Collisional Energy Transfer in the Two-Channel Thermal Decomposition of Bromoethane-1,1,2,2-d4

Article abstract of DOI:10.1021/j100226a024

The two-channel thermal decomposition of CHD2CD2Br (products HBr + C2D4, DBr + CHDCD2), along with the decomposition of CH3CH2Br, has been studied by using the technique of very low-pressure pyrolysis (VLPP).Rate coefficients were obtained at pressures both so low that only gas/wall collisions occur (over the temperature range 950-1200 K) and dilute in various bath gases (pressures up to 10 Pa) over the range 1000-1070 K.Fitting these data by solution of the appropriate reaction-diffusion integrodifferential master equation yields the gas/wall collisional efficiency, the extrapolated high-pressure rate parameters, and the gas/gas collisional energy transfer probability function, P(E,E').The extrapolated high-pressure rate coefficients are as follows: for CHD2CD2Br, 10^13.20 exp(-227.4 kJ mol^-1/RT) s^-1 (HBr elimination), 10^13.15 exp(-230.2 kJ mol^-1/RT) s^-1 (DBr elimination), and 10^13.6 exp(-221 kJ mol^-1/RT) s^-1 for CH3CH2Br, in good agreement with those obtained by other methods.Gas/wall collision efficiencies (the wall being seasoned quartz) are ca. 0.6 at 1200 K, ca. 0.8 at 1000 K (with those for the d₄ species ca. 10percent less than for the d₀), in accord with values estimated from the potential well depth.The data are moderately sensitive to P(E,E').Assuming this for downward transitions to be a function of E-E' alone, we found that this function falls off more steeply than exponential, as found previously for chloroethane.Average bromoethane/M downward energy transfer values ((ΔE_down)) are 250 (M = Ne), 600 (M = CO2), 850 (M = C2H4), and 1200 (M = benzene) cm^-1, the variation of (ΔE_down) with temperature being less than experimental uncertainty over the small experimental temperature range.