Photochemical Reactions of Alkenes and Dienes

Torsional dependence of the electronic energies of the ground state and singlet and triplet n,n states of ethene. (Adapted from references 108a and 108b.) 

The radial distribution of this orbital is large in comparison with the 2p orbitals that are the basis set for the n orbital. In essence, this is an orbital around the entire molecule. ° ° Moreover, internal conversion of the n,a, Rydberg, and k,tz states of alkenes is slow. These states may therefore exhibit their own characteristic photochemical reactions, so there can be si hcant wavelength effects on the distribution of photochemical products.  

Evidence supporting the existence of the Rydberg state was reported by Hirayama, F. Lipsky, S. J. Chem. Phys. 1975,62,576, who found weak fluorescence from a series of substituted ethenes. 

An additional complication in the photochemical reactions of alkenes in the gas phase is the possible involvement of vibrationally excited (hot) electronic ground states produced from an excited state. For a review, see Collin, G. J. Adv. Photochem. 1988,14,135. 

The 7t,7i designation refers to a singlet 7t,7c state. Similarly, refers to a triplet it,n state. These states may also be designated as (ti, ti ) and (ti, ti ), respectively. 

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Orbital Symmetry Considerations for Photochemical Reactions of Alkenes and Dienes... 

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