Negative temperature dependence

The quantum yield for the formation of the cycloaddition product has been found to be temperature dependent, increasing by a factor of approximately three as the temperature is lowered from 65 ( = 0.24) to 5°C ( = 0.69). Photolysis of mixtures of the olefin and f/my-stilbene in the presence of sensitizers yielded no cycloaddition product (42) but rather only m-stilbene. This suggests that the cycloadduct is produced via a singlet reaction. This conclusion is supported by the fact that tetramethylethylene quenches fluorescence from the /rans-stilbene singlet. A plot of l/ (42) vs. 1/[TME] (TME = tetramethylethylene) is linear. The slope of this plot yields rate constants for cycloadduct formation which show a negative temperature dependence. To account for this fact, a reversibly formed exciplex leading to (42) was proposed in the following mechanism<82) ... 

Isotactic PHEMA was found to possess negative temperature dependence in water (Oh and Jhon, 1989). While atactic PHEMA is not expected to have a strong negative temperature dependence, the mechanisms of this behavior can still exist over short ranges and may effect the phase behavior. As such, increased temperatures may also function to control the pore morphology by allowing the polymer to phase separate early on in the reaction. 

Pyrethroids have low oral toxicity to mammals, and in general their insect (topical) to mammal (oral) toxicity ratio is much higher than that of the other major classes of insecticides [25]. As the reason, at least the following mechanisms are conceivable (1) negative temperature dependence - differences in body temperature between insects and mammals makes the insect nerves much more sensitive, (2) metabolic rate - insects metabolize the insecticide more slowly than mammals, and the metabolizing enzyme systems are different, and (3) differences in body size - insects will have less chance to metabolize the insecticides before reaching the target site [26]. 

Occasionally, the rates of bimolecular reactions are observed to exhibit negative temperature dependencies, i.e., their rates decrease with increasing temperature. This counterintuitive situation can be explained via the transition state theory for reactions with no activation energy harriers that is, preexponential terms can exhibit negative temperature dependencies for polyatomic reactions as a consequence of partition function considerations (see, for example, Table 5.2 in Moore and Pearson, 1981). However, another plausible explanation involves the formation of a bound intermediate complex (Fontijn and Zellner, 1983 Mozurkewich and Benson, 1984). To... 

Examples for complex bimolecular reactions with rates exhibiting negative temperature dependencies include... 

Hence one would expect the second channel to depend on the size of the radical and to show some pressure dependence as well as a negative temperature dependence, all of which are found to be the case. 

Conversely, at the lower temperatures, the rate constant for H-abstraction is small while, at the same time, the rate of adduct decomposition is lowered. As a result, at the lower temperatures (right side of Fig. 6.11), adduct formation predominates and a negative temperature dependence, as well as a dependence on pressure is observed for the overall rate constant. In the intermediate region, both addition and abstraction are occurring at significant rates, leading to the curved OH decay plots in Fig. 6.10 and the discontinuities in the Arrhenius plots of Fig. 6.11. 

Thus, at 1 atm in air and 298 K, abstraction predominates. The addition channel (45b) would be expected to have a pressure dependence and a negative temperature dependence (see Chapter 5.A.2). Thus is consistent with the observation that the effective overall bimolecular rate constant in 1 atm of air decreases as the temperature increases from 250 to 310 K and that the fraction of the reaction that proceeds via (45a) increases from 0.24 to 0.87 over the same temperature range (e.g., Hynes et al., 1986). 

The thermal conductivities of many common liquids have a nearly linear temperature dependence with a slight negative slope. However, some important fluids, like water, have significant curvature with both positive and negative temperature dependencies in tempera-... 

In their earlier paper,105 Clyne and Thrush measured the rate constant at temperatures from 294 to 704°K and found a room-temperature value of 0.48 x 1010 M 2 sec-1 for kx + k2 with H2 as a third body whereas in their later paper," they measured rate constants between 226 and 294°K and found a room-temperature value of 1.5 x 1010 M 2 sec-1 for kx + k2 with H2 as a third body. The discrepancy was due to a numerical error in the calculations in the earlier work. They scaled all the values in the earlier paper to conform with the values in the later paper, and deduced kx + k2 = 1.5 x 1010 (7)273)-°-9 0-3 M 2 sec x. The ratio k2k3jki had an even stronger negative temperature dependence, as indeed it should, because k3 is independent of temperature and kt should have a positive temperature dependence. 

The association reactions of Si+" (2P) with acetylene and benzene have been measured by Glosik and coworkers96, who found that the rate coefficients for these reactions have strong negative-temperature dependencies. These observations were rationalized in terms of a negative entropy change in the reactions. 

Dlugokencky and Howard (24) concluded that the small negative temperature dependence that they observed (see Table I) suggests that the mechanism for this reaction involves addition of N03 to the sulfur atom... 

The apparent rate constant in (2.10), which is obtained by multiplying a true rate constant kc and the square root of an equilibrium constant, Keq, can show a law of dependence on temperature different from the simple Arrhenius law. In some cases, even a negative temperature dependence can be observed. Moreover, if both mechanisms (2.6) and (2.7)-(2.8) are active in parallel, the observed reaction rate is the sum of the single rates, and an effective reaction order variable from 1 /2 to 1 can be observed with respect to reactant A. Variable and fractionary reaction orders can be also encountered in heterogeneous catalytic reactions as a consequence of the adsorption on a solid surface [6],... 

CD molecules are partially free draining chains and the negative temperature dependence of the limiting viscosity number fn] can be attributed to the temperature dependence of the unperturbed chain dimension A. 

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