Geometric isomerism, definition

However, the foregoing observations cannot definitely be ascribed to triplet CH2 for the following reasons. Dilution of the reaction mixture with argon while maintaining constant total pressure leads to decreased deactivation efficiency, as pointed out by Frey, and therefore the loss of stereospecificity may result from geometric isomerization of the initial excited adducts rather than from a different mechanism of reaction of triplet methylene. 

Kidd and Spinney (201) ascribed the fact that only seven chemically shifted resonances are detected for the [NbCl Br6 ] system to the non-statistical formation of the mixed halogenoniobates by concluding that only the cis isomer is formed for those constituents where geometric isomerism is conceivable. This view has recently been challenged (202) and definitive proof is given for the statistical coexistence of all ten possible isomers. The failure to observe the remaining isomers is definitely due to instrumental limitations, i.e. the... 

An interesting example of geometrical isomerism about the enolio double bond has been shown (35) recently for a-hydroxymethylene-phenylacetyltropeine to which definite configurations (IX, X) could be ascribed on the basis of IR-spectra. 

Definitions. Define and illustrate the following terms (a) complex ion, (b) ligand, (c) strong-field complex, (d) weak-field complex, (e) geometrical isomerism, (f) kinetic stability, (g) thermodynamic stability. 

Carotenoids can be converted into mixtures of geometrical isomers under appropriate conditions, the most common being iodine catalyzed photoisomerization. This produces an equilibrium mixture of isomers, in general the all-trans isomers predominates. These isomers in an isomeric mixture cannot be measured separately by simple spectrophotometric determination. The usual method of subsequent measurement would be chromatographic separation, diode-array detection, and spectral analysis. In the absence of any definitive data on extinction coefficients for cfv-isomcrs, they are quantified against the all-trans isomer. Modem procedures involve the direct synthesis of c/.v-carotcnoids. 

The difference between photophysical and photochemical processes is not always clear cut. For example, the E Z isomerization of alkenes represents a photochemical process by the above definition, yet the same process might be seen as photophysical in terminal alkenes, where geometric isomers are indistinguishable in the absence of isotopic labels. 

The best-investigated example to date is that of the butenes. Cis-or trans-butene-2 do not polymerize because of severe steric compression during propagation in contrast, butene-1 can readily be polymerized with catalysts of the Ziegler-Natta t3q)e. Some of these catalysts posses definitive positional and/or geometric but not skeletal isomerization activity and they are able to convert nonpol3unerizable internal olefins to... 

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