Temperature and pressure dependence of the reaction of OH and CO: Master equation modeling on a high‐level potential energy surface

Abstract

The temperature and pressure dependence of the reaction of OH + CO has been modeled using the (energy-resolved) master equation and RRKM theory. These calculations are based on the coupled-cluster potential energy surface of Yu and co-workers (Chem Phys Lett 349, 547-554, 2001). As is well known, this reaction shows a strong non-Arrhenius behavior at moderate and low temperatures because of the stabilization of the HOCO intermediate. Kinetic simulations are in excellent agreement with experiments at temperatures above 300 K, but the agreement is only modest at temperatures below 250 K. Our calculations indicate that the contribution of tunneling to the rate constant is marginal, given the small energy difference between the transition states corresponding to formation and decomposition of the HOCO intermediate. Parametric fits to the calculated rate constants are provided for modeling purposes.

On the theory of the reaction rate of vibrationally excited CO molecules with OH radicals

“…[16][17][18][19][20][21][22][23][24][25][26][27][28] The currently accepted reaction mechanism involves a CO and OH bimolecular association to form a vibrationally excited trans-HOCO * radical, followed by a cis-trans isomerization. There are also the back reaction to reform the reactants, the forward reaction leading to H + CO 2 and the collisional stabilization of HOCO * .…”

“…[29][30][31] The non-Arrhenius behavior for the thermal rate constant has been extensively studied in experiments and theory. [3][4][5][6][7][8][9][10][11][12]16,20,21,[26][27][28] It involves a nearly activationless barrier in the entrance channel CO+ OH→ HOCO * and also in the exit channel HOCO * → H+CO 2 . There is a large H-tunneling effect in the latter, and at low temperatures there is even tunneling in the former.…”

On the theory of the CO+OH reaction, including H and C kinetic isotope effects

“…1 Reflecting its importance and the unusual temperature and pressure dependence of its rate constant, the CO+ OH→ CO 2 + H reaction has been examined extensively in many experimental [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and theoretical studies, [17][18][19][20][21][22][23][24][25][26][27][28] including experimental studies over a very wide range of temperatures and pressures. [4][5][6][7][8][9][10][11][12][13]17 Nevertheless, carbon and oxygen isotope effects have been observed [29][30][31][32][33] and the anomalous effects for these heavy-atom isotopes have not yet been treated in the literature.…”

“…The final reaction steps in competition with each other are dissociation to H and CO 2 , back reaction to OH and CO, and collisional stabilization. 5,13,16,17,21,22 As the pressure increases, the collisional stabilization of the cis-and trans-HOCO intermediates competes favorably with the dissociation channel and the back reaction. When OH and CO react in oxygen, both the dissociation channel and the collision stabilization lead to HO 2 and CO 2 , because of the follow-up reactions of H and HOCO with O 2 .…”

“…Dynamical and statistical treatments of the CO+ OH reactions have been made with various theoretical methods, 17,[20][21][22][23][24][25] including quasiclassical trajectories, quantum dynamics, transition state theory, and RRKM theory. In the present article we focus on the statistical and nonstatistical aspects of the reaction over a wide temperature region for the different experimental conditions, as well as the hydrogen and carbon isotope effects.…”