The thermodynamic equilibrium between cis- and trans-isomers in stilbene and some derivatives

Abstract

The thermodynamic cis⇌trans equilibrium in stilbene and its derivatives can be established at room temperature only by the use of catalysts such as atomic iodine. The latter has now been used to determine the above equilibrium in a number of cases. The fraction of cis-isomer at equilibrium is ca. 0·002 in stilbene and the para-substituted derivatives investigated, but increases to 0·07 in αβ-difluorostilbene and 2,4,6-trimethylstilbene, and to 0·20 in α-methylstilbene. These values were used to calculate the respective differences in free energy. Catalysed isomerization, in particular if the catalyst is a product of irradiation, may, under certain conditions, occur concurrently with direct or sensitized photoisomerization, and distort the results of the latter reactions. It is therefore important to distinguish between true photoisomerization, involving the excited states of the molecules undergoing isomerization, and photocatalysed isomerization, involving only the ground states of these molecules.