Zhan Chang-Guo, Zheng Fang, Dixon David A
Department of Medicine, College of Physician & Surgeons, Columbia University, New York, New York 10032, USA.
J Am Chem Soc. 2002 Dec 11;124(49):14795-803. doi: 10.1021/ja021026o.
The concept of aromaticity was first invented to account for the unusual stability of planar organic molecules with 4n + 2 delocalized pi electrons. Recent photoelectron spectroscopy experiments on all-metal MAl(4)(-) systems with an approximate square planar Al(4)(2-) unit and an alkali metal led to the suggestion that Al(4)(2-) is aromatic. The square Al(4)(2-) structure was recognized as the prototype of a new family of aromatic molecules. High-level ab initio calculations based on extrapolating CCSD(T)/aug-cc-pVxZ (x = D, T, and Q) to the complete basis set limit were used to calculate the first electron affinities of Al(n)(), n = 0-4. The calculated electron affinities, 0.41 eV (n = 0), 1.51 eV (n = 1), 1.89 eV (n = 3), and 2.18 eV (n = 4), are all in excellent agreement with available experimental data. On the basis of the high-level ab initio quantum chemical calculations, we can estimate the resonance energy and show that it is quite large, large enough to stabilize Al(4)(2-) with respect to Al(4). Analysis of the calculated results shows that the aromaticity of Al(4)(2-) is unusual and different from that of C(6)H(6). Particularly, compared to the usual (1-fold) pi aromaticity in C(6)H(6), which may be represented by two Kekulé structures sharing a common sigma bond framework, the square Al(4)(2-) structure has an unusual "multiple-fold" aromaticity determined by three independent delocalized (pi and sigma) bonding systems, each of which satisfies the 4n + 2 electron counting rule, leading to a total of 4 x 4 x 4 = 64 potential resonating Kekulé-like structures without a common sigma frame. We also discuss the 2-fold aromaticity (pi plus sigma) of the Al(3)(-) anion, which can be represented by 3 x 3 = 9 potential resonating Kekulé-like structures, each with two localized chemical bonds. These results lead us to suggest a general approach (applicable to both organic and inorganic molecules) for examining delocalized chemical bonding. The possible electronic contribution to the aromaticity of a molecule should not be limited to only one particular delocalized bonding system satisfying a certain electron counting rule of aromaticity. More than one independent delocalized bonding system can simultaneously satisfy the electron counting rule of aromaticity, and therefore, a molecular structure could have multiple-fold aromaticity.
芳香性的概念最初是为了解释具有4n + 2离域π电子的平面有机分子的异常稳定性而提出的。最近，对具有近似平面正方形Al₄²⁻单元和碱金属的全金属MAl₄⁻体系进行的光电子能谱实验表明Al₄²⁻具有芳香性。正方形Al₄²⁻结构被认为是一类新的芳香分子家族的原型。基于将CCSD(T)/aug-cc-pVxZ（x = D、T和Q）外推到完整基组极限的高水平从头算计算，用于计算Alₙ⁻（n = 0 - 4）的第一电子亲和能。计算得到的电子亲和能，0.41 eV（n = 0）、1.51 eV（n = 1）、1.89 eV（n = 3）和2.18 eV（n = 4），与现有的实验数据都非常吻合。基于高水平的从头算量子化学计算，我们可以估算共振能，并表明其相当大，大到足以使Al₄²⁻相对于Al₄稳定。对计算结果的分析表明，Al₄²⁻的芳香性不同寻常，与苯（C₆H₆）的芳香性不同。特别是，与苯中通常的（1重）π芳香性（可以由共享一个共同σ键框架的两个凯库勒结构表示）相比，正方形Al₄²⁻结构具有由三个独立的离域（π和σ）键合体系决定的异常“多重”芳香性，每个体系都满足4n + 2电子计数规则，导致总共4×4×4 = 64个潜在的类似凯库勒结构的共振结构，且没有共同的σ框架。我们还讨论了Al₃⁻阴离子的2重芳香性（π加σ），它可以由3×3 = 9个潜在的类似凯库勒结构的共振结构表示，每个结构有两个定域化学键。这些结果使我们提出一种用于研究离域化学键的通用方法（适用于有机和无机分子）。分子芳香性可能的电子贡献不应仅限于满足某种芳香性电子计数规则的一个特定离域键合体系。不止一个独立的离域键合体系可以同时满足芳香性的电子计数规则，因此，分子结构可以具有多重芳香性。
