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 σ240 nm(NPTO2)/ cm2 molecule–1=(6.2 ± 1.1)× 10–18 and σ240 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.