Quadricyclane radical cation isomerization reactions: A theoretical study

Abstract

AbstractOn the C7H8·+ potential energy surface studied in this work, the electrocyclic isomerization of the quadricyclane (Q·+) to the norbornadiene (N·+) radical cations has attracted much attention. Our interest, however, concerns alternative rearrangement reactions on this surface. Using quantum chemical methods, B3LYP/6‐311+G(d,p), MP2/6‐31G(d,p), and single‐point CCSD(T)/6‐311+G(d,p), a skeletal rearrangement path leading from Q·+ to the bicyclo[3,2,0]hepta‐2,6‐diene radical cation (BHD·+) is found. The barrier for this path is 4.9 kcal/mol higher at the CCSD(T)/6‐311+G(d,p)//B3LYP/6‐311+G(d,p) level than the Q·+ to N·+ barrier. Moreover, this rearrangement is found to proceed through an intermediate, bicyclo[2.2.1]hepta‐2‐ene cation (BHE·+), with a conversion barrier to BHD·+ of 8.1 kcal/mol while the barrier for conversion back to Q·+ is 12.8 kcal/mol, computed with CCSD(T)/6‐311+G(d,p)//B3LYP/6‐311+G(d,p). Interestingly, the 1H hyperfine coupling constants computed for this intermediate structure show an excellent agreement with the experiment by Williams and coworkers. This ESR spectrum, observed after irradiation of N and Q loaded CF3CCl3 matrices, was assigned to BHD·+; our calculations, however, show that it is BHE·+ that gives rise to the observed spectrum. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002