Reaction mechanisms, uncertainty

Eykholt GR, Elder CR, Benson CH. Effects of aquifer heterogeneity and reaction mechanism uncertainty on a reactive barrier. J Hazard Mater 1999 68 73-96. 

While there is agreement that the rates of clay dehydroxylations are predominantly deceleratory and sensitive to PH2G, there is uncertainty as to whether these reactions are better represented by the first-order or by the diffusion-control kinetic expressions. In the absence of direct observational evidence of interface advance phenomena, it must be concluded that the presently available kinetic analyses do not provide an unambiguous identification of the reaction mechanisms. The factors which control the rates of dehydroxylation of these structurally related minerals have not been identified. 

It is regrettable that the evidence afforded by reaction kinetics is rarely, if ever, uniquely consistent with a single mechanism or a single explanation. The results for nucleophilic aromatic substitution reactions are no exception. Legitimate questions can be raised with respect to the extent to which observations made on a particular system permit generalization to other systems. Even for the specific systems studied points of detail arise, and choices have to be made where alternatives are possible. Every such choice introduces an element of uncertainty and imposes a limitation on the extent to which the reaction mechanism is, in fact, known. 

When the residence time distribution is known, the uncertainty about reactor performance is greatly reduced. A real system must lie somewhere along a vertical line in Figure 15.14. The upper point on this line corresponds to maximum mixedness and usually provides one bound limit on reactor performance. Whether it is an upper or lower bound depends on the reaction mechanism. The lower point on the line corresponds to complete segregation and provides the opposite bound on reactor performance. The complete segregation limit can be calculated from Equation (15.48). The maximum mixedness limit is found by solving Zwietering s differential equation. ... 

Due to the heat of reaction there was an uncertainty in the temperature of up to 150° so that the results obtained with high concentrations of ammonia might be subject to some criticism. The order with respect to the diluent can hardly be reconciled with a reasonable reaction mechanism and the author himself does not put much weight on this result. 

When the objective of the modeling effort is to develop and validate a reaction mechanism, the major uncertainty in the model must reside in the detailed chemical kinetic mechanism. Under these conditions, the process must be studied either under transport-free conditions, e.g., in plug-flow or stirred-tank reactors, or under conditions in which the transport phenomena can be modeled very precisely, e.g., under laminar flow conditions. This way. 

Think of some of the largest uncertainties in reaction mechanisms and choose one of them to study in more detail (e.g., the OH-aromatic reactions, the 03-alkene reactions, etc.). Change the mechanism (file is meccm.cb4) to reflect an extreme version and see what difference this makes to the 03 isopleth, if any. Be sure to describe clearly what you are changing and how. Comment on whether the isopleth changes and how. 

In addition to water, a variety of organic liquids, including amines, carboxylic acids, and hydrocarbons, have been used as solvents in the study of the homogeneous reactions of hydrogen with metal salts. In general, there is more uncertainty about the nature of the species present in such systems than in aqueous solution and, correspondingly, it is usually more difficult to elucidate the reaction mechanisms in detail. The most extensive solvent effect studies have been made on cupric, cuprous, and silver salts. A number of the more important results are considered below. 

If there are more than two free radical intermediates, the mechanism (of a reaction) cannot be deduced by a kinetic study based upon analysis of all products and reactants (32)—the uncertainty principle of reaction mechanism. 

As the fuel complexity increases, so does the complexity and also the uncertainty of the reaction mechanism. In modeling the oxidation behavior of the large hydrocarbons, the use of semiempirical mechanisms that involve a few overall steps together with a detailed Ci-C2 subset is still a valuable approach [171]. However, for some types of problems, such as prediction of key intermediates or by-products, full mechanisms are preferred. Full oxidation mechanisms for a number of larger hydrocarbons are available in the literature (e.g., [88,92,245,327-330]), but their predictive capabilities must be evaluated carefully for specific applications. 

Experimental uncertainties and non-idealities are a concern when performing experiments to develop or validate a detailed chemical reaction mechanism. 

In summary, therefore, the detailed mechanism of the hydrolysis of carboxylic anhydrides is still in doubt and we must hope for further experimental evidence to clarify the position. As for the hydrolysis of the other carboxylic acid derivatives dealt with in this chapter, none of the mechanistic criteria, that have been used to interpret the kinetic data, gives an unambiguous interpretation, resulting in a situation where details of mechanism are open to argument. This is particularly the case for solvolysis reactions where uncertainty as to the structure and effect of the solvent preclude a firm assignment of transition state structures. This is not to say that the mechanisms are not... 

Recently, two basic questions of chemical dynamics have attracted much attention first, is it possible to detect ( film ) the nuclear dynamics directly on the femtosecond time scale and second, is it possible to direct (control) the nuclear dynamics directly as it unfolds These efforts of real-time detection and control of molecular dynamics are also known as femtosecond chemistry. Most of the work on the detection and control of chemical dynamics has focused on unimolecular reactions where the internuclear distances of the initial state are well defined within, of course, the quantum mechanical uncertainty of the initial vibrational state. The discussion in the following builds on Section 7.2.2, and we will in particular focus on the real-time control of chemical dynamics. It should be emphasized that the general concepts discussed in the present section are not limited to reactions in the gas phase. 

The application of isotope effects studies of reaction mechanism includes comparison of experimental values of isotope effects and predicted isotope effects computed for alternative reaction pathways. On the basis of such analysis some of the pathways may be excluded. Theoretical KIEs are calculated using the method of Bigeleisen and Mayer.1 55 KIEs are a function of transition state and substrate vibrational frequencies. Equilibrium isotope effects are calculated from substrate and product data. Different functionals and data sets are used in these calculations. Implementation of a one-dimensional tunnelling correction into conventional transition-state theory significantly improved the prediction of heavy-atom isotope effects.56 Uncertainty of predicted isotope effect can be assessed from the relationship between KIEs and the distances of formed or broken bonds in the transition states, calculated for different optimized structures.57 Calculations of isotope effects from sets of frequencies for optimized structures of reactants and transition states are facilitated by adequate software QUIVER58 and ISOEFF.59... 

There was also great uncertainty of the production yields for SHEs. Closely related to the fission probability of SHEs in the ground-state, the survival of the compound nuclei formed after complete fusion was difficult to predict. Even the best choice of the reaction mechanism, fusion or transfer of nucleons, was critically debated. However, as soon as experiments could be performed without technical limitations, it turned out that the most successful methods for the laboratory synthesis of heavy elements are fusion-evaporation reactions using heavy-element targets, recoil-separation techniques, and the identification of the nuclei by generic ties to known daughter decays after implantation into position-sensitive detectors [13-15],... 

Laboratory simulations of aqueous-phase chemical systems are necessary to 1) verify reaction mechanisms and 2) assign a value and an uncertainty to transformation rates. A dynamic cloud chemistry simulation chamber has been characterized to obtain these rates and their uncertainties. Initial experimental results exhibited large uncertainties, with a 26% variability in cloud liquid water as the major contributor to measurement uncertainty. Uncertainties in transformation rates were as high as factor of ten. Standard operating procedures and computer control of the simulation chamber decreased the variability in the observed liquid water content, experiment duration and final temperature from 0.65 to 0.10 g nr3, 180 to 5.3 s and 1.73 to 0.27°C respectively. The consequences of this improved control over the experimental variables with respect to cloud chemistry were tested for the aqueous transformation of SO2 using a cloud-physics and chemistry model of this system. These results were compared to measurements made prior to the institution of standard operating procedures and computer control to quantify the reduction in reaction rate uncertainty resulting from those controls. 

In the context of the present discussion, it is worth noting that virtually all the experimental systems that exhibit such "anomalous temperature-dependent transfer coefficients are multistep inner-sphere processes, such as proton and oxygen reduction in aqueous media [84]. It is therefore extremely difficult to extract the theoretically relevant "true transfer coefficient for the electron-transfer step, ocet [eqn. (6)], from the observed value [eqn. (2)] besides a knowledge of the reaction mechanism, this requires information on the potential-dependent work terms for the precursor and successor state [eqn. (7b)]. Therefore the observed behavior may be accountable partly in terms of work terms that have large potential-dependent entropic components. Examinations of temperature-dependent transfer coefficients for one-electron outer-sphere reactions are unfortunately quite limited. However, most systems examined (transition-metal redox couples [2c], some post-transition metal reductions [85], and nitrobenzene reduction in non-aqueous media [86]) yield essentially temperature-independent transfer coefficients, and hence potential-independent AS orr values, within the uncertainty of the double-layer corrections. 

From the above it is clear that kinetic rate equations are afflicted with uncertainties, which can be caused by inaccurate data and the fitting procedures used, and also by the experimental methods and the differences in catalyst material and its quality. It is not possible to conclude from the fitted rate equations, what the reaction mechanism of the catalysis is. 

In this form of voltammetry, the concentration distributions of each species in the electrode reaction mechanism are temporally invariant at each applied potential. This condition applies to a good approximation despite various processes still occurring such as mass transport (e.g. diffusion), heterogeneous electron transfer and homogeneous chemical processes. Theoretically it takes an infinite time to reach the steady state. Thus, in a practical sense steady-state voltammetric experiments are conducted under conditions that approach sufficiently close to the true steady state that the experimental uncertainty of the steady-state value of the parameter being probed (e.g. electrode current) is greater than that associated with not fully reaching the steady state. The... 

The present paper describes the most important progress that has been made within the understanding of the atmospheric chemistry of mercury within the application of theoretical calculations and experimental studies for determination of reaction coefficients and mechanisms with halogens and other reactants. There are still large uncertainties to cope with before a reliable description of dynamics and fate of mercury can be established. Theoretical calculations represent a very cost effective method to get the first information about rate constants, reaction products and as to what experimentalists should examine. Finally, theoretical calculations can document that we actually have a full understanding of the fundamental processes of atmospheric mercury. The study of lO [53] in the Antarctic opens the possibility that 1 and lO plays an important role in the oxidation of Hg . These reaction mechanisms should continue to be studied in the field and with theoretical methods. As most laboratory studies of the oxidation mercury in the atmosphere are carried out at room temperature it is very important that theoretical calculations state the temperature dependence of the various reaction steps and the thermally stability of the reaction intermediates and end products. 

Further insight into the reaction mechanism can be obtained from consideration of the conversion dependence of product ratios. These can provide more reliable information than normal selectivity plots since, although these ratios are subject to the uncertainties in the individual product yields, their accuracy is not affected by the larger uncertainty in the calculated conversion levels. Moreover, the variation in product yield ratios with conversion can provide a clearer insight into the course of secondary reactions. 

Determining the peak current density in cyclic voltammetry can sometimes be problematic, particularly for the reverse sweep, or when there are several peaks, which are not totally separated on the axis of potential. The usual way to determine the peak currents is shown in Fig. 7L. For the forward peak, the correction for the baseline is small and does not substantially affect the result. For the two reverse peaks, however, the baseline correction is quite large and may introduce a substantial uncertainty in the value of the peak current density. In fact, there is no llieory behind the linear extrapolation of the baselines shown in F/g. 7L, and this leaves room for some degree of "imaginative extrapolation." This is one of the weaknesses of cyclic voltammetry, when used as a niumtitative tool, in the determination of rate constants and reaction mechanisms. 

All these reactions are highly exothermic, and these disproportionations occur at a rate of the same order of magnitude as the corresponding recombinations.Thus none of them can be excluded a -priori. For the recombination, there are six reactions and six different products. For the disproportionations, there are nine reactions but only six products (one of which is the reactant). Here is an example of the uncertainty principle of reaction mechanisms. Each of the products is produced by three different reactions thus a kinetic study based on analysis of products alone will never characterize any of the reactions. 

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