Activation volumes for the quenching of the first electronically excited singlet state of oxygen by toluene molecules in pure toluene and four different solvents

Second-order rate constants kD, of the quenching of singlet oxygen in its a 1Δg state by toluene molecules in bulk solvent and four different solvent environments have been determined with the pressure dependence up to 1200 bar, using time-resolved phosphorescence. Experimental activation volumes ΔV‡exp were analysed taking into account contact complex formation as the initial step. The reaction volume of the formation of the contact complex ΔVC was evaluated using the hard-sphere model of Yoshimura and Nakahara. The data indicate that ΔVC is the main part of ΔV‡exp. A correlation was found between the kD values at 1 bar and the radial distribution function, as expected of the theory of Einwohner and Alder. The quenching rate constants for benzene and cyclohexane at normal pressure are in agreement with this correlation.