Dominant Role of the ð Framework in Cyclobutadiene

The extrinsic antiaromaticity of archetypal cyclobutadiene (CBD) is addressed with particular emphasis on the ó-ð separability problem. The destabilization energy E(d)CBD of CBD is obtained by appropriate homodesmotic reactions involving the open chain zigzag polyene(s). It is shown that E(d)CBD does not depend on the electron correlation and the zero-point vibrational energy contributions, since they are small and of the opposite sign. Consequently, they cancel in the first approximation. Further, it turns out that E(d)CBD can be estimated accurately enough with a very modest cc-pVDZ basis set at the Hartree-Fock (HF) level. The extrinsic antiaromatic destabilization E(ean)CBD of CBD is deduced after extracting the angular strain energy estimated to be 32 kcal/mol. The resulting E(ean)CBD value of 52 kcal/mol is in excellent agreement with the experimental thermodynamic data. If the E(ean)CBD is estimated relative to two isolated CdC double bonds, then it assumes 38 kcal/mol, which is roughly 10 kcal/mol per one ð electron. It is, therefore, safe to state that extrinsic antiaromaticity of CBD is larger than its angular strain. Although the ó and ð electrons are coupled by a mutual Coulomb interaction Vee óð, several attempts of their decoupling is made by using three partitioning schemes: stockholder, equipartition, and standard ð-electron theory recipe. The latter allocates the Vnn and Vee óð terms to the ó- and ð-electron frameworks, respectively. The nuclear repulsion term Vnn is dissected into ó and ð components in the former two partitioning schemes by using stockholder criterion. It appears that the extrinsic antiaromatic destabilization E(ean)CBD is determined by the ð-electron framework according to all three partitioning models. 1