨道（SO）效应通过准简并微扰理论处理。将用于g张量和ZFS张量计算的标量相对论（SR）Douglas-Kroll-Hess哈密顿量得到的结果与基于非相对论的计算结果进行比较。在密度泛函理论框架下，将使用完全相对论四分量矩阵狄拉克-科恩-沙姆方法计算的HFC张量与微扰SO校正的非相对论结果进行对比。这些近似方法应用于顺磁金属茂（MCp）（M = Ni、Cr、V、Mn、Co、Rh、Ir）、一种Co（II）吡唑硼酸络合物和一种Cr（III）络合物。在这些体系中，发现SR效应对于g和D来说较小。对于3d体系，发现HFC受相对论效应的影响更大。然而，对于一些3d络合物，非相对论计算结果与实验化学位移和屏蔽各向异性给出了合理的一致性。对于5d IrCp体系，标量相对论的影响很强。这种混合的从头算/密度泛函理论技术，结合用于关键HFC张量的完全相对论方法，应以合理计算成本处理电子相关和相对论效应，对于计算过渡金属体系中的pNMR屏蔽张量应该是有用的。
