Mechanism of the Reaction of OH with Alkynes in the Presence of Oxygen

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• 作者：James Lockhart ; Mark A. Blitz ; Dwayne E. Heard ; Paul W. Seakins ; Robin J. Shannon
• 刊名：The Journal of Physical Chemistry A
• 出版年：2013
• 出版时间：July 3, 2013
• 年：2013
• 卷：117
• 期：26
• 页码：5407-5418
• 全文大小：506K
• 年卷期：v.117,no.26(July 3, 2013)
• ISSN：1520-5215

文摘

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鈥揅₂H₂ 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 k_{HO鈥揅2H2+O2} = (6.17 卤 0.68) 脳 10p>鈥?2p> cmp>3p> moleculep>鈥?p> sp>鈥?p> 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鈥?5 Torr of nitrogen at room temperature. A pressure independent value of (4.21 卤 0.47) 脳 10p>鈥?2p> cmp>3p> moleculep>鈥?p> sp>鈥?p> 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鈥?5 Torr N₂) with a value of (1.87 卤 0.19) 脳 10p>鈥?1p> cmp>3p> moleculep>鈥?p> sp>鈥?p>. Analysis of biexpontial OH decays in alkyne/low O₂ conditions gives the following rate coefficients at 298 K: k_{HO鈥揅3H4+O2} = (8.00 卤 0.82) 脳 10p>鈥?2p> cmp>3p> moleculep>鈥?p> sp>鈥?p> and k_{HO鈥揅4H6+O2} = (6.45 卤 0.68) 脳 10p>鈥?2p> cmp>3p> moleculep>鈥?p> sp>鈥?p>. 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 卤 0.03) and biacetyl (0.74 卤 0.03) were determined for the propyne and but-2-yne systems, respectively. The yield of acid increases with temperature from 212鈥?00 K. Master equation calculations show that, under atmospheric conditions, the acetyl cofragment of organic acid production will dissociate, consistent with experimental observations.