Third body

Hydrogen atoms can be produced in significant quantities in the gas phase by the action of radiation on or by extreme heating of H2 (3000 K). Although hydrogen atoms are very reactive, these atoms can persist in the pure state for significant periods of time because of the inabiUty to recombine without a third body to absorb the energy of bond formation. 

The cycle accounts for 20—25% of the halogen-controUed loss (M is a third body molecule). The possible contribution of naturaUy occurring iodine compounds to ozone destmction (via I and lO radicals) is being investigated (79). 

Reaction (12-9) shows the photochemical dissodation of NO2. Reaction (12-10) shows the formation of ozone from the combination of O and molecular O2 where M is any third-body molecule (principally N2 and O2 in the atmosphere). Reaction (12-11) shows the oxidation of NO by O3 to form NO2 and molecular oxygen. These three reactions represent a cyclic pathway (Fig. 12-4) driven by photons represented by hv. Throughout the daytime period, the flux of solar radiation changes with the movement of the sun. However, over short time periods (—10 min) the flux may be considered constant, in which case the rate of reaction (12-9) may be expressed as... 

In Figure 9.7 stresses are imposed on a body showing ordinary elastic deformation only, a second body showing high elastic deformation only and a third body showing viscous deformation only. The stress is imposed at time to and held at a constant value until time t, when it is removed. Deformation... 

FIG. 5 Density profiles from the PY2 theory (fines) and computer simulations (points) for the system without (a) and with (b) third-body forces. The association energy e /k T = 1 for all profiles except for the right panel in (b), where e /ksT = 4. The bulk density is 0.605 (left panel, part (a)), 0.586 (right panel, part (a)), 0.621 (left panel, part (b)) and 0.618 (right panel, part (b)). (From Ref. 86.)... 

Deussen termed the inactive caryophyllene, as further experiments showed it to be, a-caryophyllene, and the laevo-rotatory compound, )8-caryophyllene. A third body was obtained, which yielded no blue nitrosite at all, to which Deussen assigned the name -y-caryophyllene. This body... 

InEq. (8-17), M represents a so-called third body. In gas phase reactions of atoms, M plays an essential role in conserving energy. The bulk molecules (reactant, products, added inert gases) play this role. (No third body need be involved in solution reactions, however, owing to the presence of the solvent.)... 

Such reactions are often exothermic and the role of the third body is to carry away some of the energy released and thus stabilize the product molecule. In the absence of a collision with a third body, the highly vibrationally excited product molecule would usually decompose to its reactant molecules in the timescale of one vibrational period. Almost any molecule can act as a third body, although the rate constant may depend on the nature of the third body. In the Earth s atmosphere the most important third-body molecules are N2 and O2. 

The rate of the reaction depends on the product of reactant concentrations, including the third body ... 

Ensemble or Third-Body Ejfects. These effects concern the selective blockage of a particular adsorption site by adatom deposition. This can be advantageous when the reaction mechanism contains parallel paths that can be affected differently by blocking particular sites. In some cases, the undesired reaction needs more than one free adjacent site (ensemble), and can be inhibited by blocking particular sites without decreasing the reactivity of the surface for the catalyzed reaction. 

A different mechanism seems to operate in the case of poison formation from methanol [Herrero et al., 1993]. In this case, modification of the Pt(lll) surface by Bi deposition only causes a linear decrease in the amount of poison formed, indicating the existence of a mere third-body effect. Complete inhibition of the poisoning reaction is achieved for > 0.23, i.e., before the surface is completely covered. This suggests the existence of ensemble requirements for this reaction, which need enough free contiguous Pt sites to take place. 

This effect, called the third body effect by Conway and co-workers [101], is however controversial [102], The main argument against this theory is the fact that there is a specificity of catalytic behavior for each kind of metal adatom. Even adatoms producing similar geometrical blocking effects, present different catalytic properties. So, for instance, tin and lead [97] occupy two Pt atoms, but tin produces... 

Association reactions, in particular, seem to present a severe problem for structural determination. In these reactions, an ion and a neutral species form a complex which is stabilized either by collision with a third body or, at especially low pressures, by the emission of radiation. The radiative mechanism, prominent in interstellar chemistry, is discussed below. Although some studies of radiative association have been performed in the laboratory,30,31 90 most association reactions studied are three-body in nature. It is customarily assumed that the product of three-body association is the same as that of radiative association, although this assumption need not be universally valid. 

The mechanisms that we propose to explain the H3 data has led us to revisit and to reexamine earlier data on the recombination of other ions (see Section V). The goal was to see to what extent the experimental observations in afterglow plasmas support the assumption that only binary as opposed to third-body assisted recombination is important. While this is most likely true in many cases, the evidence is not as firm as one might think. Often, the original authors stated their conclusions rather cautiously, but over the years tentative conclusions have often assumed a status that is not justified by the original data. We will point out some gaps in our knowledge which should be closed by more detailed studies. 

The anomalous plasma decay suggests that the deionization coefficient is larger at higher electron densities and higher H2 densities. The second interpretation proposes that electrons and H2 molecules can act as stabilizing third bodies and that this process involves the same long-lived intermediate complexes that are observed in merged beam experiments (see Section IV.B). 

In the following section, we will reexamine some earlier recombination measurements to see if third-body effects may have played a role. The rare gas dimer ions provide a good starting point. 

The intermediate reaction complexes (after formation with rate constant, fc,), can undergo unimolecular dissociation ( , ) back to the original reactants, collisional stabilization (ks) via a third body, and intermolecular reaction (kT) to form stable products HC0j(H20)m with the concomitant displacement of water molecules. The experimentally measured rate constant, kexp, can be related to the rate constants of the elementary steps by the following equation, through the use of a steady-state approximation on 0H (H20)nC02 ... 

Gas phase third-order reactions are rarely encountered in engineering practice. Perhaps the best-known examples of third-order reactions are atomic recombination reactions in the presence of a third body in the gas phase and the reactions of nitric oxide with chlorine and oxygen (2NO T Cl2 -> 2NOC1 2NO + 02 -> 2N02). 

If equations 4.2.25 and 4.2.26 are substituted for equations 4.2.11 and 4.2.15, respectively, in the mechanism described above, the effect is to replace kx by k [M] and k5 by /c 5[M] everywhere that they appear. Since these quantities appear as a ratio in the final rate expression, the third body concentration will drop out and kjks) becomes identical with k /k 5 The necessity for the use of the third body concentration thus is not obvious in kinetic studies of the thermal reaction. However, from studies of photochemical reaction between hydrogen and bromine, there is strong evidence that the termination reaction is termolecular. This fact and... 

CH2+ (8%), with some minor ions such as CH+ and C+ accounting for the rest. The radical reactions are mainly either recombination in the presence of a third body or insertion reactions with the parent molecule from which, all in all, a great variety of reactions can ensue (Meisels, 1968). Some examples are... 

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