Laser Flash Photolysis Studies of Benzophenone in Supercritical CO2

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Supercritical fluids (SCF's) exhibit much potential as solvents for reactions, in addition to their traditional use as extraction solvents. However, very little is known from a molecular standpoint of the effect of the supercritical solvent on reaction rates. With the objectives of understanding these molecular level interactions in SCF's, and the possibility of "tuning" chemical reactivity in mind, we have initiated a broad range of photochemical investigations in SCFs. Foremost is the use of time-resolved spectroscopies in combination with well-characterized photochemical probes to elucidate solvent/solute interactions in these unique fluids. Herein, we present our initial laser flash photolysis measurements of observed and bimolecular reaction rate constants as a function of pressure for the reaction of benzophenone triplet with isopropanol in supercritical carbon dioxide. The triplet is produced by laser flash photolysis and its disappearance measured directly with time-resolved UV-VIS absorption. The pressure effect on the bimolecular reaction rate constant is negative; i.e., the reaction rate decreases as the pressure is increased and the conditions moved further from the critical point. This is a definitive example where the reaction rate can be increased by operation near the critical point. While the results of this and subsequent studies indicate the influence of changing local compositions near the critical point, one must also consider the observed trends in terms of transition state theory and possible cage effects. The results of this study are important because they open the possibility of enhancing reaction rates by operation in a supercritical fluid near the critical point and address the important effects of the intermolecular interactions between the solvent and the reactants and transition state. A better understanding of these interactions may lead to the prediction of reactions best suited for operation in supercritical fluids.