No abstract

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.

A computational study is undertaken to provide a unified picture for various rearrangement reactions and hydrogen scrambling pathways of the toluene radical cation (1). The geometries are optimized with the BHandHLYP density functional, and the energies are computed with the ab initio CCSD(T) method, in conjunction with the 6-311+G(d,p) basis set. In particular, four channels have been located, which may account for hydrogen scrambling, as they are found to have overall barriers lower than the observed threshold for hydrogen dissociation. These are a stepwise norcaradiene walk involved in the Hoffman mechanism, a rearrangement of 1 to the methylenecyclohexadiene radical cation (5) by successive [1,2]-H shifts via isotoluene radical cations, a series of [1,2]-H shifts in the cycloheptatriene radical cation (4), and a concerted norcaradiene walk. In addition, we have also investigated other pathways such as the suggested Dewar-Landman mechanism, which proceeds through 5, via two consecutive [1,2]-H shifts. This pathway is, however, found to be inactive as it involves too high reaction barriers. Moreover, a novel rearrangement pathway that connects 5 to the norcaradiene radical cation (3) has also been located in this work.

On the C 7 H 8 ⅐ϩ potential energy surface studied in this work, the electrocyclic isomerization of the quadricyclane (Q ⅐؉ ) to the norbornadiene (N ⅐؉ ) radical cations has attracted much attention. Our interest, however, concerns alternative rearrangement reactions on this surface. Using quantum chemical methods, B3LYP/6-311ϩG(d,p), MP2/6-31G(d,p), and single-point CCSD(T)/6-311ϩG(d,p), a skeletal rearrangement path leading from Q ⅐؉ to the bicyclo[3,2,0]hepta-2,6-diene radical cation (BHD ⅐؉ ) is found. The barrier for this path is 4.9 kcal/mol higher at the CCSD(T)/6-311ϩG(d,p)//B3LYP/6-311ϩG(d,p) level than the Q ⅐؉ to N ⅐؉ barrier. Moreover, this rearrangement is found to proceed through an intermediate, bicyclo[2.2.1]hepta-2-ene cation (BHE ⅐؉ ), with a conversion barrier to BHD ⅐؉ of 8.1 kcal/mol while the barrier for conversion back to Q ⅐؉ is 12.8 kcal/mol, computed with CCSD(T)/6-311ϩG(d,p)// B3LYP/6-311ϩG(d,p). Interestingly, the 1 H 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 CF 3 CCl 3 matrices, was assigned to BHD ⅐؉ ; our calculations, however, show that it is BHE ⅐؉ that gives rise to the observed spectrum.