The Radical Cation of Methylcyclohexane

Article abstract of DOI:10.1021/j100034a021

It is shown by pulse radiolysis that the transient absorption in N2O-saturated or CO2-saturated methylcyclohexane (MCH), peaking at about 570 nm, is due to the solvent radical cation MCH⁺.From simulations with the semiempirical t^-0.6 rate law, it is concluded that MCH⁺ is of very high mobility: at 133 K (supercooled liquid) D_exp = (1.06 +/- 0.2) x 10^-6 cm² s^-1, which is ca. 400 times faster than expected from diffusion.At room temperature MCH⁺ is about 11 times faster than diffusion, and this is in perfect agreement with the conductivity of MCH⁺, as measured by Warman et al.For the high mobility of MCH⁺ an activation energy of 8.9 +/- 0.3 kJ/mol is found.The rate constant for scavenging MCH⁺ with norbornadiene (NBD) is k₂(133 K) = (1.8 +/- 0.5) x 10⁸ M^-1 s^-1.This is again 80 times faster than the diffusional rate constant.From the intercepts of the semiempirical t^-0.6 linearity plots, the free ion spectra were derived.The free ion absorbance at 133 K turns out to be 2.0 times smaller than that at room temperature.The free ion yield at low temperatures therefore was derived to be G_fi(127-153 K) = 0.06 +/- 0.015 (100 eV)^-1.From the free ion intercept at room temperature the absorption coefficient ε was determined: ε_570nm(MCH⁺) >/= 2300 M^-1 cm^-1.Without the electron scavengers N2O or CO2 the olefinic cation methylcyclohexene⁺ is found to replace MCH⁺.This indicates that some excited species, usually quenched by N2O or CO2, is the precursor of the high-mobility radical cation MCH⁺.