Prof. Laurie Butler, University of Chicago

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

Atlanta Area Chemical Physics Seminar Series

Many elementary bimolecular reactions do not proceed by a direct mechanism surmounting an energetic barrier along the reaction coordinate, but rather follow a near barrier-less addition mechanism. This addition forms unstable radical intermediates whose subsequent isomerization and dissociation dynamics determines the final product branching for the bimolecular reaction. Our experiments generate a particular isomeric form of an unstable radical intermediate along a bimolecular reaction coordinate and investigate the branching between the ensuing product channels of the energized radical as a function of its internal energy under collision-less conditions. We use both velocity map imaging and molecular beam scattering techniques, detecting the products with photoionization and electron bombardment ionization.

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 ro-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 recently characterized in molecular dynamics calculations of Bowman and co-workers.  To understand the large branching to this product channel, we consider how angular momentum and tunneling can influence the dynamics, testing our inferences with experiments on the CD2CD2OH radical.

For more information contact Prof. Christine Payne (404-385-3125).