6533b82afe1ef96bd128ca29

RESEARCH PRODUCT

Molecular orbital studies on the mechanism of catalytic isomerization of xylenes II. The photochemical process

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Abstract The feasibility of an intermediate step in the mechanism of photochemical isomerization of xylenes in acidic media is explored by using CNDO molecular orbital methods. Along the reaction path from 2,6-dimethylbenzenium ion to dimethylbicyclo[3.1.0]hexenyl cation, the energy surfaces for this intermediate step have been drawn both for the first singlet S1 and triplet T1 excited states. The energies of these excited states have been calculated by adding to the ground-state energy calculated by CNDO/2 method the excitation energy calculated by CNDO/S-CI method. An optimization of the saddle point energy is achieved by introducing the angle between methyl groups and the ring plane as an additional parameter. The optimized energy results indicate that the cyclization process occurs with no energetic barrier from the first singlet excited state, while the calculated energy barrier for the cyclization in the ground state (thermal route) is about 46 kcal · mole−1.