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Quadricyclane Radical Cation Rearrangements: A Computational Study of the Transformations to 1,3,5-Cycloheptatriene and Norbornadiene



An alternative skeletal rearrangement of the quadricyclane radical cation (Q.+) explains the side products formed in the one-electron oxidation to norbornadiene. First, the bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cation, with an activation energy of 14.9 kcal mol−1, is formed. Second, this species can further rearrange to 1,3,5-cycloheptatriene through two plausible paths, that is, a multistep mechanism with two shallow intermediates and a stepwise path in which the bicyclo[3.2.0]hepta-2,6-diene radical cation is an intermediate. The multistep rearrangement has a rate-limiting step with an estimated activation energy of 16.5 kcal mol−1, which is 2.8 kcal mol−1 lower in energy than the stepwise mechanism. However, the lowest activation energy is found for the Q.+ cycloreversion to norbornadiene that has a transition structure, in close correspondence with earlier studies, and an activation energy of 10.1 kcal mol−1, which agrees well with the experimental estimate of 9.3 kcal mol−1. The computational estimates of activation energies were done using the CCSD(T)/6–311+G(d,p) method with geometries optimized on the B3LYP/6–311+G(d,p) level, combined with B3LYP/6–311+G(d,p) frequencies.