Negative rate-dependence

Inhibitors of the slow component Iks were developed in order to circumvent the negative rate dependence of IKr blockers on the effective refractory period (Busch et al. 1996 Suessbrich et al. 1996, 1997 Bosch et al. 1998). Gogelein et al. (2000) studied the effects of a potent inhibitor of Iks channels in Xenopus oocytes and guinea pig ventricular myocytes. 

The negative rate dependence on acetic acid concentration (-0.5) indicates its strong adsorption on the catalyst, which would inhibit the access of oxygen to the catalyst surface. They assumed that the catalytic action is the formation of superoxide ion (Oj ) followed by a production of hydroperoxy radical as an active species under the reaction condition (pH = 2.9)... 

The anticatalytic effect of nitrous acid in nitration The effect of nitrous acid was first observed for zeroth-order nitrations in nitromethane ( 3.2). The effect was a true negative catalysis the kinetic order was not affected, and nitrous acid was neither consumed nor produced by the nitration. The same was true for nitration in acetic acid. In the zeroth-order nitrations the rate depended on the reciprocal of the square root of the concentration of nitrous acid =... 

The inset of Figure 11.8 shows the rate dependence on P02 (at the same PC2H4 and T) for the Rh film deposited on YSZ at various imposed potentials Uwr. The similarity between Figure 11.8 and the inset of Figure 11.8 is striking and underlines the equivalence of metal-support interactions and electrochemical promotion For low po2 values the rate is first order in P02 followed by a sharp decrease at a characteristic po2 value denoted by P02 ( Uwr ) which depends on the support (Fig. 11.8) or on the potential (inset of Fig. 11.8). Thereafter the rate becomes very low and negative order... 

At the platinum electrode the individual steps of the four-electron reaction cannot be studied separately. Slope b has its usual value of about 0.12 V, but in contrast to what is seen at the mercury electrode, the polarization is practically independent of solution pH (i.e., the potential at a given current density shifts by 0.06 V in the negative direction when the pH is raised by a unit). It follows that the reaction rate depends on hydrogen ion concentration. The step in which an electron and a proton are transferred while the 0-0 bond is broken is probably the ratedetermining step. 

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) ... 

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... 

Table IV gives values of J when the false positive rate is 5 percent and the false negative rate is either 5 percent or 1 percent. J depends on V and, since K = 1, on the sura a + a . A...

So far we have assumed that both air levels would have to be determined and therefore that two sets of J samples would have to be collected. In some cases one level may already be available from other records and can be used as a standard of comparison. Table VI shows for this special case how the probability of a false negative depends on the number of samples collected. For small differences between the measured level and the standard a small sample size has an unacceptable high false negative rate. For example, if the mean is five times the standard level and the coefficient of variation is... 

Another m3d h arises from the intuition that pressure effect is opposite to the temperature effect. This is not true in kinetics. Therefore, kinetic constants (reaction rate constants, diffusion coefficients, etc.) almost always increase with increasing temperature, but they may decrease or increase with increasing pressure. Both positive and negative pressure dependences are well accounted for by the transition-state theory and are not strange. 

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). 

This equation predicts a rate dependence on H2 partial pressure of between first- and second-order and a CO dependence of between zero- and negative first-order, as well as first-order dependence on Ru(CO)5 concentration. Based on this scheme, it may be suggested that H2Ru(CO)4 is the active catalyst species in this system. 

Figure 5.6 shows typical positive and negative dark discharge curves for pure a-Se films prepared under different conditions [2], where it can be seen that the dark discharge rate depends on substrate temperature. 

Of major interest in this review are t](y) and (O) for which a large quantity of data has now been accumulated on well-characterized polymers. Some limited information is also available on the shear rate dependence of The second normal stress function has proved to be rather difficult to measure N2 appears to be negative and somewhat smaller in magnitude than N2 82). 

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