Importance rating

Halogen radicals account for about one-third of photochemical ozone loss observed in the spring in the lower stratosphere (below 21 km) at 15—60°N latitude (76). The following three cycles (4—6) are the most important. Rate constant data are given in Reference 11. 

Adhesives are used to transfer loads and are typically designed with much higher tensile and shear strengths than sealants. The most important rating of an adhesive ia many appHcatioas is the determiaatioa of how much load it can handle. Some sealants are used as adhesives and some adhesives as sealants and thus arises the occasional blurring of their roles. If the material s primary function is the exclusion of wiad, water, dirt, etc, it is a sealant. 

The complex interplay of physicochemical and biological charactenstics that regulate the all important rate at which fluorocarbons may migrate within and finally leave the body, through the lungs and the skin, is not yet completely understood Certainly, variables are involved, such as vapor pressure, solubility m body tissues, molecular size and shape, lipid solubility, electron configuration, and critical soluQon temperatures [16, 17]... 

Instead or in addition, the reactants A and B may associate in a fast preequilibrium, or one of them may bind to a third component. Such interactions will exert important rate effects and for that reason must be accounted for. The existence of equilibria in which the reactants participate may translate to an important effect on the chemical mechanism or to a trivial one. Either way, the issue must be addressed to arrive at a reliable mechanism. The matter can grow complicated, in that the concentration variables that affect the rate may do so because they really do enter the mechanism, or because they participate in extraneous equilibria. Of course, they may play both roles. Sorting out these matters sounds complicated, but it is not difficult if one proceeds systematically. 

Because of doubts about important rate constants in the above interpretations, other mechanisms have been considered (9) Reactions 9 and 10 are consistent with most of the experimental observations ... 

As recently as 1965, Thoma and Stewart predicted that alterations in reaction rates [in the presence of the cycloamyloses] should be anticipated whose magnitude and sign will fluctuate with the reaction type, and added that at the present juncture, it is impossible to sort out confidently. . . which factors may contribute importantly to raising or lowering the activation energy of the reaction. In the short interval between 1965 and the present, a wide variety of cycloamylose-induced rate accelerations and decelerations have, indeed, been revealed. More importantly, rate alterations imposed by the cycloamyloses can now be explained with substantially more confidence. The reactions of derivatives of carboxylic acids and organo-phosphorus compounds with the cycloamyloses, for example, proceed to form covalent intermediates. Other types of reactions appear to be influenced by the dielectric properties of the microscopic cycloamylose cavity. Still other reactions may be affected by the geometrical requirements of the inclusion process. 

Fructose-1,6-bisphosphatase is an important rate-limiting step in gluconeogenesis. This gluconeogenic step antagonizes the opposite reaction that forms fructose-1, 6-bisphosphate from fmctose-6-phosphate and ATP... 

The therapeutic class that uniquely exemplifies lactone prodrugs are the statins, i.e., the cholesterol-lowering agents that act by inhibiting 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase (EC 1.1.1.34). This microsomal enzyme catalyzes conversion of HMG-CoA to mevalonate, an important rate-limiting step in cholesterol biosynthesis. Cholesterol synthesis occurs mainly... 

To answer this question one has to look at the way the rate equation is derived. A rate equation based on a certain reaction mechanism may have been derived after the introduction of some approximations valid at atmospheric pressure. If, at higher pressure these approximations are no longer valid, a continuous use of the rate equation may lead to erroneous results. As approximations usually are introduced to reduce the number of parameters, it should be evident that equations with differing numbers of parameters most probably have different algebraic forms. The omission of a critical, initially small, but increasingly more important rate constant with increasing pressure will unavoidably lead to suspect interpretations. 

This theory clearly predicts that the shape of the polymer length distribution curve determines the shape of the time course of depolymerization. For example Kristofferson et al. (1980) were able to show that apparent first-order depolymerization kinetics arise from length distributions which are nearly exponential. It should also be noted that the above theory helps one to gain a better feeling for the time course of cytoskeleton or mitotic apparatus disassembly upon cooling cells to temperatures which destabilize microtubules and effect unidirectional depolymerization. Likewise, the linear depolymerization kinetic model could be applied to the disassembly of bacterial flagella, muscle and nonmuscle F-actin, tobacco mosaic virus, hemoglobin S fibers, and other linear polymers to elucidate important rate parameters and to test the sufficiency of the end-wise depolymerization assumption in such cases. 

As is evident from these examples, computational quantum mechanics, semiempirical and ab initio methods alike, represent important new tools for the estimation of rate parameters from first principles. Our ability to estimate activation energies is particularly significant because until the advent of these techniques, no fundamentally based methods were available for the determination of this important rate parameter. It must be recognized, however, that these theoretical approaches still are at their early stages of development that is to say, computational quantum chemical methods should only be used with considerable care and in conjunction with conventional methods of estimation discussed earlier in this article, as well with experiments. 

The pH of the solution plays an important rate in the steric outcome of the reaction. Acidic conditions favor cis addition and basic conditions favor trans addition of hydrogen [22,67],... 

Since the time constants of catalytic reactions and the sorption uptake of molecules of various types on crystalline MS, e.g. zeolites, alumlnophosphates and others, are within comparable ranges, the diffusion coefficient represents one of the important rate characteristics of both catalytic and sorptive... 

Fig. 8. Most important steps in the biosynthesis of cholesterol. The reduction of 3-hydroxy-3-methyl-glutaryl-coenzyme A (HMG-CoA) to yield mevalonic acid is an important rate-limiting step and also the site of attack of the HMG-CoA-reductase inhibitors (statins).

The oxidative metabolism leads to the formation of reactive species (epoxides, quinone-imines, etc.), which can be a source of toxicity. Consequently, slowing down or limiting these oxidations is an important second target in medicinal chemistry. Thus, the metabolism of halothan (the first modern general anaesthetic) provides hepatotoxic metabolites inducing an important rate of hepatitis the oxidation of the non-fluorinated carbon generates trifluoroacetyl chloride. The latter can react with proteins and lead to immunotoxic adducts [54], The replacement of bromine or chlorine atoms by additional fluorine atoms has led to new families of compounds, preferentially excreted by pulmonary way. These molecules undergo only a very weak metabolism rate (1-3%) [54,55]. 

A sealant is a material that is installed into a gap or joint to prevent water, wind, dirt, or other contaminants from passing through the joint or gap. This joint or gap may be a fixed joint, but is often an expansion joint which may also be called a working joint. Sealants, which can also be defined by how they are tested, are rated by their ability to stretch, twist, bend, and be compressed while maintaining their bulk properties so that they do not tear apart under stress. A most important rating of a sealant in many applications is the movement ability of the sealant. The adhesion required of a sealant is simply the strength to hold the sealant in position as it is stressed and strained. 

It is true that completely independent experiments may be confused by subtle effects of inhibition by H2S, which is a byproduct in desulfurization reactions, but as discussed later, the magnitude of the inhibition at the levels produced in the experiments is not large enough to result in such drastic changes in the hydrogenation rates of biphenyl. A few well-chosen experiments in which competitive test reactions are conducted simultaneously can provide definitive information that can be used in setting reasonable limits on the ratios of the important rate constants. This is illustrated later. 

Having established reliable values for all of the important rate constants as a function of alkyl substitution on dibenzothiophenes, it is now possible to examine critically how these rate constants (and associated changes in product selectivity) are affected by other components of commercial gas oils and by the H2S that is produced during the HDS process. It is also possible to evaluate how these various rate constants are affected by changes in catalyst composition and by process conditions. Knowledge of the details of these effects can lead to novel catalyst modifications and process configurations that may be able to reach the new stricter standards of 0.05% S. These topics are discussed in later sections. However, for perspective, we will first summarize what is known about present-day catalyst compositions and catalytic mechanisms that bring about the transformations observed in HDS processes. 

Different tautomers of / -diketones may react with metal ions at quite different rates. For example, the keto form of thenoyltrifiuoroacetone does not react at all the enol form reacts by two pathways, one independent and one inversely dependent on hydrogen ion concentration. Deprotonation of the ligand is evidently an important, rate determining factor.46 Both tautomers of acetylacetone react with iron(III). The rate constants for reaction of the keto form with Fe3+ and FeOH2+ are 0.29 and 5.4 M-1 s-1 respectively and the rate constants for the reaction of undissociated enol form with Fe3+ and FeOH2+ are 5.2 and 4.4 x 103 M-1 s-1 respectively.47 Rate constants for the reaction of the enol and keto tautomers of acetylacetone with Ti3+ are 2.4 x 103 and 0.7 M-1 s-1 respectively.48... 

There is no generally accepted classification of elementary processes in heterogeneous catalysis. However, names for a few types of elementary processes are generally accepted and terminology for a partial classification [see M. Boudart, Kinetics of Chemical Processes, Chap. 2 (1968)] has received some currency. The particular reactions used below to exemplify this terminology are ones which have been proposed in the literature but some have not been securely established as occurring in nature at any important rate. 

One of the goals of this and the following chapter is to show how a knowledge of a few of the most important rate constants of radical reactions and some substituent effect trends can be used to design successful experimental conditions for applications of known radical-based synthetic methods. The design of new synthetic methods is only a small stej> beyond this. 

The kinetics of concerted thermal elimination reactions of a series of ethyl (hetero) arylcarboxylate esters (2-thienyl-, 3-thienyl-, 2-furyl, 3-furyl, 4-pyridyl-, 3-pyridyl-, and 2 -pyridylcarbo x y I ate) in the gas phase seem to indicate that there is tittle charge separation in the transition state (83) this is in contrast with the behaviour of the corresponding /-butyl and isopropyl esters for which a semi-concerted transition state (82) was proposed previously.49 Results of a kinetic study of the gas-phase elimination reactions of methylbenzoyl fonnate (84) and 3-hydroxy-3-methylbutan-2-one (85) have been compared with those for pyruvic acid (87) and benzoylformic acid (86).50 The relative rates of reaction [(86)/(87) 46, (87)/(85) = 1.1 x 105 and (86)/(82) = 1 x 106] reveal that the acidity of the hydrogen atom involved in the elimination process, rather than the initial polarization of the C—C bond which undergoes cleavage, is the important rate-controlling factor. 

Dessy et al.28 concluded by stating that nucleophilic attack on mercury was as important as electrophilic attack on carbon. In a subsequent paper, Dessy and Kim29 state, It is obvious that the important rate-determining attack lies with the H, and not with the Cl but the earlier conclusion would seem to be more in keeping with the experimental observations. ... 

The water-promoted hydrolyses of a bicyclic amide, l-azabicyclo[2.2.2]octan-2-one (87), and a planar analogue, l,4-dimethylpiperidin-2-one (88), were studied using density functional theory in conjunction with a continuum dielectric method to introduce bulk solvent effects. The aim of these studies was to reveal how the twisting of the C-N bond affects the neutral hydrolysis of amides. The results predict important rate accelerations of the neutral hydrolysis of amides when the C-N bond is highly twisted, the corresponding barrier relaxation depending on the specific reaction pathway and transition state involved.85... 

These verbal modifiers and labels help analysts to make sense of the mean values expressed by respondents. Additional approaches include the use of multiple items to assess component satisfaction scores and summing these to create an overall satisfaction index. There are some complexities with indices since it is necessary to weight the importance of the components rather than simply add them. This requirement demands that researchers collect both importance ratings and satisfaction ratings of the contributing elements. 

For the more expressive aspects of tourism experience, particularly those where the expectations are likely to be poorly defined, unfamiliar or non-existent, a Nordic School performance only approach can be recommended. Some of the strengths of this approach lie in the clarity of its task for the traveller. The researcher too benefits from the post hoc assessment if both importance ratings and any one of the benchmarking approaches reviewed are employed to facilitate data interpretation. 

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