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首页> 外文期刊>Chemistry: A European journal >Exceptionally long (>= 2.9 angstrom) CC bonding interactions in pi-[TCNE](2)(2) dimers: Two-electron four-center cation-mediated CC bonding interactions involving pi* electrons
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Exceptionally long (>= 2.9 angstrom) CC bonding interactions in pi-[TCNE](2)(2) dimers: Two-electron four-center cation-mediated CC bonding interactions involving pi* electrons

机译：pi- [TCNE]（2）（2）二聚体中异常长的（> = 2.9埃）CC键相互作用：涉及pi *电子的两电子四中心阳离子介导的CC键相互作用

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Three groups of singlet ground state [TCNE](2)(2) (TCNE=tetracyanoethylene) dimers with characteristic intradimer CC separations (r) and dihedral angles (d) [i.e., group S, (rsimilar to1.6 Angstrom; d=180degrees), L-t (rsimilar to3.5 Angstrom; d = 180degrees), and L-c (r similar to 2.9 Angstrom; d = similar to0degrees) notation: S/L: short/long bond length subscript t/c: trans/cis, respectively] are experimentally characterized. The S-t group is comprised of o-dimers of [TCNE](.-) and octacyanobutanediide, [C-4(CN)(8)](2). which have a typical, albeit long, sp(3)-sp(3) sigma bond (r similar to 1.6 Angstrom) between each [TCNE](.-) moiety and characteristic nu(CN), nu(CC), and delta(CCN) IR absorptions. The L groups are structurally characterized as pi-dimers of [TCNE](.) that are either eclipsed with r similar to 2.9 Angstrom (L-c) and the nitriles bend away from the nominal TCNE plane away from the center of the dimer by 5.0degrees (similar tosp(2.17)) or are noneclipsed with r similar to 3.5 Angstrom (L-t) and the nitriles bend toward the center of the dimer by 1.9degrees (similar tosp(2.06)). Ab initio computations on isolated dimers were used to study the formation and stability of these exceptionally long CC (greater than or equal to 2.9 Angstrom) bonding interactions as well as the process of pi-[TCNE](2)(2-) dimer formation for the L-c and L-t groups. The results Of these computational studies show that the ground-state potential curve is that of a closed-shell/open-shell singlet. depending on the distance. The short S-t group (r similar to 1.6 Angstrom) of dimers in this surface are true minimum-energy structures however, the L-t and L-c groups are unstable, although two different nonphysical minima are found when imposing a double occupancy of tire orbitals. These minima are metastable relative to dissociation into the isolated [TCNE](.-) units. Consequently, the existence of dimer dianions in crystals is due to cation ... [TCNE](-) interactions. which provide the electrostatic stabilization necessary to overcome the intradimer electrostatic repulsion. This cation-mediated pi*-pi* [TCNE] ... [TCNE] interaction complies with Pauling's definition of a chemical bond, This bonding interaction involves tire pi* orbitals of each fragment, and arise from the overlap of the b(2p) SOMO on each of the two [TCNE]. s to form a filled b(2a) [TCNE](2)(2) orbital. Although a pi dimer typically forms, if the fragments are close enough a sigma dimer can form. Due to the presence of cation-mediated intradimer CC bonding interactions the L-c group of pi-[TCNE]22 dimers exhibits experimentally observable v(CS) IR absorptions at 2191 l 2 (m) 2173 +/- 3 (s), and 2162 E3 cm(-1) (s) and v(CC) at 1364 +/- 3 cm(-1) (s) as well as a new UV-Vis feature in the range of 15 000 to 18 200 cm(-1) (549 to 667 nm) and averaging 16825 l 1180 cm(-1) (594 nm) assigned to the predicted new intradimer (1)A(1g) . B-1(10) transition and is purple on reflected light. Upon cooling to 77 K in 2-methyl tetrahydrofuran, this new band occurs at 18940 cm(-1) (528 nm) for {[Et4N] [TCNE], and the yellow solution turns deep red. Group L-t is characterized by v(CS) absorption sat 2215 l 2, 2197 +/- 3, and 2180 +/- 4 cm(-1) and v(CC) at 1209 l 9 cm(-1) (w), while group S-4 has P-CS bands at 2215 +/- 4, 2157 +/- 3 and 2107 4 cm(-1) and v(CC) at 1385 1 cm(-1) (vs). [References: 86]
机译：三组单峰基态[TCNE]（2）（2）（TCNE =四氰基乙烯）二聚体，其特征二聚体内CC间隔（r）和二面角（d）[即S组，（近似1.6埃; d = 180度），Lt（约3.5埃; d = 180度）和Lc（r约2.9埃; d =约0度）表示法：S / L：短/长键长下标t / c：反式/顺式，分别进行实验表征。 S-t基团由[TCNE]（.-）和辛酰基丁二酰亚胺[C-4（CN）（8）]（2）的邻二聚体组成。它们在每个[TCNE]（.-）部分与特征nu（CN），nu（CC）和delta之间具有典型的sp（3）-sp（3）σ键（尽管类似于1.6埃），但长（CCN）红外吸收。 L个基团在结构上被表征为[TCNE]（。）的pi二聚体，或者被r掩盖，类似于2.9埃（Lc），并且腈从标称TCNE平面向远离二聚体的中心弯曲5.0度。 （类似于sp（2.17））或不使用r类似于3.5埃（Lt）且腈向二聚体的中心弯曲1.9度（类似于sp（2.06））。使用孤立的二聚体从头算来研究这些异常长的CC（大于或等于2.9埃）键合相互作用的形成和稳定性，以及pi- [TCNE]（2）（2-）二聚体的形成过程Lc和Lt组。这些计算研究的结果表明，基态电势曲线是闭壳/开壳单线态的曲线。取决于距离。该表面上的短二聚体短S-t基团（近似于1.6埃）是真正的最小能量结构，但是L-t和L-c基团是不稳定的，尽管在强加轮胎轨道两次时发现了两个不同的非物理最小值。这些最小值相对于离解成孤立的[TCNE]（.-）单位是亚稳态的。因此，晶体中二聚二价阴离子的存在是由于阳离子[TCNE]（-）相互作用。提供克服二聚体内静电排斥所必需的静电稳定作用。这种阳离子介导的pi * -pi * [TCNE] ... [TCNE]相互作用符合Pauling对化学键的定义，该键相互作用涉及每个片段的轮胎pi *轨道，并且是由b（2p ）在两个[TCNE]上分别进行SOMO。形成一个填充的b（2a）[TCNE]（2）（2）轨道。尽管通常会形成pi二聚体，但如果片段足够接近，则会形成sigma二聚体。由于存在阳离子介导的二聚体CC键相互作用，因此pi- [TCNE] 22二聚体的Lc基团在2191 l 2（m）2173 +/- 3（s）和2162处显示实验可观察到的v（CS）IR吸收E3 cm（-1）（s）和v（CC）在1364 +/- 3 cm（-1）（s）以及15,000至18200 cm（-1）范围内的新UV-Vis功能）（549至667 nm），平均16825 l 1180 cm（-1）（594 nm）分配给预测的新二聚体（1）A（1g）。 B-1（10）过渡，在反射光下为紫色。在2-甲基四氢呋喃中冷却至77 K后，{[Et4N] [TCNE]的新谱带出现在18940 cm（-1）（528 nm），黄色溶液变为深红色。 Lt组的特征是v（CS）吸收饱和度2215 l 2、2197 +/- 3和2180 +/- 4 cm（-1），v（CC）在1209 l 9 cm（-1）（w），而S-4组的P-CS频段为2215 +/- 4、2157 +/- 3和2107 4 cm（-1），而v（CC）的频段为1385 1 cm（-1）（vs）。 [参考：86]

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