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Issue 43, 2014
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Photochemistry of aldehyde clusters: cross-molecular versus unimolecular reaction dynamics

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The unimolecular photochemistry of aldehydes has been extensively studied, both experimentally and computationally. However, less is known about the role of cross-molecular photochemical processes in the condensed-phase photolysis of aldehydes. The triplet-state photochemistry of pentanal in its pentameric (n = 5) cluster was investigated as a model for photochemical reactions of aliphatic aldehydes in atmospheric aerosols. This study employs “on the fly” dynamics simulations using a semi-empirical MRCI electronic code for the singlet and triplet states involved. Previous studies have shown that the triplet-state photochemistry of an isolated pentanal molecule is dominated by Norrish I and II reactions. The main findings for the cluster are: (1) 55% of the trajectories lead to a unimolecular or cross-molecular reaction within a timescale of 100 ps; (2) cross-molecular reactions occur in over 70% of the reactive trajectories; (3) the main cross-molecular processes involve an H-atom transfer from the CHO group of the excited pentanal to an O atom of a nearby pentanal; and (4) the unimolecular Norrish II reaction is suppressed by the cluster environment. The predictions are qualitatively supported by experimental results on the condensed-phase photolysis of an aliphatic aldehyde, undecanal. The computational approach should be useful for predicting the mechanisms of other condensed-phase organic photochemical reactions. These results demonstrate a major role of cross-molecular processes in the condensed-phase photolysis of carbonyls. The cross-molecular reactions discussed in this work are relevant to photolysis-driven processes in atmospheric organic aerosols. It is expected that the condensed-phase environment of an organic aerosol particle should support a multitude of similar cross-molecular photochemical processes.

Graphical abstract: Photochemistry of aldehyde clusters: cross-molecular versus unimolecular reaction dynamics

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Publication details

The article was received on 16 Jul 2014, accepted on 16 Sep 2014 and first published on 16 Sep 2014

Article type: Paper
DOI: 10.1039/C4CP03130J
Author version available: Download Author version (PDF)
Citation: Phys. Chem. Chem. Phys., 2014,16, 23861-23868
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    Photochemistry of aldehyde clusters: cross-molecular versus unimolecular reaction dynamics

    D. Shemesh, S. L. Blair, S. A. Nizkorodov and R. B. Gerber, Phys. Chem. Chem. Phys., 2014, 16, 23861
    DOI: 10.1039/C4CP03130J

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