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

Abstract

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, N₂ and CO₂ were employed as bath gases. Benzyl radicals were generated by H abstraction from toluene by Cl atoms formed via laser photolysis of Cl₂ 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^ET_∞(T) = (4.1 ± 0.3) × 10⁻¹¹ (T/300 K)^−0.23 cm³ molecule⁻¹ s⁻¹. Although the reaction has clearly reached the “high-pressure limit” (k^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 ∼5 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 < N₂ ≈ 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.