Absolute Rate Coefficient of the OH + CH₃C(O)OH Reaction at T = 287−802 K. The Two Faces of Pre-reactive H-Bonding

Abstract

The rate constants for the reaction OH + CH3C(O)OH --> products (1) were determined over the temperature range 287-802 K at 50 and 100 Torr of Ar or N2 bath gas using pulsed laser photolysis generation of OH by CH3C(O)OH photolysis at 193 nm coupled with OH detection by pulsed laser-induced fluorescence. The rate coefficient displays a complex temperature dependence with a sharp minimum at 530 K, indicating the competition between a reaction proceeding through a pre-reactive H-bonded complex to form CH3C(O)O + H2O, expected to prevail at low temperatures, and a direct methyl-H abstraction channel leading to CH2C(O)OH + H2O, which should dominate at high temperatures. The temperature dependence of the rate constant can be described adequately by k1(287-802 K) = 2.9 x 10(-9) exp{-6030 K/T} + 1.50 x 10(-13) exp{515 K/T} cm3 molecule(-1)(s-1), with a value of (8.5 +/- 0.9) x 10-13 cm3 molecule(-1)(s-1) at 298 K. The steep increase in rate constant in the range 550-800 K, which is reported for the first time, implies that direct abstraction of a methyl-H becomes the dominant pathway at temperatures greater than 550 K. However, the data indicates that up to about 800 K direct methyl-H abstraction remains adversely affected by the long-range H-bonding attraction between the approaching OH radical and the carboxyl -C(O)OH functionality.

Kinetics of the •OH-radical initiated reactions of acetic acid and its deuterated isomers

“…In the photooxidation reaction system, the CH 3 C(O)O • radical and its deuterated isomers undergo fast decomposition forming the methyl radical and CO 2 [2][3][4]. The further fate of the minor primary product • CH 2 C(O)OH (and its deuterated counterparts) has been examined in the present study by product analysis.…”

“…The reference compounds were methanol (for the kinetic study of CH 3 Quoted uncertainties (1σ) have been estimated by the statistical errors of the sampling procedure (~ 10%). They do not include the uncertainties in the reference rate constants, which are estimated to be about 15% for the methanol + OH reaction [12] and at least 30% for the methanol-d 4 + OH reaction.…”

“…They do not include the uncertainties in the reference rate constants, which are estimated to be about 15% for the methanol + OH reaction [12] and at least 30% for the methanol-d 4 + OH reaction. Table 2 gives a comparison of all the rate constant values that have been reported for reactions (1) -(4) in previous works [3,4,[6][7][8][16][17][18]. The rate constant for the acetic acid + OH reaction has been determined in several investigations.…”

“…Below _________________________ * Corresponding author. E-mail: tomas@ensm-douai.fr ~ 500 K, the rate constant presents a slightly negative temperature dependence, while above ~ 600 K, it was found to display a very sharp increase with temperature [3,4]. With the aid of quantum chemical computations, this characteristic behaviour was attributed to the formation of a pre-reactive complex between • OH and CH 3 C(O)OH stabilized at low temperature and to a change in the reaction mechanism when going to higher temperatures.…”

Absolute rate constant and O(<sup>3</sup>P) yield for the O(<sup>1</sup>D)+N<sub>2</sub>O reaction in the temperature range 227 K to 719 K

“…The reactor on the right is a larger heatable stainless steel cell that has been used for many previous studies for C 2 H, OH, and CF 2 reactions (Elsamra et al, 2005;Khamaganov et al, 2006;Dils et al, 2004). It can cover a temperature range of 290 K to 850 K. A glass window is used as exit window for the chemiluminescence, detected perpendicular to the axis of the laser beam.…”