Photolysis of cis-pent-2-ene at 185 nm

Abstract

On photolysis at 185 nm, cis-pent-2-ene decomposes to form hydrogen, more than twenty hydrocarbons and a polymer. By observing the relative effects of oxygen and nitrogen on the product yields it was possible to distinguish between products formed directly in primary reactions and those formed from free radicals. There are two major paths of breakdown : (i) cleavage of the C—C bond β to the double bond, and (ii), the formation of butadiene by elimination of the elements of methane. There are two other minor primary processes; the primary quantum yields have been estimated.

As with the butenes, the product yields are reduced by adding argon, nitrogen or carbon dioxide but in contrast to the earlier results with but-2-ene, increase of reactant pressure while reducing the quantum yields of formation of most products, increases the quantum yield of formation of the trans isomer. A mechanism is proposed in which the excited state intermediate, which is thought to be the first Rydberg state, can either decompose or be deactivated to the original isomer by collision with an inert gas molecule. Collision with another pentene molecule results in geometrical isomerization.