Kinetic Investigation of the Reactions of Mg(¹S), Ca(¹S), and Sr(¹S) Atoms with NO₂ over the Temperature Ranges 303−836, 303−916, and 303−986 K, Respectively

The kinetics of the second-order reactions Mg( 1 S) + NO 2 (X 2 A 1 ) MgO + NO, Ca( 1 S) + NO 2 (X 2 A 1 ) CaO + NO, and Sr( 1 S) + NO 2 (X 2 A 1 ) SrO + NO have been investigated in a fast-flow reactor in the temperature ranges of, respectively, 303−836, 303−916, and 303−986 K. Solid magnesium, calcium, and strontium pellets were thermally evaporated to generate the corresponding alkaline earth metal atoms in the gas phase. Their decays as a function of the added NO 2 concentration were followed by means of atomic absorption spectroscopy (AAS) at 285.2 nm for magnesium, 422.7 nm for calcium, and 460.7 nm for strontium atoms. All reactions show an Arrhenius behavior and the rate constants are given by k 1 Mg = [(1.4 ± 0.2) × 10 -11 ] exp(−3.4 ± 0.6 kJ mol -1 / RT ) cm 3 molecule -1 s -1, k 1 Ca = [(1.5 ± 0.6) × 10 -9 ] exp(−2.9 ± 1.2 kJ mol -1 / RT ) cm 3 molecule -1 s -1, and k 1 Sr = [(1.2 ± 0.1)x 10 -9 ] exp(−0.9 ± 0.3 kJ mol -1 / RT ) cm 3 molecule -1 s -1 . The results will be discussed in terms of the electron jump mechanism. Since the Mg/NO 2 reaction is too slow to proceed via this mechanism, a classical oxygen atom abstraction is suggested. In the case of the Ca/NO 2 and the Sr/NO 2 reactions, the experimental rate constants are too high to be quantitatively explained by the classical electron jump mechanism. The modified electron jump mechanism which takes into account long distance forces between the reagents gives a better agreement with the experimental values.