Laser flash photolysis investigations of diffusion-controlled reactions in supercritical fluids

Abstract

Laser flash photolysis has been used to investigate the triplet-triplet annihilation (TTA) process of benzophenone (Ph[sub 2]C=O) and the self-termination reaction of benzyl radical (PhCH[sub 2]) in supercritical CO[sub 2] and ethane. Kinetic measurements were performed at various pressures above the critical pressure along two isotherms, one close to the critical temperature of the solutions (35[degrees]C) and one further removed (50[degrees]C). The second-order rate constants obtained indicate that each reaction occurs at the diffusion limit when spin statistical factors are considered. No evidence of enhanced cage effects due to supercritical solvent clustering about diffusive encounter pairs or enhanced solute/solute interactions were observed in these experiments. Additionally, the photocleavage of dibenzyl ketone and the rate constants for decarbonylation of phenylacetyl radical (PhCH[sub 2]CO) have been examined under the above conditions and do not show any anomalous behavior or cage effects. 37 refs., 8 figs.