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New insights into frustrated Lewis pairs: structural investigations of intramolecular phosphane-borane adducts by using modern solid-state NMR techniques and DFT calculations

机译:沮丧的路易斯对的新见解:使用现代固态NMR技术和DFT计算的分子内膦-硼烷加合物的结构研究

摘要

Covalent bonding interactions between the Lewis acid and Lewis base functionalities have been probed in a series of “frustrated Lewis pairs” (FLPs) (mainly substituted vinylene linked intramolecular phosphane–borane adducts), using solid-state nuclear magnetic resonance techniques and accompanying DFT calculations. Both the 11B NMR isotropic chemical shifts and nuclear electric quadrupolar coupling parameters turn out to be extremely sensitive experimental probes for such interactions, revealing linear correlations with boron–phosphorus internuclear distances. The principal component Vzz of the 11B electric field gradient tensor is tilted slightly away (∼20°) from the boron–phosphorus internuclear vector, leading to an improved understanding of the remarkable reactivity of the FLPs. Complementary 31P{1H}-CPMAS experiments reveal significant 31P–11B scalar spin–spin interactions (1J ≈ 50 Hz), evidencing covalent bonding interactions between the reaction centers. Finally, 11B{31P} rotational echo double resonance (REDOR) experiments show systematic deviations from calculated curves based on the internuclear distances from X-ray crystallography. These deviations suggest non-zero contributions from anisotropic indirect spin–spin (J anisotropy) interactions, thereby offering additional evidence for covalent bonding.
机译:路易斯酸和路易斯碱官能团之间的共价键相互作用已通过固态核磁共振技术和伴随的DFT计算在一系列“沮丧的路易斯对”(FLP)(主要是取代的亚乙烯基连接的分子内膦-硼烷加合物)中进行了探索。 。事实证明,11B NMR各向同性化学位移和核电四极耦合参数都是此类相互作用的极其灵敏的实验探针,显示出与硼-磷核间距离的线性相关性。 11B电场梯度张量的主分量Vzz从硼-磷核间矢量略微倾斜(约20°),从而使人们对FLP的显着反应性有了更好的了解。互补的31P {1H} -CPMAS实验揭示了显着的31P-11B标量自旋-自旋相互作用(1J≈50 Hz),证明了反应中心之间的共价键相互作用。最后,11B {31P}旋转回波双共振(REDOR)实验表明,基于与X射线晶体学的核间距,计算出的曲线存在系统偏差。这些偏差表明各向异性间接自旋-自旋(J各向异性)相互作用的非零贡献,从而为共价键合提供了更多证据。

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