Phenothiazine radical-cations: electron transfer equilibria with iodide ions and the determination of one-electron redox potentials by pulse radiolysis
The establishment of an equilibrium reaction between iodine radical-anions, I2–˙, the protonated form of the phenothiazine, promethazine, PZH+, its related radical-cation, PZH2+˙, and iodide ions, I–, has been observed directly by pulse radiolysis [reaction (i)]. From measurements of the radical-anion and radicalcation I2–˙+ PZH+⇌ PZH2+˙+ 21–(i) equilibrium concentrations at different iodide and promethazine concentrations a value for the equilibrium constant K3= 94 mol dm–3 has been attained. A closely similar value K3= 85 mol dm–3 has been derived from a kinetic analysis of the radical concentrations as equilibrium was approached. Taking the mean value K3= 90 ± 5 mol dm–3 and a reported value of K8= 8.85 × 10–6 mol dm–3 for the equilibrium reaction (ii) the difference in redox potentials given by equation (iii) can be calculated. Values of I2–˙⇌ I˙+ I–(ii), ΔE°=E°(PZH2+˙/PZH+)–E°(I˙/I–)=–0.415 V (iii)E°(PZH2+˙/PZH+)=+0.865 V and E°(I˙/I–)=+1.31 V have been reported previously. Our experimental result agrees very well with the calculated ΔE°=–0.445 V based on these independently derived redox potentials. Measurements of absolute rate constants for the reaction of PZH2+˙ and related radical-cations from chlorpromazine (3) and metiazinic acid (2) with ascorbate (4)(AH–) and α-tocopherol (5)(α-T) indicate that such electron transfer equilibria are likely to exist also with other phenothiazine cations and strongly reducing compounds. The following bimolecular rate constants have been obtained: k(PZH2+˙+ AH–)= 8.2 × 108, k(PZH2+˙+α-T)= 1.1 × 108, k(MZ±˙+ AH–)= 9.0 × 107, and k(MZ±˙+α-T)= 1.3 × 108 mol–1 dm3 s–1.