The high pressure photochemistry of alkenes. III. The 184.9 nm photoisomerization processes in acyclic alkenes

Abstract

We have made a systematic study of the 184.9 nm photoisomerization of the gaseous acyclic alkenes. Apart from the cis-trans isomerization (geometric isomerization), we have also observed the formation of products arising from the 1,3-hydrogen and methylene shifts (structural isomerization). 1-Alkenes do not show evidence of structural isomerization. This kind of isomerization increases with an increase in the number of alkyl substituents around the double bond. These observations, combined with those from the literature, may be explained on the basis of the following: (a) the 1π,π* state is involved in the cis–trans isomerization process; (b) the 1π,R(3s) state is responsible for the methylene shifts; (c) another singlet state is required for the 1,3-hydrogen shift; (d) this last state is either at an energy level higher than that of the Rydberg state or the hot ground state. Finally, the photoexcited molecules, through internal conversion, may convert from one state to another, and their lifetime is long enough to be stabilized by collision.