engleski

On the Correlation Energy of pi-Electrons in Planar Hydrocarbons

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.

antiaromaticity; aromaticity; correlation energy; dynamical correlation; nondynamical correlation

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