Study of bipyridyl radical cations. Part III. Reaction of paraquat radical cation with oxygen and with chromic ions in methanol

Abstract

We have studied methanol solutions of the radical cation of paraquat, 1,1′-dimethyl-4,4′-bipyridylium dichloride. The u.v.–visible spectrum changes with temperature in a very similar way to that for the corresponding morphamquat system. E.s.r. experiments show that, although above –20 °C in methanol solution, there is only one paramagnetic species (the paraquat radical cation, PQ⁺˙), a second paramagnetic species develops at lower temperatures, in contrast to the behaviour of the morphamquat system. The reaction of methanol–paraquat radical cation solutions with oxygen has been followed by the stopped-flow technique over a range of temperature from +40 to –80°C. At +40 °C the reaction is second-order in radical cation, with a velocity constant 8.0 × 10⁸ 1² mol^–2s^–1. The mechanism of this reaction is discussed. We have also studied the reaction of paraquat radical cation with Cr³⁺ ions in methanol over the temperature range 0–40 °C. The reaction is first order both in Cr³⁺ and in PQ⁺˙ It does not go to completion but reaches an equilibrium position. The thermodynamic constants for this equilibrium have been determined and also the thermodynamic constants of activation for the reaction of Cr³⁺ with PQ⁺˙ in methanol at +25 °C.