NMR Research Group, Department of Physics, University of Oulu, P.O. Box 3000, FIN-90014, Finland.
Phys Chem Chem Phys. 2010 Mar 20;12(11):2679-92. doi: 10.1039/b923506j. Epub 2010 Jan 27.
The series of methyl halides, CH(3)X (X = F, Cl, Br, and I), is prototypic for demonstrating the s.c. normal halogen dependence of light-atom nuclear magnetic resonance shielding constants in the presence of halogen atoms of varying electronegativity. We report a systematic experimental and first-principles theoretical study of the (13)C and (1)H shielding tensors in this series. The experimental shielding constants were obtained from gas-phase NMR experiments and the anisotropies were determined using liquid crystal NMR spectroscopy. After taking into account rovibrational effects and solute-solvent interactions, this provided the currently best experimental estimates for the full shielding tensors. Quantum chemical calculations were carried out at ab initio and density functional theory levels, involving relativistic corrections taken into account at the leading-order Breit-Pauli perturbation level. Anharmonic and harmonic vibrational corrections were performed. The main trends of the shielding constants and anisotropies of the nearby light (13)C and (1)H nuclei as functions of the halogen mass, were confirmed to be mainly due to relativistic spin-orbit effects. For carbon, also the scalar relativistic effects are important for quantitative results. Thermal averaging at 300 K decreases the magnitude of all the parameters but exhibits partial cancellation between the nonrelativistic and smaller relativistic rovibrational averages. For the shielding anisotropy, the relativistic terms add to the negative rovibrational effect. Overall, the current experimental and theoretical results are in excellent agreement for all the shielding parameters, setting a standard for further investigations of normal halogen dependence.
该系列的甲基卤化物，CH(3)X（X=F、Cl、Br 和 I），是展示在不同电负性卤素原子存在下轻原子核磁共振屏蔽常数的 s.c. 正常卤素依赖性的典型范例。我们报告了对此系列的（13）C 和（1）H 屏蔽张量的系统实验和第一性原理理论研究。实验屏蔽常数是从气相 NMR 实验中获得的，各向异性是通过液晶 NMR 光谱学确定的。在考虑到 rovibrational 效应和溶质-溶剂相互作用之后，这为完整的屏蔽张量提供了目前最好的实验估计值。量子化学计算是在从头算和密度泛函理论水平上进行的，涉及到在领先阶 Breit-Pauli 微扰水平上考虑相对论修正。进行了非谐和调和振动修正。作为卤素质量的函数，附近轻（13）C 和（1）H 核的屏蔽常数和各向异性的主要趋势被证实主要归因于相对论自旋轨道效应。对于碳，标量相对论效应对于定量结果也很重要。300 K 的热平均会降低所有参数的幅度，但在非相对论和较小的相对论 rovibrational 平均值之间会产生部分抵消。对于屏蔽各向异性，相对论项会增加负的 rovibrational 效应。总体而言，所有屏蔽参数的当前实验和理论结果都非常吻合，为进一步研究正常卤素依赖性设定了标准。
