Photodissociation dynamics of the reaction CF2Br2+hν→CF2+ 2Br. Energetics, threshold and nascent CF2 energy distributions for λ=223–260 nm

Abstract

The dissociation dynamics of the reaction CF2Br2+hν→CF2+2 Br have been studied for a variety of dissociation energies, Ediss=460–535 kJ mol−1 (corresponding to λ=260–223 nm). The laser induced fluorescence spectrum of nascent CF2 products was measured for various dissociation energies within this range. Analysis of the spectra yielded the CF2 vibrational distribution and average rotational energy. The translational energy of CF2 was measured ia the Doppler broadening of various fully resolved rovibronic transitions. The most detailed analysis of energy disposal in the CF2 fragments was carried out at Ediss=486 kJ mol−1 (or λ=246 nm). At this energy each degree of freedom of CF2 had an average energy of Evib=0.4±0.2 kJ mol−1, Erot=2.5±0.5 kJ mol−1, and Etrans=24±3 kJ mol−1. These CF2 energies, coupled with the available thermochemical data, allow us to determine unambiguously that CF2 production must be accompanied by the production of two atomic Br fragments. A photofragment excitation spectrum of CF2Br2, probing for the production of CF2 fragments, provided a reaction threshold of 460±3 kJ mol−1 (corresponding to 260±1.5 nm). The range of previously published reaction enthalpies varies from 392 to 438 kJ mol−1, all of which are substantially below the observed threshold. Additionally, at Ediss=486 kJ mol−1, the energy of the CF2 fragment was 27 kJ mol−1 on average, already in excess of the available 26 kJ mol−1, and without considering the kinetic energy of the recoiling Br atoms. We rationalise these data by proposing that the reaction might have a small barrier in the exit channel. The observed threshold corresponds to the top of the barrier (460 kJ mol−1), while the final energy in the fragments is determined by the asymptotic reaction energy (∽424 kJ mol−1). Simple dynamical models are presented to show that the proposed mechanism is reasonable. Key future experiments and calculations are identified that would enable a clearer picture of the dynamics of this reaction.