High pressure range of addition reactions of HO. II. Temperature and pressure dependence of the reaction HO+CO⇔HOCO→H+CO2

Abstract

Thermal rate constants of the complex-forming bimolecular reaction HO+CO■HOCO→H+CO2 were measured between 90 and 830 K in the bath gas He over the pressure range 1–700 bar. In addition, the vibrational relaxation of HO in collisions with CO was studied between 300 and 800 K. HO was generated by laser photolysis and monitored by saturated laser-induced fluorescence. The derived second-order rate coefficients showed a pronounced pressure and complicated non-Arrhenius temperature dependence. Above 650 K, the disappearance of HO followed a biexponential time law, indicating thermal instability of collisionally stabilized HOCO. By analyzing the corresponding results, an enthalpy of formation of HOCO of ΔHof,0=−(205±10) kJ mol−1 was derived. On the basis of energy- and angular-momentum-dependent rates of HOCO formation, activated complex properties for the addition reaction HO+CO→HOCO were derived from the limiting high-pressure rate constants; with the limiting low-pressure rate constants, activated complex properties for the dissociation HOCO→H+CO2 could be fitted as well. The observed transitions between low- and high-pressure limiting rate constants were well reproduced with these molecular parameters and collisional contributions; some evidence for rotational effects in collisional energy transfer was found. The surprisingly successful theoretical modeling of all available experimental data (80–2800 K, 0.0001–700 bar) allows for a satisfactory data representation of the rate coefficients over very wide ranges of conditions.

Using <sup>14</sup>C, <sup>13</sup>C, <sup>18</sup>O and <sup>17</sup>O isotopic variations to provide insights into the high northern latitude surface CO inventory

“…The rate constant k CO+OH = 1.5 × 10 −13 × (1 + 0.6p atm ) cm 3 molec. −1 s −1 (DeMore et al, 1997) is dependent on pressure, but shows almost no temperature dependence at atmospheric pressure (Fulle et al, 1996). Therefore, k CO+OH cannot cause no- , 1997).…”

CO2FORMATION IN QUIESCENT CLOUDS: AN EXPERIMENTAL STUDY OF THE CO + OH PATHWAY

“…Route (2) has been extensively studied in the gas phase, both experimentally (Frost et al 1993;Fulle et al 1996;Baulch et al 2005) and theoretically (Yu et al 2001;Chen & Marcus 2005;Sun & Law 2008), due to its importance in atmospheric and combustion chemistry. Recently, reaction (2) was experimentally studied in the solid phase by reflection-absorption infrared spectroscopy (RAIRS) for the first time, with positive results (Oba et al 2010).…”

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

“…The conversion of CO to CO 2 by reaction with an OH radical is one of the principal oxidation reactions in the atmosphere 1 and has been the subject of many experimental [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] and theoretical studies. [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.…”

“…[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.…”

“…The latter was obtained by microwave discharge in an N 2 / Ar mixture to form excited N͑ 4 S͒, which then reacted with NO. The vibrational temperature T v of the CO molecules was determined from the optical absorption spectrum of the vibrational-rotational states of CO͑v , J͒ at or below 1800 K. On the other hand, the rotational and translational temperatures remained at room temperature.…”