On the Correlation Energy of p-Electrons in Planar Hydrocarbons
On the Correlation Energy of p-Electrons in Planar Hydrocarbons
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关于平面烃中p电子的相关能
DOI：
10.1021/jp0015473
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发表时间：
2000
影响因子：
2.9
通讯作者：
I. Petanjek
中科院分区：
文献类型：
--
作者：
Z. Maksić;D. Barić;I. Petanjek
我们考虑了各种平面碳氢化合物中π电子的非动力学E（ND）^pi和动力学E（D）^pi相关能。前者可以方便地计算内的CASSCF形式主义，通过使用适度的基集。用CASPT 2方法研究了关联能的动力学部分。似乎E（D）^pi对所采用的基组敏感。我们还发现，从头计算的E（ND）^pi和E（D）^pi值遵循非常简单的加和规则，这使得我们可以很好地估计π电子的非动力学和动力学关联效应，只需计算碳原子和氢原子。在苯（2.1 kcal/mol）、萘（3.3 kcal/mol）和环丁二烯（~ 3.0 kcal/mol）中发现了与E（D）^pi的加和性的小偏差，这表明在将加和性规则应用于（反）芳族分子时必须小心。非动力学关联E（ND）^pi在具有较大π电子离域的体系中比在线性多烯中表现出更明显的偏离可加性。介绍了一种新的静电学+相关性解释的（反）芳香性，它揭示了一个古老的，但中心的化学问题的新的光。还建议应区分内芳香性和外芳香性。计算结果表明，在环丁二烯（CBD）过渡结构（TS）中，Hartree-Fock核-电子相互作用V_ne和电子-电子相互作用V_ee以及非动力学关联比在基态（GS）中有利得多.然而，CBD（TS）中的压倒性作用似乎是核排斥V_nn的增加，比（GS）高86.8 kcal/mol。因此，CBD在GS中呈现矩形几何形状的倾向阿利亚是因为核排斥力的显著减轻而发生。而苯的GS则相反，正六边形中占主导地位的V_ne超过了由D_6h形成引起的V_ee和V_nn排斥的增加。通过将CBD（GS）和苯的E（ND）^pi与相应的线性多烯的E（ND）^pi进行比较，得到了有趣和反直觉的结果。前者比1，3-丁二烯高8千卡/摩尔，而后者比己三烯低5.7千卡/摩尔。发现CBD和苯相对于1，3-丁二烯的（反）芳（脱）稳量分别为40.7和28.4kcal/mol。事实证明，这两种化合物上的V_ne吸引力明显更高（即，不太有利）。然而，这在苯中被更有利的V_ee和V_nn项过度补偿，而CBD的情况并非如此。这种差异使得苯是外芳香的，CBD是外反芳香的。
We considered nondynamical E(ND)^pi and dynamical E(D)^pi correlation energies of pi electrons in a wide variety of planar hydrocarbons. The former could be conveniently calculated within the CASSCF formalism by using modest basis sets. The dynamical part of the correlation energy was studied by the CASPT2 method. It appeared that E(D)^pi was sensitive to the employed basis set. It is also found that the ab initio E(ND)^pi and E(D)^pi values follow very simple additivity rules, which allow fairly good estimates of the nondynamical and dynamical correlation effects of pi-electrons, simply by counting carbon and hydrogen atoms. Small deviations from the additivity of E(D)^pi are found in benzene (2.1 kcal/mol), naphthalene (3.3 kcal/mol) and cyclobutadiene (-3.0 kcal/mol) indicating that some care has to be exercised in applying the additivity rules to (anti)aromatic molecules. Nondynamical correlation E(ND)^pi exhibits even more pronounced deviations from the additivity in systems characterized by larger pi-electron delocalization than in linear polyenes. A novel electrostatics + correlation interpretation of (anti)aromaticity is introduced, which sheds new light on an old but central problem of chemistry. It is also suggested that one should distinguish between endo- and exo-aromaticity. An interesting result emerging from the present calculations is given by the fact that the Hartree-Fock nuclear-electron V_ne and electron-electron V_ee interactions as well as the nondynamical correlation are much more favourable in the cyclobutadiene (CBD) transition structure (TS) than in its ground state (GS). It appears, however, that the overwhelming effect in CBD(TS) is an increase in the nuclear repulsion V_nn, which is higher by 86.8 kcal/mol than in the (GS). Consequently, the propensity of CBD to assume a rectangular geometry in the GS occurs inter alia because of a dramatic relief in the nuclear repulsion. The opposite is the case in the GS of benzene, where the dominating V_ne in the regular hexagon prevails over an increase in V_ee and V_nn repulsions caused by the D_6h formation. Intriguing and counterintutitive results are obtained by comparing the E(ND)^pi of the CBD(GS) and benzene with that of corresponding linear polyenes. The former is higher by 8 kcal/mol than in the 1, 3-butadiene, whereas the latter is lower by 5.7 than that in hexatriene (in kcal/mol). It is found that (anti)aromatic (de)stabilization of CBD and benzene relative to the 1, 3-butadiene are 40.7 and 28.4 kcal/mol, respectively. It turns out that V_ne attraction on both compounds is appreciably higher (i.e., less favourable) than that in the reference molecule 1, 3-butadiene. However, this is overcompensated in benzene by more advantageous V_ee and V_nn terms, whilst it is not the case for CBD. This difference makes benzene exo-aromatic and CBD exo-antiaromatic.