Mercury-photosensitized cis—trans isomerization of olefins in the gas phase physical chemistry experiments for the advanced undergraduate laboratory

Abstract

Some of the most important problems and concepts evoked by the gas phase mercury(Hg{6( 3P 1)})-photosensitized cis—trans isomerization of olefins are reviewed through a set of five physical chemistry experiments for the advanced undergraduate laboratory. The nature and shape of the so-called 253.7 nm resonance line of mercury under various conditions are discussed. A short description of the Hg{6( 3P 1)}-photosensitized isomerization of 2-butene (previously proposed as a gas phase actinometer) is given. Quantum yields of other photochemical reactions are determined. Relative efficiencies of energy transfer from Hg{6( 3P 1)} atoms to various olefins are obtained by competitive photosensitizations of these compounds. The photostationary steady state (SS) concentration ratio α ss = [ trans]/[ cis] of a 1,2-disubstituted olefin is determined from the time dependence of the isomer concentration ratio. The ratio α ss is related to the relative cross section for energy transfer from Hg{6( 3P 1)} to the geometric isomers and to the natural decay ratio of the triplet olefin to the ground state cis or trans forms.