Activation energy-lowering

Table 1 shows a summary of the apparent activation energies for various catalytic conditions. The apparent activation energy of HDPE mixed with pure MCM-41 is significantly lower than that of HDPE only, indicating that pure MCM-41 is likely to demonstrate catalytic activity. As the A1 content increased, the apparent activation energy significantly decreased. Al-MCM-41-P demonstrated activation energies lower than those demonstrated by Al-MCM-41 -D at the same Si/Al. 

The application of very low temperatures to detect, to thermally trap, and to characterize intermediates in enzyme-catalyzed reactions. This is made possible by the fact that each individual, elementary step in a reaction pathway has its own activation energy. Lowering the temperature reduces the fraction of molecules that can react in certain steps, thereby permitting otherwise reactive species to accumulate. 

A carboxylic acid is even more stable than an a-hydroxyketone, which is why the thermodynamic factor accounts for 86% of the activation energy lowering of 2-hydroxyallyl vinyl ether (2-hydroxy-AVE) ... 

Negative apparent activation energy, lower rate at higher temperature... 

Frey concludes from the activation energy lowering that the allylic resonance... 

The cis-trans isomerization of 4-methyl-2-oxo-2/T-l,3,2-dioxaphos-phorinans (123) and (124) has been studied by n.m.r. spectroscopy on a sample of 90% /ra j -phosphorinan. The reaction is first-order, with an activation energy lower than the inversion barrier in tertiary phosphines. In view of this, a mechanism involving deprotonation and inversion of the phosphite anion is proposed. 

Thermal characteristics of encapsulates are important, especially for aroma active compounds, from the viewpoint of their release in thermally processed food. In one of the recent publications, the thermal release of vanillin encapsulated in Carnauba wax microcapsules was studied by isothermal thermogravimetric analysis at a temperature range of 170°C-210°C. Kinetic studies revealed that the release is not a single-step reaction but a complex kinetic process that can satisfactorily be described by the Avrami-Erofe ev kinetic model A3. More importantly, thermal release of vanillin encapsulated into Carnauba wax proceeded with an activation energy lower than 40 kJ moL, indicating that the Carnauba wax microcapsules release vanillin relatively easily and thus suggesting that the Carnauba wax can be suitably used as a carrier for aromas especially in the food industry. 

The clue to the rationalization of these results came from theoretical calculations at the DPT level of theory. These calculations showed that the two most stable dienamine conformers were the E-s-trans-2>E and the E-s-trans-3Z, which differed in energy by 1.4kcal/mol. Assuming that these two isomers were readily interconvertible by reprotonation to the iminium ion followed by deprotonation, the experimentally observed diastereomer mixture could be explained. However, the calculations showed that the "y-amination of these two intermediates was endothermic. On the other hand, the less stable s-cis conformers were predicted to react in an exothermic fashion with diethyl azodicarboxylate via a Diels-Alder cycloaddition. The reaction of the E-s-cis-3E conformer was the more exothermic one, and it showed an activation energy lower than that of the E-s-cis-3Z conformer by 6.5 kcal/mol. Hydrolysis of this adduct would then lead to the observed "y-amination product, with (R) configuration (Scheme 2.18). 

Hydrogen abstraction from tertiary carbon has an activation energy lower by approximately 15 kj mol than an abstraction reaction on secondary carbon (Ref. 2, p. 386), and is the predominant mode of formation of hydroperoxides in PP. In addition to energy considerations, the hydrogen abstraction rate constant is dependent on steric factors and polymer conformation. It is found, for instance, that fes (Scheme 15.1) in solution is lower in a theta solvent than in a good solvent, owing to increased steric repulsion in a contracted molecular coil [7]. 

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