Radiative association

CHXNH. Many of the reactions involve radiative association. Dissociative electron-ion recombination then yields neutrals such as CH (methane), C2H OH (ethanol) and CH CN (acetonitrile) [158]. It is often joked... 

Cheng Y-W and Dunbar R C 1995 Radiative association kinetics of methyl-substituted benzene ions J. Rhys. Chem. 99 10 802-7... 

The first step in interstellar chemistry is the production of diatomic molecules, notably molecular hydrogen. Observations of atomic hydrogen in dense clouds show that this species cannot be detected except in a diffuse halo surrounding the cloud, so that an efficient conversion of H into H2 is necessary. In the gas phase this might be accomplished by the radiative association reaction,... 

If the H3 ion reacts with atomic carbon, an analogous series of reactions leads to the methyl ion, CH3, although the initial reaction to form CH+ has not been studied in the laboratory. The methyl ion does not react rapidly with H2 but does undergo a relatively slow radiative association reaction,... 

Another variation in the initial chemistry occurs for the case of sulfur. The reaction between S and H3 to form SH+ and H2 is exothermic although it has not been studied in the laboratory. The SH+ product does not react exothermically with H2 and only a rather slow radiative association, in which an electronic spin flip must occur, can produce H3S+, the precursor to HS and HjS.40... 

Perhaps the synthesis of hydrocarbons is best understood. There are three main classes of reactions leading to complex hydrocarbons carbon insertion, condensation, and radiative association. Carbon insertion reactions are between C+ ions and smaller hydrocarbon neutrals viz.,... 

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. 

Ion-molecule radiative association reactions have been studied in the laboratory using an assortment of trapping and beam techniques.30,31,90 Many more radiative association rate coefficients have been deduced from studies of three-body association reactions plus estimates of the collisional and radiative stabilization rates.91 Radiative association rates have been studied theoretically via an assortment of statistical methods.31,90,96 Some theoretical approaches use the RRKM method to determine complex lifetimes others are based on microscopic reversibility between formation and destruction of the complex. The latter methods can be subdivided according to how rigorously they conserve angular momentum without such conservation the method reduces to a thermal approximation—with rigorous conservation, the term phase space is utilized. 

Based on the study of simple radiative association processes that are statistical in nature, one can conclude that even with small binding energies, as the size of the reactants becomes sufficiently large, radiative association becomes 100% efficient. Using the phase-space approach,96 Herbst and Dunbar99 have studied the rate of radiative association reactions between hydrogen-rich hydrocarbons of the type,... 

Herbst and Dunbar" have investigated the effects of exit channel barriers on association reactions of type 43 and have shown that, depending on the size of the barrier, the efficiency of radiative association reactions as a function of N can be strongly curtailed. For example, at 10 K and a nonpolar neutral reactant, they found for a system with a well depth of 2 eV and an exothermic channel barrier of 1.0 eV, N = 130 atoms for 100% sticking efficiency, approximately 10 times the corresponding value of N in the absence of a competitive exothermic channel. 

The mechanism for the formation of complex hydrocarbons through fullerenes is loosely taken from Helden et al.119 and Hunter et al.,120 and is depicted in Figure 2. As in the work of Thaddeus,117 linear carbon clusters grow via carbon insertion and radiative association reactions, although in this case a large number of additional reactions involving neutral atoms such as C, O, and H and neutral molecules such as H2 are also included. Reactions with H and H2 serve to produce... 

The process of radiative association has recently been reviewed by McMahon16 and by Dunbar.17 The observed neutral molecules are then produced by the dissociative recombination of these product ions with electrons, e.g. ... 

This isomeric form is of interest from an interstellar point of view since the isomer, CH3OH2, is a possible route, via dissociative electron-ion recombination, to the observed methanol.14 A proposed reaction68 leading to this isomer is the radiative association,... 

This would reveal possible routes to the C2H5CNH+ that is believed to be produced in the analogous radiative association reactions that occur in interstellar gas clouds. 

Binary ion-molecule reactions are indicated by thin arrows (c.t. indicates charge transfer), the radiative association reaction of C+ with H2 is indicated by the thick arrow and the dissociative recombination reactions are indicated by dashed arrows leading to the neutral molecules inside the compound brackets (e indicates free electrons). The molecules indicated in bold are known (observed) interstellar molecules. 

Radiative association Collision of two atoms or molecules to form a new chemical bond stabilised by radiating the energy of bond formation to produce the molecule in its ground state. 

Again the radiative association kinetics described above allow a direct comparison for some realistic values of k and k. For most chemically activated systems at the threshold for unimolecular dissociation, the observed radiative rate constants are of the order of 10-100 s and hence are much below the values expected for k of about 10 s . Therefore, the first limit is most likely to be valid, with the interesting conclusion that the observed unimolecular dissociation rate constant will depend only on the photon density and the absorption cross section (rate constant) at a given wavelength. 

Radiative association is the combination of ion and molecule to form a metastable complex that is then stabilized by emission of a photon ... 

Standard hydrocarbon estimation. Measurement of the low-pressure radiative association rate constant is sufficient for assignment of if k and k are independently known, from the relation, ... 

Figure 7 shows how the experimental radiative association rate constant of 1.7 X 10 cm molecule s combines with the calculated curve to give a fitted binding energy of 1.53 0.10 eV, where the uncertainty is estimated from the uncertainty in the k that was used in the PST calculation. The previous thermochemistry for this complex, as analyzed in Ref 52, gives a value of 1.6 0.3 eV. 

Figure 8. Radiative association rate constants measured for dimerization of methyl-substituted benzene ions. The binding energies derived from these measurements are shown in Table 5.

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