Stilbenes, rotational barriers

CaY zeolite has been found either to act as a reagent to reduce stilbenes to 1,2-diarylethanes or to act as a catalyst and isomerize (Z)-stilbenes to the more stable E-form, depending on the number of Bronsted acid sites present in the zeolite. It has also been suggested that Z-to-E one-way isomerization of (Z)-stilbene through proton addition-elimination and electron-transfer processes occurs in acidic zeolite cavities. The isomerization reaction of (E)- and (Z)-a-phenylcinnamic acid molecules has been studied " at the level of semiempirical quantum chemical methods. The calculations revealed that the (Z)-a-phenylcinnamic acid is slightly more stable than the -isomer. Kinetic studies have been made on the thermal Z-E isomerization of C(40)-carotenoids, and the rotational barriers for Z-E isomerization of different proline analogues have been investigated by dynamic H NMR spectroscopy. The effects of... 

On the basis of the available experimental and computational data, the authors came to several conclusions. The StDNP probe undergoes anisotropic rotation in the 6 cP solvent solution vith a correlation time x = l.l ns. This is much faster than theoretically predicted from the rotation barrier in vacuum about the C—N bond connecting the DNP and stilbene fragments (x,. = 1.9 ns). Therefore, the rigid label moves in solution as a vhole. 

Theoretical calculations have been made on stilbene which are relevant to photoisomerization dynamics. MNDO calculations of stilbene potential energy properties shows no evidence of a doubly excited "phantom" state but a singly excited state with adiabatic rotation around the central ethylene bond has only a small barrier on this path23T Calculations of dipole moments, optical spectra, and second order hyperpolarizability coefficients of some mono- and disubstituted stilbene molecules allows the design of useful nonlinear optical molecules 38. 

Stilbene is a molecule which exists in two isomeric states because of the large energy barrier associated with rotation around the central double bond (see fig. 7.13). It has been known for some time [22] that when stilbene is excited to its first singlet state by absorption of UV light, it rapidly fluoresces to the ground state but with some molecules in the other isomeric form. Thus, if c -stilbene is irradiated with ultraviolet light, one obtains approximately 50% tran -stilbene and... 

An experimental value for the activation barrier for the isomerisation of cw-2-butene 2.104 is 259 kJ mol-1 (62 kcal mol-1). Phenyl groups stabilise radical centres, and the barrier to rotation in stilbenes 2.105 is correspondingly reduced from that in 2-butene to 179 kJ mol-1 (43 kcal mol-1). Steric interaction between the cA-vicinal substituents raises the energy of the planar structure, and contributes to lowering the barrier to rotation. In a fairly extreme example, the bifluorenylidene 2.106 benefits from both effects, and the barrier falls to 95 kJ mol-1 (23 kcal mol-1).136... 

Hochstrasser used picosecond spectroscopy to analyze stilbene isomerization. Irradiation of the trans isomer with a pulse at 265 nm produces an intermediate with a A ax of 584 nm and a lifetime of 68 ps. As described above, these isomerization reactions proceed via twisting of the double bond in the excited state, and in this case the intermediate with the 68-ps lifetime is the (Tr,Tr ) state. In contrast, the lifetime of the intermediate formed from cis-stilbene is less than 1 ps. Apparently there is a barrier to rotation on the excited state for the trans isomer, with a much lower barrier or no barrier for the cis isomer. 

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