Observation of unique pressure effects in the combination reaction of benzyl radicals in the gas to liquid transition region

The combination reaction of two benzyl radicals has been studied in a wide range of pressures (0.01-1000 bar) and temperatures (250-400 K) in various bath gases (Ar, N-2 and CO2). The measured second-order combination rate constants of benzyl radicals were independent of the pressure and the bath gas below 1 bar, as expected for a limiting "high pressure' rate constant of a termolecular combination process. However, the reaction becomes steadily faster when the pressure in Ar is further raised until it finally starts to decrease when densities corresponding to diffusion controlled kinetics are reached. Such a unique pressure dependence was more strongly accentuated in CO2 and at lower temperatures. Our results seem to provide first clear indications for a contribution of the radical-solvent interaction in the combination reaction kinetics of such large radicals as benzyl in the gas to liquid transition range.