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Analytical Chemistry

Chirality's Atmospheric Role

Oxidation reactions of chiral organic molecules with ozone on aerosol particles in the atmosphere depend on the molecules’ stereochemistry

by Mitch Jacoby
September 28, 2009 | APPEARED IN VOLUME 87, ISSUE 39

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Accessibility of the olefin group in the (S) diastereomer (right) leads to double the rate of surface ozonolysis over the (R) compound.
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Accessibility of the olefin group in the (S) diastereomer (right) leads to double the rate of surface ozonolysis over the (R) compound.

The probability that chiral organic molecules coating aerosol particles will undergo atmospheric reactions with ozone depends on the molecules' stereochemistry, according to a study in the Journal of the American Chemical Society (DOI: 10.1021/ja904206t). Some man-made and natural components of the atmosphere are surface-active and chiral. Yet until now the role of enantioselectivity in atmospheric heterogeneous oxidation and its effect on the composition of the atmosphere have not been studied. Northwestern University's Grace Y. Stokes, Franz M. Geiger, and coworkers modeled those processes, which are thought to be related to cloud formation, by preparing surface-bound chiral olefins and treating them with ozone in a concentration range selected to model the troposphere. Specifically, the researchers adsorbed quinuclidine diastereomers onto silica surfaces and probed these model compounds' rates of ozonolysis with a surface-sensitive spectroscopy method. The group found that diastereomers that orient their reactive C=C bonds away from the surface undergo surface ozonolysis twice as fast as molecules with olefin groups oriented toward the surface. Therefore, chiral organic molecules could serve to distinguish between anthropogenic and biogenic sources of certain organic carbon emissions, the team proposes.

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