The kinetics of the acid-base equilibrium of 4-carboxybenzophenone ketyl radical. A pulse radiolysis study

Abstract

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 λ max = 650 nm (assigned to the ketyl radical anion) was observed immediately after the pulse which was subsequently converted to an absorption band with λ 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- · + H 2O → CBH · + OH -) and by protons (CB-· + H + → CBH ·). The respective rate constants are: 2.5 × 10 2 dm 3 mol -1 s -1 and 6.8 × 10 10 dm 3 mol -1 s -1. The rate constants for the reverse reactions are: k(CBH · + OH -) = 8.7 × 10 9 dm 3 mol -1 s -1 and k(CBH · → CB- · + H +) = 430 s -1.