Temperature dependence of the rate constants for the reactions O(3P) + 2,3-dimethyl-2-butene and O(3P) + NO + M determined by a phase shift technique

Abstract

Absolute values of the rate constant k2 for the reaction of O(3P) atoms with 2,3-dimethyl-2-butene are determined over the temperature range 298–481 °K using modulated photosensitized decomposition of N2O to generate O(3P) atoms. The rate constants obtained from the phase shift measurements fit the Arrhenius expression, k2 = (1.24±0.12) ×1010 exp(774±76 cal mole−1/RT) in units of l mole−1⋅sec−1. The indicated uncertainties are least squares standard deviations. Possible explanations for the negative Arrhenius activation energy are reviewed. Also, the rate constant k3 for the reaction O(3P) + NO + M is redetermined for M=N2O. Over the temperature range 298–473 °K, the results fit the Arrhenius expression, k3 = (6.12±0.45) ×109 exp(1230±55 cal mole−1/RT), where the units are l2 mole−2⋅sec−1 and the uncertainties are the least squares standard deviations.

Reaction of oxygen atoms with olefins

“…In the phase shift technique [65,66,76,84], oxb-gen atoms are generated by mercury photosensitized decomposition of N20 at 253.7 nm, and, in the presence of NO, the following reactions occur: Table II reported by Sato [87,88], using pulse radiolysis-resonance absorption, and the relative values of McClenny [89] are generally in good agreement with the mean values determined by photochemical methods for the series of olefins in Table II. The data in Table II [75] in contrast to the phase shift 4722 cal/mol) [76] and FP-CL results 4515 cal/mol) [77].…”

“…(2 -C4H 8 [76,77,86], trimethylethylene [13], tetramethylethylene [84,86]). Although it is convenient to divide the discussion into separate theories or mechanisms, it is possible that there may be more than one contributing factor.…”

“…The figure shows the extent of curvature which can be expected from the derived expression for AT, which appears to be sufficient to account for the potential curvature consistent with the experimental plots in Figure 2, with the exception of the data for C2H 4 and tetramethylethylene, which lie outside the range cov- ~e limiting inverse temperature dependence consistent with simple collision theory is T -3/2, assuming an energy dependent cross-section M~ich is greater than zero at only one collision energy [86]. ~ms collision theory, although requiring perhaps an unrealistic reaction cross-section, is capable of encompassing the data for tetramethylethylene, for which a temperature dependence of T -I was obtained by the phase shift technique [84] (although the temperature dependence of T -2, obtained by flash photolysis-resonance fluorescence measurements at only two closely spaced temperatures [86], is beyond the reach of collision theory).…”

Radical–Molecule Complexes: Changing Our Perspective on the Molecular Mechanisms of Radical–Molecule Reactions and their Impact on Atmospheric Chemistry

“…± molecule complexes has been of fundamental importance to the understanding of a number of significant chemical processes. Singleton and Cvetanovic, [63] in an attempt to describe the negative temperature dependence that they observed for the O( 3 P) cis-CH 3 CH 2 CH 2 CH 3 reaction as well as other reactions with negative temperature dependencies (O( 3 P) (CH 3 ) 2 CC(CH 3 ) 2 [64] and O( 3 P) (CH 3 ) 2 CHCH 3 [63] suggested the involvement of an intermediate O( 3 P) ± olefin complex (atom ± molecule complex), which formed early in the reaction path, before the transition state. [63] The formation of this complex, they postulated, would be reversible and it would be bound by not more that a few kilocalories.…”

A Two Transition State Model for Radical−Molecule Reactions: Applications to Isomeric Branching in the OH−Isoprene Reaction

“…For example, negative activation energies have been noted for a wide variety of olefin reactions with Cl atoms, [1][2][3][4] with O( 3 P) atoms, [5][6][7][8][9] and with hydroxyl radicals. 10 The role of negative activation energies has also been widely discussed in the reactions of hydrocarbon radicals with hydrogen halides and the related reverse reactions.…”