A study of bipyridyl radical cations. Part II. The reaction of morphamquat {bis-N-[(2,6-dimethylmorpholin-4-yl)carbonylmethyl]-4,4′-bipyridylium} radical cation with oxygen in methanol

Abstract

The equilibrium, 2MQ⁺˙⇌(MQ⁺˙)₂ between monomeric morphamquat radical cation, MQ⁺˙, and diamagnetic dimer molecules, (MQ⁺˙)₂, has been examined by u.v. spectroscopy over the temperature range +81 to –88°, yielding ΔG°–13.4 kJ mol^–1, ΔH°–55.3 kJ mol^–1, and ΔS°–141 J K^–1 mol^–1, which confirm data from our e.s.r. study. The reaction of the morphamquat radical cation with oxygen has been followed by the stopped-flow technique over a range of temperature from +40 to –80°. At +40° the reaction is second-order in monomer and first-order in oxygen with a velocity constant of 1.6 × 10⁸1² mol^–2 s^–1, whereas at –80° it is first-order in dimer and first-order in oxygen with a velocity constant of 84 l mol^–1 s^–1. The mechanism of these reactions is discussed.