Recombination of benzyl radicals: dependence on the bath gas, temperature, and pressure

The combination reaction C6H5CH2 + C6H5CH2 (+M) --> C14H14 (+M) was studied over the pressure range 0.03 - 900 bar and the temperature range 250 - 400 K. Helium, argon, xenon, N-2 and CO2 were employed as bath gases. Benzyl radicals were generated by H abstraction from toluene by Cl atoms formed via laser photolysis of Cl-2 at 308 nm. Benzyl radicals were monitored by time-resolved transient UV absorption at 253 nm. The observed combination rates were pressure independent over the range 0.04 - 0.45 bar in CO2 and 0.03 - 5 bar in argon which identifies a limiting high-pressure'' value of rate constant within the energy-transfer mechanism, k(infinity)(ET)(T) = (4.1 +/- 0.3) x 10(-11) (T/300 K)(-0.23) cm(3) molecule(-1) s(-1). Although the reaction has clearly reached the high-pressure limit'' (k(infinity)(ET)) at pressures below 1 bar, a further increase of the rate constants was observed when the pressure of the bath gas was raised above similar to5 bar. At much higher pressures finally the rate constants decrease when diffusion-controlled kinetics takes over. The degree of enhancement of the combination rates beyond the high-pressure limit'' was found to depend on the bath gas, increasing in the order He < N-2 approximate to Ar < CO2. The enhancement was most prominent at low temperatures. Measurements of transient UV absorption spectra of benzyl radicals, over the pressure range 5 - 30 bar of CO2 at 300 K, confirmed that an only small pressure-dependent solvatochromic shift of benzyl radicals cannot be responsible for the observation of enhanced rate constants. Instead, the results clearly point toward a significant effect of van der Waals clustering, i.e. of radical-complex'' formation, on the combination reaction kinetics in the gas - liquid transition range. An analysis in terms of statistical adiabatic channel/classical trajectory calculation (SACM/CT) appears to provide a consistent description.