Stilbene isomerization volume

Figure 27. Inclusion of trans-stilbene in X type and in ZSM-5 zeolites. Required free volume for geometric isomerization is present in supercages of X zeolite and such is absent in ZSM channels. Extensive free volume in ZSM-5 channel is present along the molecular axis, but that is of no use for the reaction to occur.

From this point of view, a brief comparison of the acyloxylation of cis or trans stil-benes under electrochemical and chemical conditions is also relevant. Anodic oxidation (Pt) of cis or trans stilbenes in the presence of acetic or benzoic acid gives predominantly mesa diacylates of hydroxybenzoin or, if some water is present, threo monoacylate. None of stereoisomeric erythro monoacylate and rac diacylate was obtained in either case. There was no evidence of isomerization of cis-to-trans stilbene under the electrolytic conditions employed (Mango Bonner 1964 Koyama et al. 1969). The sequence of reaction steps in Scheme 2-23 was proposed. Adsorption-controlled one-electron oxidation of the substrate takes place. Then an adsorption-controlled rotation proceeds of cis stilbene monoadduct into thermodynamically more stable trans benzoxonium ion. The trans benzoxonium ion is the common intermediate for conversions of both cis and trans stilbenes and, of course, for all the final products (Scheme 2-24). Hence, oxidized molecules of stilbene are, at the electrode, involved in a reaction with acylate ions. There is no passing into the solution volume, with the following electron being exchanged there with unoxidized molecules of stilbene. 

Solvent reorientation and isomerization of trans-stilbene in alkane solutions has been studied by ps time scale anisotropic absorption and polarization239 Coupling of solute and solvent decreases as the size of the solvent molecules increases. The applicability of currently favoured models for the activated barrier crossing in the photoisomerization of stilbene is discussed, A method for measuring quantum yields in the photoisomerization of trans-stilbene gives high accuracy without use of a chemical actinometer . Evidence has been found for dynamic solvent effects on the photoisomerization of 4,4 -dimethoxystilbene in which the effects of temperature and hydrostatic pressure were made in n-alkane and n-alkyl alcohol. A ps laser time-resolved study fits frequency dependent solvent shifts but gives results inconsistent with the free volume model. Photophysical and theoretical studies of trans and 9-... 

Isomerization needs some extra sweep volume. The volumes for the two mechanisms of azobenzene should be quite different—ca. 0.25 nm for rotation and ca. 0.12 nm for in version. This bears out in restricted spaces. In some zeolites azobenzene can isomerize whereas stilbene does not. Kuriyama and Oishi found that there are two separate AH versus AS lines for azobenzenes isomerizing by inversion (azobenzene type) and rotation (pseudo-stilbene type). ... 

No additional free volume has to be applied by the solvent for cis trans photoisomerization of sterically hindered stilbenes if the molecular volume, occupied by both isomers, is equal. For nonplanar a- or 2-substituted stilbenes and < > , . are practically unaffected by viscosity and temperature [178], From the fact that d>f increases strongly at high viscosities, an isomerization pathway via intersystem crossing from a higher excited singlet state has been inferred by Sharafy and Muszkat [178]. Cis trans photoisomerization of 4-hydroxystilbene in micellar solution was used to induce viscosity changes [190],... 

Kramers paper spurred an enormous amount of research on the theory of activated rate processes, especially in the physics community, as evidenced in numerous textbooks see, for example, Refs. 13 and 14. However, as noted by Landauer (15) in his subjective description of the history of noise activated escape from metastable states, up till the end of the seventies, the physical chemistry community largely ignored the theory of rates introduced by Kramers. The first experimental measurements of viscosity effects on activated rate processes were performed on the isomerization of frans-stilbene to c/s-stil-bene by Fischer and co-workers in 1968 (16). These authors were not aware of Kramers work and interpreted their results in terms of the free volume necessary for isomerization to occur. Since then, experimental work has proliferated see, for example, the recent textbook (17). 

Clear evidence for the large conformational change upon isomerization of stilbene (tetramethoxystilbene) dendrimers of generation GO, Gl, G2, and G4 was presented [105]. From the time-resolved studies on energy and molecular volumes changes, it was found that the conformational change upon the trans- to cis-isomerization completed with the decay of the Ti state for G0-G2. The diffusion coefficient (D)... 

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