Radiative stabilization reaction

It was experimentally found that the overall process of ion-molecule association reactions can more descriptively be schemed [1] as follows collisional stabilization reaction (Eq. 2.1), radiative stabilization reaction (Eq. 2.2), and elimination reaction (Eq. 2.3) from ion-molecnle complexes. 

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. 

In specific applications, it is critically important to know which isomer is produced in a particular situation in order to ascertain its further reactivity. Indeed, further reactivity, in the form of rate coefficients and product ion distributions, both identifies which reactions generate the same isomeric forms and gives information to enable the isomeric forms to be identified (often by determining the energetics and comparing them with theoretical calculations). One such application is to molecular synthesis in interstellar gas clouds. In the synthesis of the >115 molecules (mainly neutral -85%) detected in these clouds,14 a major production route is via the radiatively stabilized analog of the collisional association discussed above,15 viz. ... 

Bass et al. (1981) published phase space theory models of the reaction CHj + HCN -> (CH3 HCN) + hv, analyzing, in particular, radiative stabilization of the complex. Important work on radiative stabilization has also been published by Dunbar (1975), Herbst (1976) and Woodin and Beauchamp (1979). 

A versatile route for the gas-phase synthesis of various gold(I) complexes is provided by the reaction of Au+ with hexafluorobenzene. While IE F ) = 9.91 eV is large enough to prevent ET, C6F6 has a sufficient number of rovibronic states to allow for efficient formation of the Au(C6F6)+ complex via radiative stabilization in the low-pressure regime according to Reaction (7.5) (Schroder et al. 1995) ... 

Chemical reactions take place in ionized clusters containing acetylene or other triple-bonded moieties. The reaction heat liberated upon formation of the new bond(s) is dissipated by condensation (i.e. elimination of a neutral residue from the cluster), unless collisional or radiative stabilization is possible. 

Radiative Association Reactions The study of radiative association reactions, (Eq. 2.2), has been of considerable interest [6-8] in chemical kinetics, planetary and interstellar chemistry, flames, and a variety of other areas. The kinetic study makes it possible to model the formation of complex molecular species in the interstellar science. At the very low molecular number densities in interstellar environments, the probability of formation of the products of association reactions by collisional stabilization is very low. Therefore, the radiative association process becomes an extremely important one for the production of the complex molecular species observed by astronomical physicist. The methodology is either flowing afterglow (FA) or Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. For the study of the apparent bimolecular rate constant for formation of association products as a function of pressme of a third body (N), the pressure should be set up to be sufficiently high in order to release the energy in the associated complex. Under the high pressure conditions collisional stabilization has competed with and usually dominated over radiative associatioiL As a result, the radiative association rate was then extrapolated from the intercept of a plot of apparent rate constant versus pressure of a third body, N. 

Clearly, emission of radiation is needed to stabilize the excited (CHg ) ion this radiation has not yet been observed. The radiative association rate coefficient for reaction (11) i.e. k ad H) been determined to )3e 1.1 X lO cm s at 13K (Barlow et al 1984), although this is in conflict with a more recent experimental value obtained at a higher temperature by Gerlich (1987). Most of the kj. values for radiative association reactions other than for reaction (11) are inferred from laboratory measurements of the analogous collisional (ternary)... 

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. 

In order to avoid such ambiguities, the definition of chemical species will depend on the simple concept of stability. In the absence of chemical reactions, a chemical species will last indefinitely. Thus an ion is a distinct chemical species, and an electron transfer reaction must be seen as a chemical change. However, an electronic excited state of an atom or molecule must inevitably decay back to the ground state, so the processes of excitation, emission and non-radiative deactivation are photophysical processes. 

The last decade has witnessed an intense interest in the theory of radiative association rate coefficients because of the possible importance of the reactions in the interstellar medium and because of the difficulty of measuring these reactions in the laboratory. Several theories have been proposed these are all directed toward systems of at least three or four atoms and utilize statistical approximations to the exact quantum mechanical treatment. The utility of these treatments can be partially gauged by using them to calculate three body rate coefficients which can be compared with laboratory measurements. In order to explain these theories briefly, it would be helpful to write down equations for the mechanism of association reactions. Consider two species A+ and B that come together with bimolecular rate coefficient kj to form a complex AB+ which can then be stabilized radiatively with rate coefficient kr, be stabilized collisionally with helium with rate coefficient kcoll, or redissociate with rate coefficient k j ... 

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