Indirect dissociation

Butler D A and Hayden B E 1995 The indirect channel to hydrogen dissociation on W(100)c(2 2)Cu—evidence for a dynamical precursor Chem. Phys. Lett. 232 542... 

There are many compounds in existence which have a considerable positive enthalpy of formation. They are not made by direct union of the constituent elements in their standard states, but by some process in which the necessary energy is provided indirectly. Many known covalent hydrides (Chapter 5) are made by indirect methods (for example from other hydrides) or by supplying energy (in the form of heat or an electric discharge) to the direct reaction to dissociate the hydrogen molecules and also possibly vaporise the other element. Other known endothermic compounds include nitrogen oxide and ethyne (acetylene) all these compounds have considerable kinetic stability. 

The concentration of fluoride in drinking water may be determined indirectly by its ability to form a complex with zirconium. In the presence of the dye SPADNS, solutions of zirconium form a reddish colored compound, called a lake, that absorbs at 570 nm. When fluoride is added, the formation of the stable ZrFe complex causes a portion of the lake to dissociate, decreasing the absorbance. A plot of absorbance versus the concentration of fluoride, therefore, has a negative slope. 

The determination of the degree of dissociation of cotarnine ° and the good agreement with the values derived from measurements of electrical conductivity with those from the spectrophotometric methods is indirect evidence that no significant part of the undissociated cotarnine is in the amino-aldehyde form. In the conductance calculation, the undissociated part was neglected. If this included a significant amount of amino-aldehyde (i.e., a secondary base), there would be a noticeable discrepancy in the degree of dissociation obtained by the two methods. 

Significant stimulated emission is only found for the pristine side of the sample. From these results it was concluded that the photoinduccd absorption that suppresses die stimulated emission is directly or indirectly caused by the presence of oxygen-related defects. It was shown earlier that the effect of photooxidation on the emission properties of PPV can be explained by the formation of carbonyl-groups that act as sLrong electron acceptors leading to an efficient dissociation of the plioh excited slate 29). It can be concluded that the dissociated pair near the defcci leads to the strong photoinduccd absorption. The observation that... 

Dissociative chemisorption was considered to be either direct, when the incoming diatomic molecule has sufficient energy to surmount the barrier without being trapped into the molecular state, or indirect, when it passes via the molecular (precursor) state into the dissociated state. If the dissociated state is not immediately equilibrated with the lattice, the fragments will move across... 

The role of OSC materials in NO conversion is more complex. Most metals, specially Rh, are able to decompose NO, but this reaction is rapidly inhibited by O species resulting from this decomposition [60]. On Rh, for instance, Rh-0 species are replaced by Rh-NO+ ones in which NO is no longer dissociated [61]. O species may react with adsorbed species of the reducer (CO, HC) to form C02. The first role of the OSC materials could be to liberate metal sites by accepting O species. Indirect effects can also occur, with the Ce3+/Ce4+ redox system being able to regulate the metal state (zero-valent or ionic) during richness oscillations. 

The above kinetics studies of the thermal reactions provide powerful indirect evidence for the operation of a limiting dissociative mechanism in this solvent and for the formation of a reactive intermediate such as IV. Such studies also allow one to evaluate the relative reactivities of that intermediate with different substrates. For example, k.g/kg, the ratio of the rate constants for reaction of IV with CO or PPI13 in 25° THF, was determined to have the value 15 ... 

In this section we deal with the first of the physical effects which impinge on reactivity — the influences which heats of reaction and bond dissociation energies have on the course of chemical reactions. Both heats of reaction and bond dissociation energies are enthalpy values that are experimentally determined by thermochemical methods, in the first case usually by direct calorimetric methods, in the second by more indirect techniques 22). 

Excited states of hydrocarbon molecules often undergo nondissociative transformation, although dissociative transformation is not unknown. In the liquid phase, these excited states are either formed directly or, more often, indirectly by electron-ion or ion-ion recombination. In the latter case, the ultimate fate (e.g., light emission) will be delayed, which offers an experimental window for discrimination. A similar situation exists in liquid argon (and probably other liquefied rare gases), where it has been estimated that -20% of the excitons obtained under high-energy irradiation are formed directly and the rest by recombination (Kubota et al., 1976). 

In careful experiments by pulse radiolysis, the maximum G value of ozone production is 13.8, of which 6.2 comes from ionization and eventual neutralization, each such sequence giving two O atoms. If the remaining yield is attributed to the dissociation of excited states, either directly or indirectly, then the total yield of excitation will be about the same as that of ionization, 3.8 in this case, because each dissociation also gives two O atoms. 

Figure 3 Different processes for losing energy from the excited state (1) direct CL (2) molecular dissociation (3) chemical reaction with other species (4) intramolecular energy transfer (5) intermolecular energy transfer (in case of a fluorophore, indirect CL) (6) isomerization (7) physical quenching. (Adapted from Ref. 1.)...

Peroxyoxalate-based CL reactions are related to the hydrogen peroxide oxidation of an aryl oxalate ester, producing a high-energy intermediate. This intermediate (l,2-dioxetane-3,4-dione) forms, in the presence of a fluorophore, a charge transfer complex that dissociates to yield an excited-state fluorophore, which then emits. This type of CL reaction can be used to determine hydrogen peroxide or fluorophores including polycyclic aromatic hydrocarbons, dansyl- or fluores-camine-labeled analytes, or, indirectly, nonfluorescers that are easily oxidized (e.g., sulfite, nitrite) and quench the emission. The most widely used oxalate... 

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