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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Solid-state NMR spectra and long intradimer bonds in the pi-[TCNE](2)(2-) dianion
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Solid-state NMR spectra and long intradimer bonds in the pi-[TCNE](2)(2-) dianion

机译：π-[TCNE]（2）（2-）二价阴离子的固态NMR光谱和较长的二聚物内键

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摘要

The principal C-13 chemical-shift values for the pi-[TCNE](2)(2-) dimer anion within an array of counterions have been measured to understand better the electronic structure of these atypical chemical species in several related TCNE-based structures. The structure of pi-[TCNE](2)(2-) is unusual as it contains two very long C-C bond lengths (ca. 2.9 angstrom) between the two monomeric units and has been found to exist as a singlet state, suggestive of a (1)A(1)g (b(2u)(2)b(1g)(0)) electronic configuration. A systematic study of several oxidation states of [TCNE](n) (n = 0, 1-, 2-) was conducted to determine how the NMR chemical-shift tensor values change as a function of electronic structure and to understand the interactions that lead to spin-pairing of the monomer units. The density functional theory (DFT) calculated nuclear shielding tensors are correlated with the experimentally determined principal chemical-shift values. Such theoretical methods provide information on the tensor magnitudes and orientations of their principal tensor components with respect to the molecular frame. Both theoretical and experimental ethylenic chemical-shielding tensors reveal high sensitivity in the component, delta(perpendicular to), lying in the monomer molecular plane and perpendicular to the pi-electron plane. This largest shift dependence on charge density is observed to be about -111 ppm/e(-) for delta(perpendicular to). The component in the molecular plane but parallel to the central C=C bond, delta, exhibits a sensitivity of approximately -43 ppm/ e(-). However, the out-of-plane component delta'(perpendicular to) shows a minimal dependence of -2.6 ppm/e(-) on the oxidation state (n) of [TCNE](n). These relative values support the claim that it is changes within the ethylenic pi- electrons and not the sigma-electrons that best account for the dramatic variations in bonding and shift tensors in this series of compounds. Concerning the intraion bonding, relatively weak Wiberg bond orders between the two monomeric components of the dimer correlate with the long bonds linking the two [TCNE center dot](-) monomers. The chemical-shift tensors for the cyano group, compared to the ethylene shifts, exhibit a reduced sensitivity on the TCNE oxidation state. The experimental principal chemical-shift components agree ( within typical errors) with the calculated quantum mechanical shieldings used to correlate the bonding. The embedded ion model (EIM) was used to investigate the typically large electrostatic lattice potential in these ionic materials. Chemical-shielding principal values calculated with the EIM model differ from experiment by +/- 3.82 ppm on average, whereas in the absence of an electrostatic field model, the experimental and theoretical results agree by +/- 4.42 ppm, which is only a modest increase in error considering the overall ionic magnitudes associated with the tensor variations. Apparently, the effects of the sizable long-range electrostatic fields cancel when the shifts are computed because of lattice symmetry.
机译：已测量了一系列抗衡离子中pi- [TCNE]（2）（2-）二聚体阴离子的主要C-13化学位移值，以更好地了解几种基于TCNE的相关非典型化学物种的电子结构结构。 pi- [TCNE]（2）（2-）的结构与众不同，因为它在两个单体单元之间包含两个非常长的CC键长度（约2.9埃），并且发现它以单重态存在，这表明a（1）A（1）g（b（2u）（2）b（1g）（0））电子配置。对[TCNE]（n）（n = 0，1-，2-）的几种氧化态进行了系统研究，以确定NMR化学位移张量值如何随电子结构变化并了解相互作用。导致单体单元的旋转配对。密度泛函理论（DFT）计算的核屏蔽张量与实验确定的主要化学位移值相关。这种理论方法提供了有关其主要张量分量的张量大小和方向相对于分子框架的信息。理论上和实验上的乙烯化学屏蔽张量都显示出位于单体分子平面内且垂直于pi电子平面的delta（垂直于）分量的高灵敏度。对于δ（垂直于），观察到对电荷密度的最大位移依赖性约为-111ppm / e（-）。分子平面中与中心C = C键平行的组分δ表现出大约-43 ppm / e（-）的灵敏度。但是，面外成分δ'（垂直于）显示出-2.6 ppm / e（-）对[TCNE]（n）的氧化态（n）的最小依赖性。这些相对值支持这样的说法，即在该系列化合物中，键合π电子和σ电子的变化最能说明键合和位移张量的剧烈变化。关于离子内键合，二聚体的两个单体组分之间相对弱的Wiberg键序与连接两个[TCNE中心点]（-）单体的长键相关。与亚乙基位移相比，氰基的化学位移张量对TCNE氧化态的敏感性降低。实验的主要化学位移成分（在典型误差范围内）与用于使键相关的计算量子力学屏蔽相符。嵌入式离子模型（EIM）用于研究这些离子材料中通常较大的静电晶格势。用EIM模型计算出的化学屏蔽主值与实验平均相差+/- 3.82 ppm，而在没有静电场模型的情况下，实验和理论结果相差+/- 4.42 ppm，这只是一个中等水平考虑到与张量变化相关的整体离子强度，误差增加。显然，由于晶格对称性，在计算位移时，相当大的远程静电场的影响会消失。

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