Reaction mechanism discrimination

This mechanistic question is one of the examples of the success of density functional theory methods in organometallic chemistry. Earlier work on the reaction mechanism could not discriminate between the two alternatives. Analysis of the different orbitals based on extended Hiickel calculations came to the result that the [3+2] pathway is more likely, but could not exclude the possibility of a [2+2] pathway [13]. Similar conclusions where obtained from the results of Hartree-Fock calculations in combination with QCISD(T) single point calculations [21], Attempts to use Ru04 as a model for osmium tetraoxide indicated that the formation of an oxetane is less favorable compared to the [3+2] pathway, but still possible [22, 23],... 

Even in this case, traditionally, a logarithmic form is used [Equation (9)] to treat kinetic data. The intercept of the linear plot (Figure 3) gives the value of k0 and the slope the value of ZAZB, and this is very useful information for ionic reactions to discriminate between kinetically equivalent mechanisms ... 

In appropriate cases spectroscopic investigation of reaction intermediates can give highly precise information about their orientation and position in the lattice. Although such information is usually not sufficient to establish the reaction mechanism in full detail, there can be more than enough to discriminate between a realistic and an unrealistic computer simulation. 

A periodically forced system may be considered as an open-loop control system. The intermediate and high amplitude forced responses can be used in model discrimination procedures (Bennett, 1981 Cutlip etal., 1983). Alternate choices of the forcing variable and observations of the relations and lags between various oscillating components of the response will yield information regarding intermediate steps in a reaction mechanism. Even some unstable phase plane components of the unforced system will become apparent through their role in observable effects (such as the codimension two bifurcations described above where they collide and annihilate stable, observable responses). 

The fundamental concept of chemical kinetics is that of reaction mechanism. In the broad sense, the word mechanism ("detailed , "intimate ) is the comprehensive interpretation of all experimental data accumulated on the complex reaction process. In this mechanism, one should discriminate individual stages and reaction steps, give characteristics for intermediates, describe transition states of individual steps, provide energy levels of substances, etc. As far as catalytic reactions are concerned, one should characterize surface properties, examine the adsorption character, etc. "I want to know everything about a complex chemical reaction this is the way one must understand chemists when they speak about their intention to investigate a detailed mechanism. Whether it is possible to realize such good intentions at a modern theoretical and experimental level will be another question. 

Let us emphasize the most essential conclusion that can be drawn in this section a sufficient condition for the uniqueness of steady states in catalytic reactions is the absence of interaction steps for various intermediates in the detailed reaction mechanisms. Their presence is a necessary condition for the multiplicity of steady-state values for the catalytic reaction rates. This principal statement possesses an evident discrimination property. If some experiment is characterized by the multiplicity of steady states and its interpretation suggests a law of acting surfaces, the description of this experiment implies a detailed mechanism that must contain interaction steps of various intermediates. 

The construction of this matrix is sufficiently described in the original studies [33,43] and therefore it is not necessary to repeat it here. We recall only that the form of the matrix depends on the actual reaction mechanism of the transformation R - P so that it is precisely via this matrix that the possibility of discriminating between various reaction mechanisms enters into play. 

This intuitive parallel can be best demonstrated by the example of electrocye-lic reactions for which the values of the similarity indices for conrotatory and disrotatory reactions systematically differ in such a way that a higher index or, in other words, a lower electron reorganisation is observed for reactions which are allowed by the Woodward-Hoffmann rules. In contrast to electrocyclic reactions for which the parallel between the Woodward-Hoffmann rules and the least motion principle is entirely straightforward, the situation is more complex for cycloadditions and sigmatropic reactions where the values of similarity indices for alternative reaction mechanisms are equal so that the discrimination between allowed and forbidden reactions becomes impossible. The origin of this insufficiency was analysed in subsequent studies [46,47] in which we demonstrated that the primary cause lies in the restricted information content of the index rRP. In order to overcome this certain limitation, a solution was proposed based on the use of the so-called second-order similarity index gRP [46]. This... 

The effect of enantioselectivity reversal serves as an additional experimental observation that gives a possible clue for the reaction mechanism. By the proposed additive-product interactions it was predicted that even poor stereoselectivity and discriminating capability of the catalytic additive can give rise to enantioselectivity reversal. This also gives a possible kinetic explanation for the effect of miscellaneous chiral additives in the Soai reaction and their role as potent chiral initiators. 

Using the thermochemical estimates given above, along with the considerable body of available thermochemical and kinetic data, several plausible reaction pathways in coal and model compound reactions will now be examined. This analysis is intended to discriminate between feasible and unlikely reaction mechanisms. It should be kept in mind that absolute rate constant estimates are often only very approximate, and we are testing ideas, not proving them. 

From the calorimetric results of the study of surface interactions, it has been deduced (Section VI, C) that on the surface of the gallium-doped nickel oxide, as in the case of NiO(250°), two reaction paths are probable (mechanisms I and II). The actual reaction mechanism on NiO(10 Ga)(250°) is discriminated, as in the former case (Fig. 24), by... 

Examination and discrimination of hypotheses Each of the proposed hypotheses can pose of detecting those features that mig t be expected for the dif ferent classes of mechanisms. Kinetic equations are derived at this stage for the stationary reaction route rates and (in case of linear mechanisms) equations are obtained for the rate of substance formation or depletion or, in the most general case, systems of differential equations are written. Examination of the kinetic models allows us to devise a plan for model (hypothesis) discrimination using chemical, physicochemical, and kinetic methods. It is worthwhile discussing in more detail these methods f< r discriminating reaction mechanism hypotheses. 

In view of these considerations, it may not be possible to determine accurately the detailed reaction mechanism in such photochromic ABC systems. However, under some conditions, the experiments can be arranged in such a way as to discriminate between similar reaction mechanisms and extract the relevant parameters. This will include varying the incident photon flux /q, the irradiation wavelength X, the duration of irradiation tirr, the temperature, and the initial concentrations. Appendixes 4 and 5 show how the particular thermal or photochemical processes in ideal photochromic systems of the ABC type can be identified. 

From a postulated reaction mechanism (the model), a rate equation can be derived and used to analyse the experimental data. If the obtained fit is not statistically significant, the scheme is rejected. In complex systems, several schemes can produce compatible rate expressions and the problem of model discrimination is of primary importance. 

Discriminating Between Reaction Mechanisms Metal Cluster Rearrangements... 

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