Physics  College of Engineering 

From Quantum Mechanics to Atmospheric Chemistry: Probing Radical Intermediates of Bimolecular Rx

Date(s): 2/20/2013 11:00 AM - 2/20/201312:00 PM
Location: Library Grand Reading Room
Contact: Robert Fisher 508-999-8353

Many elementary bimolecular reactions do not proceed by a direct mechanism surmounting an energetic barrier along the reaction coordinate (giving the activation energy discussed in an introductory chemistry course). Instead they follow a near barrier-less mechanism through a radical intermediate whose subsequent unimolecular and bimolecular reactions determine the branching between chemical product channels.

The talk focuses on a class if reactions important in atmospheric chemistry, the reaction of OH radicals with unsaturated organic molecules. The reaction of OH with volatile organic compounds in the atmosphere is the first step in an oxidation mechanism that forms secondary organic aerosols. Our work on the OH + ethene reaction focuses on the vibrationally excited C2H4OH radical intermediate that forms in the addition/elimination mechanism for this bimolecular reaction. We produce the radical intermediate photolytically, characterize the internal energy distribution of the radicals, and then measure the branching between the product channels. We first develop and test a model for predicting the partitioning between rotational and vibrational energy in the radicals prepared photolytically, one that explicitly includes the change in impact parameter due to the vibrational modes of the photolytic precursor. We then measure the product branching ratios and compare them with the theoretically predicted branching between the H + ethenol, methyl + formaldehyde, and H + acetaldehyde product channels. The data reveal an additional and unexpected product channel that we investigate further with molecular dynamics calculations. For young physicists, you may enjoy considering how angular momentum and tunneling can influence these processes.

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