Kinetics and spectral characteristics of transient species formed on one-electron oxidation of acridine-1,8-dione in aqueous solution pulse radiolysis study

Abstract

The ˙OH radicals in a neutral aqueous solution of acridine-1,8-dione (AD) have been shown to react with a bimolecular rate constant of 6.1 × 10⁹ dm³ mol^–1 s^–1 and form two transient absorption bands with λ_max= 320 and 650 nm. The 650 nm band is observed to decay by first-order kinetics with k= 2.2 × 10³ s^–1 and the 320 nm band showed mixed kinetics. Both H˙ and O˙^– react with AD to form transient absorption bands with λ_max= 330 and 680 nm, and are assigned to a radical species formed on H atom abstraction from a CH₃ group. One-electron oxidants such as Br₂˙^–, N₃˙^– and I₂˙^– react with AD and form transient absorption bands with λ_max= 310 and 650 nm. They are assigned to a radical species formed on deprotonation of solute radical cations at neutral pH. Based on these studies, ˙OH radicals at neutral pH are inferred to react with AD by H atom abstraction from two different sites: 38% from a CH₃ group and 62% from a CH₂ group which on protonation results in a solute radical cation. The 320 nm band formed on reaction of ˙OH radicals is resolved into two bands with λ_max= 310 nm (radical species formed on deprotonation of solute radical cation) and 330 nm (radical formed on H atom abstraction from CH₃ group). Cl₂˙^– is able to oxidize AD and form transient absorption bands with λ_max= 305 and 560 nm which are assigned to the solute radical cation AD˙⁺ in acidic solutions. The pK value for the deprotonation of the solute radical cation is found to be 2.3. The I₂˙^–/2I^– redox couple established an equilibrium with AD/AD˙⁺, and the oxidation potential for the AD/AD˙⁺ couple was determined to be 1.02 ± 0.03 V close to the half-wave potential, E_1/2, value obtained electrochemically (1.17 V).