MO-THEORETICAL INVESTIGATION ON THE PHOTODISSOCIATION OF CARBON OXYGEN BONDS IN AROMATIC-COMPOUNDS

The homolytic photodissociation process of carbon-oxygen bonds in aromatic compounds of the common type R-OPh (R=carbon fragment) was investigated by using molecular orbital theory. Orbital and state correlation diagrams for this reaction were constructed and are used in a qualitative interpretation of experimental solution data. State energies and bond dissociation enthalpies were calculated with MNDOC-CI and AM1/AM1-HE methods. Further information was obtained from the calculation of potential hypersurfaces for the dissociation from the ground (S0), the S1 and the T1 states. For this purpose the MNDOC-CI method with full geometry optimization at the CI level of theory was applied. As model compounds para-substituted phenoxyacetones, allylphenyl ether and phenylacetate were investigated. From the results a unified description of the three reactions is deduced using the dissociative pi-sigma* state as a common feature of the primary process. Starting from the nondissociative excited states (n-pi* or pi-pi*) the reaction proceeds along a reaction barrier with a calculated height of 20-40 kcal/mol to the dissociative part of the potential hypersurface. A correlation of the calculated barrier height with estimated experimental rate constants for the photodissociation is obtained. The findings presented for the beta-cleavage of phenoxyacetones and the photo-Fries rearrangement suggest that these processes should not be classified as typical ketone photoreactions. Structural effects, the reaction multiplicity and the influence of substituents in the aromatic moiety on the reaction efficiency are discussed. From the calculated data a general model for pi-sigma* photodissociation processes is deduced which agrees very well with experimental observations.