PHOTODIMERIZATION OF CYCLOHEXENE AND METHANE BY DECATUNGSTATE ANIONS - MOLECULAR-ORBITAL THEORY

A molecular orbital study has been made of the photodimerization of cyclohexene by excited W10O324-, yielding 3,3'-dicyclohexene to explain the observations of Yamase. The overall process is radical monomer coupling. Radical formation by an adiabatic H transfer from cyclohexene to O- on the O 2p to W 5d charge transfer photoexcited oxyanion is described. This process is highly activated, just as on metal oxide surfaces, because of the stability which comes when an electron from the CH bond reduces the hole in the oxyanion O 2p band during H abstraction. Since α-H abstraction is stabilized by the formation of the allylic π orbital, the product selectivity can be understood. Calculated H abstraction activation energies for olefinic, α, and β CH bonds in cyclohexene and for a CH bond in CH4 are higher, but they are low enough that in the case of CH4 it can be suggested that dimerization could be looked for in future experiments using photoactivated oxyanions. Based on the calculated electronic structures, it is possible to envision a nonadiabatic electron transfer to O- during mild thermal collisions between a CH bond and the clusters yielding an organic radical cation followed by proton transfer at a later time. Polar solvents would enhance the probability for this mechanism. © 1990, American Chemical Society. All rights reserved.