Quantum yield unsaturated aldehydes

High quantum and chemical yields of ODPM rearrangements are obtained with substrates in which the ly-enone chromophore is part of a conformationally rigid molecular assembly which at the same time guarantees adequate orbital overlap of the C=C and rr-bonds. Whereas in bicyclic and bridged 3, y-unsaturated ketones these prerequisites are widely met, acyclic p,7-unsaturated ketones usually rearrange inefficiently since other channels of energy dissipation from the triplet state predominate. Exceptions to this rule are substrates in which the C= ir-bond is part of a styrene or an a, -enone moiety (Section 2.6.3.1). In this context it should also be noted that p,7-unsaturated aldehydes, except for one case, are ODPM unreactive (Section 2.6.3.2). 

A pressure increase, brought about by an increase in the concentration of the ketone or by the addition of an inert gas, enhances the formation of the unsaturated aldehyde as compared to that of CO. The value of awehyde increases at the expense of 0co> thus, the ketone consumption yield is independent of pressure. This seems to be generally valid in the photolysis of the cyclic ketones it was confirmed, for instance, for cyclopentanone °, cyclohexanone 2-methyl cyclohexanone and 2,6-dimethyl cyclohexanone . An increase in wavelength also favours the formation of the aldehyde as compared to decarbonylation in the photolysis of cyclobutanone , cyclopentanone and cyclohexanone . At 3130 A, the decrease in temperature has a similar effect on the product distribution in the photolysis of cyclopentanone , cyclohexanone , 2-methyl cyclohexanone , and 2,6-dimethyl cyclohexanone to that caused by the increase in wavelength or pressure. However, at shorter wavelengths, the quantum yields seem to be independent of temperature . ... 

Unfortunately, the product quantum yields for cyclohexanone photodecomposition are insufficient to allow reasonable estimates of the y-values for its photolysis in the troposphere. However, it can be concluded that the unsaturated aldehydes, affected very little by oxygen addition, probably will be important products under these conditions. 

In contrast to kinetic models reported previously in the literature (18,19) where MO was assumed to adsorb at a single site, our preliminary data based on DRIFT results suggest that MO exists as a diadsorbed species with both the carbonyl and olefin groups being coordinated to the catalyst. This diadsorption mode for a-p unsaturated ketones and aldehydes on palladium have been previously suggested based on quantum chemical predictions (20). A two parameter empirical model (equation 4) where - rA refers to the rate of hydrogenation of MO, CA and PH refer to the concentration of MO and the hydrogen partial pressure respectively was developed. This rate expression will be incorporated in our rate-based three-phase non-equilibrium model to predict the yield and selectivity for the production of MIBK from acetone via CD. 

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