Isomerization thennal

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although confonnational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thennally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, k = A. The quantity /cg T//i from transition state theory provides... 

Thennal isomerization, also occurs, but only with prolonged heating and only in the presence of excess glycine. Quantum yields of photoisomerization, however, are independent of the free glycine. In addition, when isomerizations are carried out in the presence of radioactively labeled free glycine, the thermal trans product contains labeled glycine, whereas the photochemical product contains none. Therefore, the two reactions must proceed by different mechanisms, the thermal reaction by an intermolecular pathway and the photochemical isomerization by an intramolecular pathway involving a twist mechanism. 

Pudovik, A.N., and Gazizov, T.K., Thennal isomerization of diethyl acetyl phosphite and reactions of dialkyl acetyl phosphites with a-oxo phosphonic acid esters, Zh. Obshch. Khim., 38, 140, 1968 J. Gen. Chem. USSR (Engl. Transl.). 38. 139. 1968. 

Thennal Neutron Cross Sections Cross seclions for neubon caplute teaclions in unils of barns cm or millibatns (mb). Proton, alpha ptoduclion and fission teaclions ate designated by [Pg.1925]

Pig. 13. Thennal backward cis -+ trans isomerization of azobenzene at room temperature in polycarbonate films UV irradiated in advance onto the film (O). UV irradiated during casting ( ), as wen as of azobenzene in ethylacetate ( ), ethanol (A), and dichloroethane ( ) UV irradiated in advance... 

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