Kinetic investigation of the reaction of ground state lead atoms Pb(6 3P 0) with alkyl bromides at elevated temperatures by time-resolved atomic resonance absorption spectroscopy

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We present a kinetic investigation of the reactions between ground state lead atoms Pb(6p 2( 3P 0)) and a range of alkyl bromides RBr in the temperature range 640 – 760 K. Pb(6 3P 0) was generated by the pulsed irradiation of the PbBr 2 vapour in equilibrium with the solid at the elevated temperatures and monitored by time-resolved atomic resonance absorption spectroscopy at λ = 283.3 nm (Pb(7 3P 1°) ← Pb(6 3P 0)). Absolute second-order rate constants k RBr for reaction were measured at various temperatures, yielding the following Arrhenius parameters ( k RBr = A exp(− E/RT)) (errors 1σ): ▪ For the molecules 1-bromobutane and 1,3-dibromopropane rate data could be determined only at single temperatures, yielding k C 4H 9Br = (2.3 ± 0.2) × 10 −15 cm 3 molecule −1 s −1 ( T = 639 K) and k C 3H 6Br 2 = (3.3 ± 1.0) × 10 −14 cm 3 molecule −1 s −1 ( T = 740 K). The activation energies are compared with reaction endothermicities and used to show that the bond energy D(PbBr) is not more than 2.5 eV. The rate data extrapolated to room temperature indicate that the previously reported values of k RBr at that temperature are too high and presumably result from the effect of secondary reactions of Pb(6 3P 0) with photofragments. The diffusion of Pb(6 3P 0) in helium is studied in detail leading to a value of D(Pb(6 3P 0)−He) = 0.48 ± 0.03 cm 2 s −1, extrapolated to standard temperature and pressure. To the best of our knowledge, the present measurements constitute the first characterization of absolute rate data for atomic abstraction reactions by lead atoms at elevated temperatures.