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

Abstract

The thermodynamic cis r '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 ap-difluorostilbene and 2.4.6-trimethylstilbene, and to 0.20 in a-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.

The photochemistry of electron donor–acceptor (EDA) complexes in micellar solutions. I. The stilbene–methylviologen system

“…These photoprocesses have been camed out, in homogeneous solution, by means of straightforward excitation (2), triplet sensitization (3,4), via radical cations (5) or exciplexes (6), and through the excitation of charge-transfer complexes formed by stilbenes and organic neutral molecules (7,8). There have been, however, very few studies in microheterogeneous media (micelles, semiconductor particles, etc.…”

Iodine‐Catalyzed Isomerization of Dimethyl Muconate

“…This mechanism may be applicable to diene isomerization,t hough recent work has highlighted additional complexity of catalyzed diene isomerization because of persistent questionsr egarding regioselectivity,c atalyst association strength,and energy barriers, among other factors. [33,35] To address this knowledge gap, we employ an integrated computational and experimental approach to investigate the mechanistic aspects of the iodine-catalyzed isomerization of ccDMMt o ctDMM and ttDMM. [33,35] To address this knowledge gap, we employ an integrated computational and experimental approach to investigate the mechanistic aspects of the iodine-catalyzed isomerization of ccDMMt o ctDMM and ttDMM.…”

“…[30] Currently,t here is ad earth of mechanistic information for this chemistry,w hich, if addressed, could advance strategies to improveb oth the isomerizationy ield ande fficiency. [33,35] To address this knowledge gap, we employ an integrated computational and experimental approach to investigate the mechanistic aspects of the iodine-catalyzed isomerization of ccDMMt o ctDMM and ttDMM. Computationally,i somer stabilities and free energy barriers were evaluated for the major species involved in the isomerization mechanism.…”

The photochemistry of EDA complexes in micellar solutions. II. The stilbene–methylviologen system in the presence of oxygen

“…An increase of medium acidity decreases this inhibition. A possible explanation is that the radical cation cS'+, from the equivalent of step [2] of the reaction scheme, converts irreversibly to tS'+:…”