Features of the potential energy surface for the reaction of HO₂ radical with acetone

Abstract

ABSTRACT:The mechanism of the reaction of acetone with HO 2 has been studied by quantum chemical computations. Different stationary points on the potential energy surface (PES) of the reaction have been characterized. These stationary points are the reactants, products, molecular complexes, and transition states. Three pathways have been studied: two H-abstraction channels and one HO 2 -addition channel. The MP2 level of theory with the 6-311G(d,p) basis set was employed for geometry optimization. The electronic energies was obtained at the PMP2, PMP4, and CCSD(T) level of theory with the 6-311G(d,p) basis set on the computed geometries. The addition pathway is clearly the more favorable, contrary to the acetone ϩ OH system. The pre-reactive hydrogenbonded complexes have been characterized and show a large red shift between the OOH stretching frequency in the HO 2 radical and the one in the HO 2 fragment of intermolecular complexes. Our addition rate constant k ϩ at T ϭ 298 K (3.49 ϫ 10 Ϫ16 cm 3 s Ϫ1 ) is consistent with previous experimental results (giving an upper limit of the rate constant of 6 ϫ 10 Ϫ16 cm 3 s Ϫ1 at 298 K).

Direct Measurement of the Equilibrium Constants of the Reaction of Formaldehyde and Acetaldehyde with HO2 Radicals

“… have directly detected the peroxy radical HOCH 2 O 2 by IR and near IR spectroscopy and have found that the appearance rate from the reaction is consistent with direct formation by reaction (R1b). The experimental results are consistent with calculations, which indicate that the barrier for reaction (R1b) is high in the case of acetone , but much lower than the energy of the initial reactants, HO 2 + CH 2 O, in the case of formaldehyde . Therefore, we have evaluated our experimental data by considering a single equilibrium reaction.…”

Does acetone react with HO<sub>2</sub> in the upper-troposphere?

“…Also displayed on Fig. 4 are lines representing K 1 (T ) values from Hermans et al (2004) and Cours et al (2007), neither of which are well-reproduced by the results from this work. Striking however, is the coincidence of the equilibrium constants derived in this work, with the predictions for K 1a from Aloisio and Francisco (2004), and the recent experimental determinations from Grieman et al (2011).…”

“…2). This simulation (indistinguishable from simulations using values of k 1 and K 1 from Cours et al, 2007) clearly underestimates the impact of [CH 3 C(O)CH 3 ] = 1 × 10 15 molecule cm −3 on the observed OH profile (the blue circles), suggesting a good experimental sensitivity, and that CH 3 C(O)CH 3 does indeed interact strongly with HO 2 at this temperature. Note that the small differences in the simulations with [CH 3 C(O)CH 3 ] = 0 (black solid line), and the k 1 = 0 green, dotted line were principally due to an increased OH removal rate via reaction with the added acetone (R4), and to a lesser extent scavenging of the Cl precursor (R11).…”

“…2-3 provide some evidence for the theoretical predictions of Hermans et al (2004) of a significant forward rate coefficient with k 1 (200-210 K) ≈3 × 10 −12 cm 3 molecule −1 s −1 and a strongly temperature dependent reverse process (R1). The lower limiting forward rate coefficient is clearly not consistent with the much smaller k 1 ∼ 10 −16 cm 3 molecule −1 s −1 predicted by Cours et al (2007). To isolate and quantify the effects on observed OH that were attributable to (R1), and hence better constrain our estimates of k and K, numerical simulations of the datasets were conducted.…”

Acetone in the upper troposphere/lowermost stratosphere measured by the CARIBIC passenger aircraft: Distribution, seasonal cycle, and variability

“…The oceans act as either sources or sinks, depending on light, microbial activity, and water temperatures [ Wisthaler et al , 2002; de Laat et al , 2001; Warneke and de Gouw , 2001; Singh et al , 2003]. Besides dry deposition to ocean and land surfaces, major sinks are the reaction with OH, photodissociation in the UV [ Folkins et al , 1998; Wennberg et al , 1998], and reaction with HO 2 at low temperatures in the upper troposphere/lowermost stratosphere (UT/LMS) [ Hermans et al , 2004, 2005; Cours et al , 2007].…”