Pyrolysis of Ethyl Iodide as Hydrogen Atom Source: Kinetics and Mechanism in the Temperature Range 950-1200K

Ethyl iodide is a well known H atom precursor in shock tube experiments. In the present work, we study peculiarities, when C2H5I is used under conditions, where its decomposition is not longer fast compared to the consecutive bimolecular reactions of the H atoms. On the basis of shock tube experiments with detection of H and I atoms by resonance absorption spectrometry, accompanied by quantum chemical (CCSD(T)/6-311G(d, p)//CCSD(T)/6-311G( d, p)) and statistical rate theory calculations, we propose a small mechanism (5 reactions, 7 species) and kinetic data, which allow an adequate description of C2H5I pyrolysis as a H atom source down to temperatures between 950 and 1200K at pressures ranging from 1 to 4 bar: C2H5I -> C2H5 + I (1), k(1) = 9.9 x 10(12) exp(-23 200K/T) s(-1); C2H5 + M -> C2H4 + H + M (2), k(2) = 1.7 x 10(-6) exp(-16 800K/T) cm(3) s(-1) [D. L. Baulch et al., J. Phys. Chem. Ref. Data 34 (2005) 757]; C2H5I -> C2H4 + HI (3), k(3) = 1.7 x 10(13) exp(-26 680K/T) s(-1); H + HI -> H-2 + I (4), k(4) = 7.9 x 10(-11) exp(-330K/T) cm(3) s(-1) [D. L. Baulch et al., J. Phys. Chem. Ref. Data 10 (Suppl. 1) (1981) 1]; C2H5I + H -> C2H5 + HI (5), k(5) = 7.0 x 10(-9) exp(-3940K/T) cm(3) s(-1). The latter bimolecular abstraction step turned out crucial for an adequate description of the hydrogen atom concentration-time profiles in the above mentioned temperature and pressure range for initial concentrations [C2H5I](0) > 2 x 10(13) cm(-3) corresponding to mole fractions > 1 ppm.