Pressure Dependence in the Methyl Vinyl Ketone+OH and Methacrolein+OH Oxidation Reactions: An Electronic Structure Study

Abstract

High-level electronic structure calculations were carried out for the study of the reaction pathways in the OH-initiated oxidations of methyl vinyl ketone (MVK) and methacrolein (MACR). For the two conformers of MVK (called synperiplanar and antiperiplanar), the addition channels of OH to the terminal and central carbon atom of the double bond dominate the overall rate constant, whereas the abstraction of the methyl hydrogen atoms has no significant kinetic role. In the case of MACR, only the antiperiplanar conformer is important in its reactivity. In addition, the lower Gibbs free energy barrier for MACR corresponds to the aldehydic hydrogen abstraction reaction, which will be somewhat more favorable than the addition processes. The subtle balance between the different pathways (additions versus abstractions) serves to give an understanding of the pressure dependence of the rate constants of these tropospheric oxidation processes.

Atmospheric Reaction of Cl + Methacrolein: A Theoretical Study on the Mechanism, and Pressure- and Temperature-Dependent Rate Constants

“…The optimized geometries of the two conformers have the C s symmetry as shown in Figure 1. The energy of ap conformer is lower than that of the sp conformer by 3.5 kcal mol −1 at the HL level of theory, which is in consistent with the theoretical results performed by Ochando-Pardo et al, who obtained the ap:sp ratio of 99.6:0.4, 8 and experimental results (97.5:2.5 and 100:0 at room temperature) also showed that the ap conformer of MACR has significant weight in its reactivity. 47 So only the ap conformer was taken into account for the mechanism and kinetics study of the Cl + MACR reaction, just like researchers did with the OH + MACR reaction.…”

“…47 So only the ap conformer was taken into account for the mechanism and kinetics study of the Cl + MACR reaction, just like researchers did with the OH + MACR reaction. 8,9 From the geometric structure of MACR shown in Figure 1, we can see that the Cl atom may react with MACR by the following possible paths: (1) addition, the Cl atom adds to the CC or the CO double bonds; (2) hydrogen atom abstraction, the Cl atom abstracts the aldehyde-H, the methyl-H, or the methylene-H atoms; (3) group abstraction, the Cl atom abstracts the −CH 3 or the −HCO group. In order to acquire the comprehensive reaction mechanism on which the kinetics calculation is based, we tried to calculate all of the above possible paths.…”

Methyl Vinyl Ketone+OH and Methacrolein+OH Oxidation Reactions: A Master Equation Analysis of the Pressure‐ and Temperature‐Dependent Rate Constants

“…See ref. [20] for a complete electronic structure discussion of the whole potential-energy surface. MVK + OH system: As can be observed in Figure 1, two conformers of the MVK molecule were taken into account.…”

“…In this reaction only the ap conformer was taken into account because the population of the sp conformer is negligible at all the temperatures studied, in good agreement with experimental results. [20,[53][54][55] Determination of the location of the terminal addition transition-state structure of MACR, TSadd-t-ap2, is again awkward because the C À O distance between the fragments was found to be too long with MPW1K (3.2 ) and too short with MP2 (2.1 ). The same strategy as used for the TSadd-t-ap1 species of MVK was applied, the single point CCSD(T)/cc-pVTZ(Q)//MP2/6-31 + GA C H T U N G T R E N N U N G (d,p) energy barrier height finally being lowered 1.5 kcal mol…”

“…This negative temperature dependence turned out to be more negative than that measured by Kleindienst et al (E a /R = ÀA C H T U N G T R E N N U N G (175AE52) K) between 300 and 423 K at 50 torr, [19] and by Gierczak et al (E a /R = ÀA C H T U N G T R E N N U N G (379AE46) K) between 234 and 373 K at 20-100 torr. [15] Previously [20] we carried out high-level electronic structure calculations, exploring the potential-energy surfaces of the addition and hydrogen-abstraction reactions involved in the overall R1 and R2 reactions. To our knowledge, that was the first time that these reactions had been studied theoretically, at least using high-level ab initio methods.…”