Shock wave and modelling study of the dissociation pathways of (C2F5)3N

Abstract

The thermal decomposition of perfluorotriethylamine, (C₂F₅)₃N, was investigated in shock waves by monitoring the formation of CF₂. Experiments were performed over the temperature range of 1120–1450 K with reactant concentrations between 100 and 1000 ppm of (C₂F₅)₃N in the bath gas Ar and with [Ar] in the range of (0.7–5.5) × 10⁻⁵ mol cm⁻³. The experiments were accompanied by quantum-chemical calculations of the energies of various dissociation paths and by rate calculations, in particular for the dissociation of C₂F₅via C₂F₅ → CF₃ + CF₂. The overall reaction can proceed in different ways, either by a sequence of successive C–N bond ruptures followed by fast C₂F₅ decompositions, or by a sequence of alternating C–C and C–N bond ruptures. A cross-over between the two pathways can also take place. At temperatures below about 1300 K, yields of less than one CF₂ per (C₂F₅)₃N decomposed were observed. On the other hand, at temperatures around 2000 K, when besides the parent molecule, CF₃ also dissociates, yields of six CF₂ per (C₂F₅)₃N decomposed were measured. The rate-delaying steps of the dissociation mechanism at intermediate temperatures were suggested to be the processes (C₂F₅)NCF₂ → (C₂F₅)N + CF₂ and (CF₂)N → N + CF₂. The reduction of the CF₂ yields at low temperatures was tentatively attributed to a branching of the mechanism at the level of (C₂F₅)₂NCF₂, from where the cyclic final product perfluoro-N-methylpyrrolidine, (C₄F₈)NCF₃, is formed which was identified in earlier work from the literature.