Quadricyclane radical cation Q(+): Formation and isomerization in liquid methylcyclohexane

The radical cation of quadricyclane (Q) was studied by pulse radiolysis at 133 K with methylcyclohexane (MCH) as solvent, saturated with N2O. The know n solvent radical cations, MCH+ and its precursor M+*, are expected to prod uce Q(+) by charge transfer. Although Q(+) was known not to absorb in the v isible lambda-range, there was a very early absorption band at lambda(max) = 720 nm (the transient is called Q(+)*), which eventually transformed into the cation of norbornadiene (NBD+) with lambda(max) = 650 nm. The analysis of the geminate ion kinetics with the semiempirical t(-0.6) kinetic law re vealed that Q(+)* decays faster than the isomer NBD+ is built up. Q(+)* mus t be a precursor to the nonabsorbing Q(+), which eventually isomerizes to N BD+, followed by a back reaction with Q to re-form Q(+). The quantitative a nalysis revealed that a substantial amount of the cations is lost before NB D+ is formed. This loss to a fragment or isomer (called F+) occurs from Q()*. As this loss dropped drastically for very low [Q], Q(+)* must increasin gly be bypassed by lowering [Q]. It turns out that Q(+)* is produced from M +* only (the higher energy precursor of the solvent radical cation MCH+) in competition with the transformation of M+* to MCH+, the latter becoming do minant at low [Q], increasingly producing Q(+) directly from MCH+ without g oing through Q(+)*. The loss yield (F+) correspondingly loses. The complete mechanism is given (Scheme 3). All the rate constants and the free ion con tributions of all cations were determined and, together with the known G(fi ) value, the absorption coefficients were derived. Comparing these results with a mechanism proposed recently by Adam et al. (J. Am. Chem. Sec. 1995, 117, 9693) suggests that Q(+)* corresponds to their cation Q(+)(l), where t he lateral bonds are oxidized, Q(+) to their cation Q(+)(i), where the inte rnal bonds are oxidized, and F+ the Q(+) isomer BHD+ (the bicyclo[3.2.0]hep ta-2,6-diene cation). The precursor ion M+* of the solvent, which is respon sible for the Q(+)* production must be of higher energy than MCH+; however, its structure remains unknown. The two precursor cations, M+* and Q(+)*, a re critically compared and discussed.