Product Branching from the CH2CH2OH Radical Intermediate of the OH + Ethene Reaction

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• 作者：Britni J. Ratliff ; Bridget W. Alligood ; Laurie J. Butler ; Shih-Huang Lee ; Jim Jr-Min Lin
• 刊名：Journal of Physical Chemistry A
• 出版年：2011
• 出版时间：August 25, 2011
• 年：2011
• 卷：115
• 期：33
• 页码：9097-9110
• 全文大小：1260K
• 年卷期：v.115,no.33(August 25, 2011)
• ISSN：1520-5215

文摘

Using a crossed laser-molecular beam scattering apparatus and tunable photoionization detection, these experiments determine the branching to the product channels accessible from the 2-hydroxyethyl radical, the first radical intermediate in the addition reaction of OH with ethene. Photodissociation of 2-bromoethanol at 193 nm forms 2-hydroxyethyl radicals with a range of vibrational energies, which was characterized in our first study of this system ( J. Phys. Chem. A 2010, 114, 4934). In this second study, we measure the relative signal intensities of ethene (at m/e = 28), vinyl (at m/e = 27), ethenol (at m/e = 44), formaldehyde (at m/e = 30), and acetaldehyde (at m/e = 44) products and correct for the photoionization cross sections and kinematic factors to determine a 0.765:0.145:0.026:0.063:<0.01 branching to the OH + C₂H₄, H₂O + C₂H₃, CH₂CHOH + H, H₂CO + CH₃, and CH₃CHO + H product asymptotes. The detection of the H₂O + vinyl product channel is surprising when starting from the CH₂CH₂OH radical adduct; prior studies had assumed that the H₂O + vinyl products were solely from the direct abstraction channel in the bimolecular collision of OH and ethene. We suggest that these products may result from a frustrated dissociation of the CH₂CH₂OH radical to OH + ethene in which the C鈥揙 bond begins to stretch, but the leaving OH moiety abstracts an H atom to form H₂O + vinyl. We compare our experimental branching ratio to that predicted from statistical microcanonical rate constants averaged over the vibrational energy distribution of our CH₂CH₂OH radicals. The comparison suggests that a statistical prediction using 1-D Eckart tunneling underestimates the rate constants for the branching to the product channels of OH + ethene, and that the mechanism for the branching to the H₂O + vinyl channel is not adequately treated in such theories.