Flash photolysis study of the spectra and self-reactions of neopentylperoxy and t-butylperoxy radicals

Abstract

The self-reaction of neopentylperoxy radicals, neo-C₅H₁₁O₂(NPTO₂): NPTO₂+ NPTO₂→ 2t-C₄H₉CH₂O + O₂(1a), → t-C₄H₉CHO + t-C₄H₉CH₂OH + O₂(1b), → t-C₄H₉CH₂OOCH₂C₄H₉-t + O₂(1c), has been studied from 248 to 373 K and from 50 to 760 Torr total pressure. The neopentylperoxy radicals fromed via channel (1a) react, under most experimental conditions, by unimolecular decomposition: t-C₄H₉CH₂O + M → t-C₄H₉+ HCHO + M. (2). The t-butyl radicals so formed are rapidly converted into t-butylperoxy radicals under the conditions employed in this work; these radicals are unreactive on the timescale of the NPTO₂ decay and enable the branching ratio for reaction (1) to be determined via their UV absorption. The overall rate constant for reaction (1) displays a strong negative temperature dependence, being well described by k₁/cm³ molecule^–1 s^–1= 3.02 × 10^–19(T/298)^9.46 exp(4260/T) over the temperature range studied here. The non-terminating channel (1a) becomes increasingly important with increasing temperature, with β=(197 ± 67)exp[-(1658 ± 98)/T], where β is the ratio of those radicals which react via the non-terminating channel (1a) to those which react via the terminating channels (1b) and (1c). By measuring the reduction in the fraction of NPYO₂ radicals converted to t-butylperoxy radicals with increasing oxygen concentration, rate constants for reaction (2) were determined, giving E₂/kJ mol^–1= 42.7 ± 2.1. The UV spectra of NPTO₂ and t-C₄H₉O₂ have been determined relative to that of CH₃O₂; both are similar in shape and magnitude to those of other alkylperoxy radicals, displaying maxima around 240 nm, with σ_{240 nm}(NPTO₂)/ cm² molecule^–1=(6.2 ± 1.1)× 10^–18 and σ_{240 nm}(t-C₄H₉O₂)/cm² molecule^–1=(4.7 ± 0.8)× 10^–18. The self-reaction of t-butylperoxy radicals: 2t-C₄H₉O₂→ 2t-C₄H₉O + O₂(3), was also briefly studied, resulting in k₃/cm³ molecule^–1 s^–1≈ 1.0 × 10^–11 exp(–3894/T). Errors are 1σ.