Anti-Arrhenius behavior

Reaction behavior that fails to agree with the Arrhenius temperature dependence. Enzyme-catalyzed reactions have been reported to display anti-Arrhenius behavior. As the temperature rises, the temperature dependence of the reaction rate does exhibit a normal Arrhenius pattern however, at elevated temperatures, the reaction velocity falls off as a result of denaturation. 

A reaction for which the mechanism changes in a temperature-dependent manner may also exhibit anti-Arrhenius behavior. See Arrhenius Equation Arrhenius Plot, Nonlinear... 

ANTI-ARRHENIUS BEHAVIOR ARRHENIUS EQUATION ARRHENIUS PLOT, NONLINEAR Antibiosis,... 

Figure 2 Anti-Arrhenius behavior of the kinetics of a host-guest complex during enclathration.

It is perhaps noteworthy that the use of the term anti-Arrhenius behavior in such reaction systems is probably inappropriate or misleading. When one says that the rate constants, k, follow anti-Arrhenius behavior, it apparently means that k corresponds to a reaction step that has negative energy of activation and by definition and physical reality, energy of activation can never be negative for any elementary reaction step. The apparent anti-Arrhenius plots are obtained when the rate constants used to construct the Arrhenius plots are the functions of various rate and equilibrium constants for various irreversible and reversible reaction steps in the overall reactions. It is almost certain for most micellar-mediated reactions that micellar effects on reaction rates are not caused by the decrease or increase in the energy of activation for the rate-determining steps of these reactions. For example, reported anti-Arrhenius behavior of reaction rate ... 

The thermoregulated phase-transfer function of nonionic phosphines has been proved by means of the aqueous-phase hydrogenation of sodium cinnamate in the presence of Rh/6 (N =32, R = n-CsHu) complex as the catalyst [16]. As outlined in Figure 2, an unusual inversely temperature-dependent catalytic behavior has been observed. Such an anti-Arrhenius kinetic behavior could only be attributed to the loss of catalytic activity of the rhodium complex when it precipitates from the aqueous phase on heating to its cloud point. Moreover, the reactivity of the catalyst could be restored since the phase separation process is reversible on cooling to a temperature lower than the cloud point. 

Fig. 2 Atmospheric-pressure hydrogenation of sodium cinnamate using Rh/6 (N = 32, R = n-CsH,) as the catalyst in water at different temperatures. The cloud point of the catalyst is 64 °C. Anti-Arrhenius kinetic behavior results due to the inversely temperature-dependent water-solubility of the nonionic phosphine [16].

Nonenzymatic smart systems, that show a nonlinear response to a stimulus, took advantage of the variation of temperature. For instance, the reaction between p-tolyl sulfide and peroxomonosulfate or m-chloroperbenzoic acid shows the expected linear dependence of the logarithm of the measured rate constant on the reciprocal temperature (the Arrhenius behavior). However, in the presence of a thermoresponsive poloxamer H(OCH2CH2)27-(OCH(CH3) CH2)6i(OCH2CH2)2 OH, which forms micelles as the temperature is increased, anti-Arrhenius or hyper-Arrhenius behaviors are observed." ... 

Andersen Corporation, 79, 87-90 Angular rotation, 632 Anisotropic composites, 236 Annealing, 22, 24, 338 Antagonistic behavior of antioxidants, 529 Antibacterials, 415 Anti-microbial agents, 30, 210, 413 Antimony oxide, 471, 476 Antimony trioxide, 471, 476 Antioxidants, 28, 30, 37, 91, 133, 208, 429, 494, 495, 526, 605, 629 Apparent core density, 222 Apparent overall density, 222 Apparent shear rate, 619, 626, 627, 629 Apparent shear stress, 618, 619 Apparent viscosity, 618, 619 Arabinoxylans, 95 ARES, 640, 641 Arkema, 166, 168 Arrhenius equation, 633 Arrhenius plot, 513 Arsenic salt, 416 Asbestos, 80... 

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