Experimental Study of the Reactions of Br Atoms with Thiirane and Nitrosyl Chloride.

Abstract

The kinetics of Br-atom reactions with C₂H₄S and ClNO were studied as a function of temperature at a total pressure of 2 Torr of helium using a discharge-flow system combined with mass spectrometry: Br + C₂H₄S → SBr + C₂H₄ (1) and Br + ClNO →BrCl + NO (2). The rate constant of reaction (1) was determined at T = 340-920 K by absolute measurements under pseudo-first-order conditions, either by monitoring the kinetics of Br-atom or C₂H₄S consumption in excess of C₂H₄S or of Br atoms, respectively, and by using a relative rate method: k₁ = (6.6 ± 0.7) × 10^-11 exp(-(2946 ± 60)/T) cm³molecule^-1s^-1 (where the uncertainties represent the precision at the 2σ level, the estimated total uncertainty on k₁ being 15% at all temperatures). The rate coefficient of reaction (2), determined either from the kinetics of the formation of the reaction product, BrCl, or from the decays of Br-atoms in an excess of ClNO, showed non-Arrhenius behavior, being practically independent of temperature below 400 K and increasing significantly at temperatures above 500 K. The measured rate constant is well reproduced by a sum of two exponential functions: k₂ = 1.2 × 10^-11 exp(-19/T) + 8.0 × 10^-11 exp(-1734/T) cm³ molecule^-1 s^-1 (with an estimated overall temperature-independent uncertainty of 15%) at T = 225-960 K.