THE KINETICS OF THE ACID-BASE-EQUILIBRIUM OF 4-CARBOXYBENZOPHENONE KETYL RADICAL - A PULSE-RADIOLYSIS STUDY

Pulse radiolysis of nitrogen saturated aqueous solutions of 4-carboxybenzophenone below pH 8 has been used to generate the 4-carboxybenzophenone ketyl radical anion to allow the direct observation of the protonation of this radical in neutral and moderately acidic solutions. A transient absorption band with lambda(max) = 650 nm (assigned to the ketyl radical anion) was observed immediately after the pulse which was subsequently converted to an absorption band with lambda(max) = 570 nm (assigned to the ketyl radical). The kinetics of this conversion was of pseudo-first-order with a rate constant linearly dependent on the H+ concentration. These findings are interpreted in terms of a protonation of the ketyl radical anion by water (CB-. + H2O --> CBH. + OH-) and by protons (CB-. + H+ --> CBH.). The respective rate constants are: 2.5 x 10(2) dm(3) mol(-1) s(-1) and 6.8 x 10(10) dm(3) mol(-1) s(-1). The rate constants for the reverse reactions are: k(CBH. + OH-) = 8.7 x 10(9) dm(3) mol(-1) s(-1) and k(CBH. --> CB-. + H+) = 430 s(-1).