title: Mechanism of the reaction of OH with alkynes in the presence of oxygen.
abstract: Previous work has shown that the branching ratio of the reaction of OH/C2H2/O2 to glyoxal and formic acid is dependent on oxygen fraction, and a significant component of the product yield under atmospheric conditions is formed from reaction of chemically activated OH-C2H2 adduct. In this article, isotopic substitution is used to determine the mechanism of the OH/C2H2/O2 reaction resolving previous contradictory observations in the literature. Using laser flash photolysis and probing OH concentrations via laser induced fluorescence, a rate coefficient of kHO-C2H2+O2 = (6.17 ± 0.68) × 10(-12) cm(3) molecule(-1) s(-1) is determined at 298 K from the analysis of biexponential OH decays in the presence of C2H2 and low concentrations of O2. The studies have been extended to propyne and but-2-yne. The reactions of OH with propyne and but-2-yne have been studied as a function of pressure in the absence of oxygen. The reaction of OH with propyne is in the fall off region from 2-25 Torr of nitrogen at room temperature. A pressure independent value of (4.21 ± 0.47) × 10(-12) cm(3) molecule(-1) s(-1) was obtained from averaging the eight independent measurements at 25 and 75 Torr. The reaction of OH with but-2-yne at 298 K is pressure independent (5-25 Torr N2) with a value of (1.87 ± 0.19) × 10(-11) cm(3) molecule(-1) s(-1). Analysis of biexpontial OH decays in alkyne/low O2 conditions gives the following rate coefficients at 298 K: kHO-C3H4+O2 = (8.00 ± 0.82) × 10(-12) cm(3) molecule(-1) s(-1) and kHO-C4H6+O2 = (6.45 ± 0.68) × 10(-12) cm(3) molecule(-1) s(-1). The branching ratio of bicarbonyl to organic acid in the presence of excess oxygen also shows an oxygen fraction dependence for propyne and but-2-yne, qualitatively similar to that for acetylene. For an oxygen fraction of 0.2 at 298 K, pressure independent yields of methylglyoxal (0.70 ± 0.03) and biacetyl (0.74 ± 0.03) were determined for the propyne and but-2-yne systems, respectively. The yield of acid increases with temperature from 212-500 K. Master equation calculations show that, under atmospheric conditions, the acetyl cofragment of organic acid production will dissociate, consistent with experimental observations.
authors.affiliation: School of Chemistry, University of Leeds , Leeds, LS2 9JT, U.K.
authors.forename: James
authors.initials: J
authors.lastname: Lockhart
authors.forename: Mark A
authors.initials: MA
authors.lastname: Blitz
authors.forename: Dwayne E
authors.initials: DE
authors.lastname: Heard
authors.forename: Paul W
authors.initials: PW
authors.lastname: Seakins
authors.forename: Robin J
authors.initials: RJ
authors.lastname: Shannon
mesh_terms: D000480:Alkynes / Q000737:chemistry*; D005561:Formates / Q000138:chemical synthesis / Q000737:chemistry; D006037:Glyoxal / Q000138:chemical synthesis / Q000737:chemistry; D006878:Hydroxides / Q000737:chemistry*; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry*
chemical_list: D000480:Alkynes; D005561:Formates; D006878:Hydroxides; C030544:formic acid; D006037:Glyoxal; C031356:hydroxide ion; D010100:Oxygen