Photolysis Mapping Studies of Aliphatic Carbonyl Compounds

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Abstract
The photolysis mapping technique is a marriage of an old radical detecting system with the following principle: If polarized light is used for photodissociation, the flux of fragments will be anisotropic because the molecular absorption is anisotropic in the molecular axis system. The simplest aliphatic carbonyl compounds (formaldehyde, acetaldehyde, propionaldehyde, and acetone) were photodissociated using polarized ultraviolet light. The gases were held at such a low pressure that the fragments travel to the walls without collision in the gas phase. On the walls the radicals were detected by the old mirror removal technique. There is a substantial anisotropy of removal. The sense of the anistropy is different for the different molecules, being perpendicular for formaldehyde and acetone, and parallel for acetaldehyde and propionaldehyde. These results are interpreted by assuming that the carbonyl compounds all become nonplanar in the excited state and the ultimate direction of the fragments can be inferred from the initial motion. In formaldehyde the departing hydrogen atom leaves in a direction perpendicular to the plane as does the methyl in acetone. On the other hand, in acetaldehyde and propionaldehyde the hydrogen atom leaves the molecular plane but remains attached to the central carbon atom while the departing alkyl radicals leave in the molecular plane, parallel to the transition dipole moment.

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