Formation of the trityl cation in pulse irradiated solutions of triphenylmethyl chloride in cyclohexane. Evidence for an intermediate radical cation

Abstract

The absorption spectrum observed on nanosecond pulse irradiation of triphenylmethyl chloride, ϕ₃ CCl, solutions in cyclohexane results from the formation of three intermediates: the triphenyl-methyl radical (absorption maximum 340 nm), the trityl cation (absorption maximum between 400 and 450 nm) and a second positive ion species (absorption maximum 360 nm). The free radical, formed by electron attachment to ϕ₃ CCl with a rate constant of 3.7 × 10¹² dm³ mol^–1 s^–1, undergoes a reversible reaction with O₂ for which the forward and backward rate constants are 1.2 × 10⁹ dm³ mol^–1 s^–1 and 1.0 × 10⁷ s^–1, respectively at 20°C. In a 10^–3 mol dm^–3 solution the trityl cation grows in over several hundred ns while over the same timescale the absorption at 360 nm decays. The rate constant for reaction of the cyclohexane radical cation with ϕ₃ CCl is 2.5 × 10¹¹ dm³ mol^–1 s^–1. The initial product of this reaction, which is considered to be responsible for the 360 nm absorption, is thought to be the radical cation ϕ₃ CCl⁺, which is converted to the trityl cation by further reaction with ϕ₃ CCl with a rate constant of 5 ± 2 × 10⁹ dm³ mol^–1 s^–1. The trityl cation transfers its charge to TMPD with a rate constant of 8.7 × 10⁹ dm³ mol^–1 s^–1.