NMR Research Group, Department of Physics, P.O. Box 3000, FI-90014 University of Oulu, Finland.
Phys Chem Chem Phys. 2013 Oct 28;15(40):17468-78. doi: 10.1039/c3cp51904j.
Secondary isotope effects on nuclear shielding provide an experimentally well-defined reference point of quantum-chemical methodology. We carry out a quantum-mechanical treatment of thermal rovibrational motion in the linear CSe2 molecule and combine it with relativistic modeling of (77)Se and (13)C nuclear magnetic shieldings. The effects of electron correlation are studied at nonrelativistic (NR) ab initio and both NR and relativistic density functional theory (DFT) levels. Fully relativistic 4-component Dirac-DFT (D-DFT) as well as Breit-Pauli perturbation theory (BPPT) are used to produce the relativistic shielding surfaces. Quantitative agreement with the experimental secondary isotope effects can be obtained by a piecewise combination of a high-level ab initio NR shielding surface with the more approximate, albeit reasonable-quality relativistic corrections by DFT at either Dirac or BPPT levels of theory, when operating at the basis-set limit. Using a high-quality potential energy surface is also crucial, which further underlines the highly demanding nature of modeling of the isotope shifts.
次级同位素效应对核屏蔽提供了一个实验上定义良好的量子化学方法的参考点。我们对线性 CSe2 分子中的热振动运动进行了量子力学处理，并将其与（77）Se 和（13）C 核磁屏蔽的相对论建模相结合。在非相对论（NR）从头算和 NR 和相对论密度泛函理论（DFT）水平上研究了电子相关的影响。完全相对论的 4 分量狄拉克-DFT（D-DFT）以及 Breit-Pauli 微扰理论（BPPT）用于产生相对论屏蔽表面。通过在基组极限下将高水平的非相对论 NR 屏蔽表面与更近似但合理质量的相对论修正（通过 DFT 在 Dirac 或 BPPT 理论水平上）分段组合，可以获得与实验次级同位素效应的定量一致。使用高质量的势能面也至关重要，这进一步强调了对同位素位移进行建模的高度要求的性质。
