High-mobility ions in cyclohexane. A transient absorption study

Transient absorption kinetics in radiolysis of N2O-saturated cyclohexane has been studied (0.1-100 ns; 300-800 nm). The spectra indicate the involvement of at least three cations (ions I, II, and III), only one of them having abnormally high mobility. Ion II is probably the cyclohexene radical cation, and ion III might be the dimer olefin ion. These two ions absorb as much as ion I at 450-500 nm. While ion II and ion III are scavenged by ethanol and triethylamine with a rate constant of approximate to 10(10) mol(-1) dm(3) s(-1), the scavenging of ion I proceeds with rate constants of approximate to 9 x 10(10) and 2.3 x 10(11) mol(-1) dm(3) s(-1), respectively. The spectrum of ion I is similar to the spectrum of the radical cation of cyclohexane isolated in low-temperature matrices. We were not able to observe the absorption from ion I at delay times longer than 50 ns. A corresponding fast growth of the absorption from solute radical cations of pyrene and perylene was observed. The data (simulated using continuum-diffusion and Monte Carlo approaches) indicate that the scavenging constant is approximate to 4 x 10(11) mol(-1) dm(3) s(-1); the lifetime of the precursor of the aromatic radical cations is approximate to 30 ns. This short lifetime cannot be explained by a reaction with radiolytic products or by homogeneous recombination, and it seems to be incompatible with identification of the long-lived high-mobility ions observed in conductivity experiments as the radical cation of cyclohexane. A mechanism in which the mobile radical cation is in equilibrium with a normally-diffusing ion is examined in an attempt to resolve this conundrum.