The potential energy surfaces for the electrocyclic reactions of o-xylylene to benzocyclobutene were calculated by ab initio molecular orbital methods. The transition states of two reaction pathways, conrotatory and disrotatory, for the electrocyclic reaction of o-xylylene were obtained. The difference of energy barriers on the conrotatory and the disrotatory pathways is extremely low: 7.4 kcal/mol as determined by complete active space self-consistent field (CASSCF) calculations and 8.0 kcal/mol as determined by complete active space second-order Moller-Plesset perturbation (CAS-MP2) calculations. The energy difference due to the orbital phase such as the orbital symmetry rules was also estimated; 3.5 kcal/mol at the CAS-MP2 calculations. These mechanisms along the conrotatory and the disrotatory reaction pathways were analyzed by the configuration interactions (Cl)-localized molecular orbital (LMO)-CASSCF along the intrinsic coordinate (IRC) pathway (CiLC-IRC) method.
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