Channels of reaction

As for the samples which were not only stored but also irradiated at different temperatures (in particular, at 77 and 120 K), further, more detailed investigations [39] of the effect of temperature on the kinetics of reaction (4) atT > 77 K, as well as a comparison of these data with those on the reaction etr with other acceptors in water alkaline glasses [39,40], showed the non-activated electron tunneling to be the predominant channel of reaction (4) only at sufficiently low temperatures (T < 93 K). At 93 K and above, the dominant channel appears to be the activated tunnel transfer with the activation energy Ea = 3.1 0.4kcalmol (Chap. 5, Sect.2.3). 

The structure of the product BaCl is close to Ba +Cl . The product channel of reaction 7 thus correlates to the double electron-transfer intermediate, whose strueture is probably CPBa CL [91, 92]. 

In subsequent work, Bayes (9) found that formation of allene was suppressed by NO and O2, well-known triplet scavengers, at wavelengths above 260 nm. No suppression was observed below 260 nm. From this observation he concluded that C2O in a low-lying singlet state was the photodecomposition product below 260 nm, but that triplet C2O was formed above this wavelength. This behavior with 2 and NO is similar to that found with singlet and triplet methylene. The relative Importance of the two channels of reaction 1 which lead to singlet or triplet C2O is in some doubt near 260 nm, however, since Cundall et al. (13) found that butene-2 suppressed the major part of processes which yield CO at 253.7 nm while Bayes found that 2 and NO had little effect at 254 nm on allene formation. 

K, compare stochastic and deterministic formulas (6-13) and (6-15) for the probability of the adiabatic channel of reaction (6-2) limiting the Zeldovich mechanism of NO synthesis in plasma. Explain the difference in temperature dependences of the probabilities for the stochastic and deterministic approaches. 

Smith et al. (1996) investigated the products of the OH-initated oxidation of cinnamaldehyde, generating OH by photolysis of CH3ONO. The yield of benzaldehyde formed in the presence of NO and air was 0.90 0.39, after corrections were made for photolysis of cinnamaldehyde and for reaction of benzaldehyde with OH. There are two probable channels, involving abstraction of the aldehydic H and addition to the double bond in the substituent group. In the former route, the initially formed acyl radical, CeHsCHCHCO, reacts with O2 to form the peroxy radical, and then NO to form benzaldehyde, at least in part. The two y3-hydroxy alkyl radicals formed by OH addition to the olefinic double bond react with O2 followed by NO to form -hydroxy alkoxy radicals, which will then dissociate to form benzaldehyde formaldehyde. Thus, the formation of benzaldehyde is compatible with both channels of reaction, but it was not feasible to assign channel yields. 

The mechanism of photodecomposition of 2,3-dimethylcyclobutanone is thought to involve several channels of reaction (Calvert et al., 2008) ... 

Venkatesh P K, Dean A M, Cohen M H and Carr R W 1999 Master equation analysis of intermolecular energy transfer in multiple-well, multiple-channel unimolecular reactions. II. Numerical methods and application to the mechanism of the C. + O2 reaction J. Chem. Phys. Ill 8313... 

Rather than using transition state theory or trajectory calculations, it is possible to use a statistical description of reactions to compute the rate constant. There are a number of techniques that can be considered variants of the statistical adiabatic channel model (SACM). This is, in essence, the examination of many possible reaction paths, none of which would necessarily be seen in a trajectory calculation. By examining paths that are easier to determine than the trajectory path and giving them statistical weights, the whole potential energy surface is accounted for and the rate constant can be computed. 

A single-channel manifold also can be used for systems in which a chemical reaction generates the species responsible for the analytical signal. In this case the carrier stream both transports the sample to the detector and reacts with the sample. Because the sample must mix with the carrier stream, flow rates are lower than when no chemical reaction is involved. One example is the determination of chloride in water, which is based on the following sequence of reactions. ... 

This leads to the possibiUty of state-selective chemistry (101). An excited molecule may undergo chemical reactions different from those if it were not excited. It maybe possible to drive chemical reactions selectively by excitation of reaction channels that are not normally available. Thus one long-term goal of laser chemistry has been to influence the course of chemical reactions so as to yield new products unattainable by conventional methods, or to change the relative yields of the products. 

The differential reactor is simple to construct and inexpensive. However, during operation, care must be taken to ensure that the reactant gas or liquid does not bypass or channel through the packed catalyst, but instead flows uniformly across the catalyst. This reactor is a poor choice if the catalyst decays rapidly, since the rate of reaction parameters at the start of a run will be different from those at the end of the run. 

Reactions of Complex Ions. For reactions of systems containing H2 or HD the failure to observe an E 1/2 dependence of reaction cross-section was probably the result of the failure to include all products of ion-molecule reaction in the calculation of the experimental cross-sections. For reactions of complex molecule ions where electron impact ionization probably produces a distribution of vibrationally excited states, kinetic energy transfer can readily open channels which yield products obscured by primary ionization processes. In such cases an E n dependence of cross-section may be determined frequently n = 1 has been found. 

Rosenstock (55) pointed out that the initial formulation of the theory failed to consider the effect of angular momentum on the decomposition of the complex. The products of reaction must surmount a potential barrier in order to separate, which is exactly analogous to the potential barrier to complex formation. Such considerations are implicit in the phase space theory of Light and co-workers (34, 36, 37). These restrictions limit the population of a given output channel of the reaction com-... 

In prokaryotes, each reaction of Figure 34-2 is catalyzed by a different polypeptide. By contrast, in eukaryotes, the enzymes are polypeptides with multiple catalytic activities whose adjacent catalytic sites facilitate channeling of intermediates between sites. Three distinct multifunctional enzymes catalyze reactions 3, 4, and 6, reactions 7 and 8, and reactions 10 and 11 of Figure 34-2. 

Five of the first six enzyme activities of pyrimidine biosynthesis reside on multifunctional polypeptides. One such polypeptide catalyzes the first three reactions of Figure 34-2 and ensures efficient channeling of carbamoyl phosphate to pyrimidine biosynthesis. A second bifunctional enzyme catalyzes reactions 5 and 6. 

The catalytic combustor provides heat for the endothermic reforming reaction and the vaporization of liquid fuel. The endothermic reforming reaction is carried out in a parallel flow-type micro-channel of the reformer unit. It is well known that the methanol steam reforming reaction for hydrogen production over the Cu/ZnO/AbOs catalyst involves the following reactions [10]. Eq. (1) is the algebraic summation of Eqs. (2) and (3). 

It is challenging experimentally to study two pathways leading to a single product channel for the simple reason that the products in either case are structurally identical. Nevertheless, there are several methods, each applicable to certain classes of reactions, that can distinguish the presence of multiple pathways. 

In the study of reaction mechanisms, it is almost always easier to detect final products than reaction intermediates. Although it is often the case that each detected product channel represents one pathway on the PES, this chapter demonstrates many examples where this assumption fails. 

The study of multiple pathways leading to a single product channel provides a stringent test of our understanding of the potential energy surface and the calculations that use it to predict reaction outcomes. Although there are not many examples to date of pathway competitions, the increasing prominence of such systems, coupled with advances in experiment and theory that facilitate their study, promises a rich future in this normally hidden facet of reaction mechanisms. 

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