Radiative association reaction

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,... 

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... 

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. 

Modified thermal (Bates 1983) or phase space (Herbst 1985c) calculations of radiative association rates indicate, as expected, an inverse temperature dependence and a direct dependence on the complexity of the reaction partners. Thus, if theory is to be believed, the importance of radiative association is enhanced by complex molecules reacting in cold clouds. Let us consider two important examples in the synthesis of interstellar methane (Huntress and Mitchell 1979). Although methane can only be observed with difficulty via radioastronomical methods (by centrifugal distortion induced rotational transitions) because it does not possess a permanent dipole moment, its synthesis is an important one because methane is a precursor to more complex hydrocarbons which can be and have been detected. This synthesis can proceed via the following series of normal and radiative association reactions, most of which have been studied in the laboratory ... 

Somewhat after this synthesis appeared in the literature, it was criticized by Mitchell, Huntress, and Prasad (1979) who proposed a synthesis involving radiative association reactions between protonated nitrogen-containing ions and hydrocarbon neutrals. In this approach the synthesis of the lowest members of the series proceeds by... 

Both of these approaches involve collisions of two large species. According to model calculations by one of us (Herbst 1983), neither of these syntheses can reproduce the observed high abundance of HC3N in sources such as the nearby dense interstellar Cloud TMC-1. However, this negative assessment for the first mechanism relies on a laboratory rate coefficient for reaction (3.39) at room temperature that shows the reaction to be slow slow ion-molecule reactions at room temperature often become more rapid at lower temperature (Rowe et al. 1984). Likewise, the negative assessment for the second mechanism is based on a theoretical calculation for the rate coefficient of the radiative association reaction. 

It was suggested by first Smith and Adams (1978) and then Huntress and Mitchell (1979) that radiative association reactions involving the methyl ion CH (synthesized via Reaction 3.20) and a host of small neutral species could lead to a variety of more complex ion precursors to organic molecules. The rate coefficients of these radiative association reactions have since been calculated by Bates (1983) and Herbst (1980a, b 1985 a, c). Let us consider some of these reactions and the subsequent dissociative recombination reactions ... 

Other radiative association reactions leading to formic acid, ethanol, and sundry species are discussed in Leung, Herbst, and Huebner (1984). Calculations of radiative association rate coefficients for ion-molecule systems with large numbers of atoms will be necessary to extend gas phase mechanisms to the syntheses of still larger species. Unfortunately, such calculations are often rendered difficult by the lack of suitable thermodynamic, structural, and vibrational data on the product ions which are needed as input into the calculations. A somewhat easier approach is to estimate the radiative association rate coefficient from higher temperature laboratory three-body rates (Smith et al. 1983). Even so, this approach cannot be used for most reactions of interest involving more complex reactants because of a paucity of laboratory measurements. It is clear that more laboratory work will always be needed ... 

In dense clouds however reactions with are of importance Black and Dalgarno (1973) suggested the radiative association reaction between and... 

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. 

Woodin RE, Beauchamp JL. Bimolecular infrared radiative association reactions. Attachment of LG to carbonyl compounds in the gas phase. Chem Phys. 1979 41 1-9. 

Bass LM, Kemper PR, Anicich VG, Bowers MT. Ion-molecule radiative association reactions. A statistical phase space theory model. J Am Chem Soc. 1981 103 5283-92. 

ABSTRACT. Radiative association reactions are reactive processes in which two smaller gas phase species collide to form a larger molecule while emitting a photon. These reactions are thought to be important in the synthesis of molecules in both diffuse and dense interstellar clouds. Mo ls of interstellar clouds require the rate coefGdents of a variety of radiative association reactions as input yet few experimental studies of these processes have been undertaken. Therefore, Ae role of theory in the detmnination of radiative association rate coefficients is paramount. Most experimental studies of association reactions are at sufficiently high pressure that the mechanism for association is collisional rather than radiative. Yet even collisional (tonary) association studies yield valuable information about radiative association processes. In this review, we consider the nature of association reactions - both radiative and ternary - and discuss experimental and thecaetical tqrproachesm the detomination of rate coefficients of radiative association reactions. 

But, no radiative association reactions in this latter class appear to be important in interstellar clouds. 

In the first class of interstellar radiative association reactions, the neutral reactant is molecular hydrogen and the process of radiative association acts as a detour when normal ion-molecule reactions cannot occur. As one example, the reaction... 

In the second class of radiative association reactions, the neutral species involved is a heavy molecule and the piocess of radiative association leads to the formation of complex molecular ions (Smith and Adams 1978 Huntress and Mitchell 1979). An example is the reaction... 

Superimposed on the plot of kg vs. [C] in Fig. 1 are some of the experimental techniques used to study ion-molecule association reactions in the laboratory. The low pressure ion trap technique (designated TRAP) of Dunn and co-workers (see, e.g. Barlow, Dunn, and Schauer 1984a,b) operates at sufficiendy low densities that there is no doubt that only radiative association is being observed. Unfortunately, these experiments are difficult and only a few have been performed. In only one - the radiative association reaction between CH3" and H2 (Barlow, Dunn, and Schauer 1984a,b) - was an actual value rather than an upper limit determined. A strength of this technique is that it can be utilized at temperatures as low as 10 K if H2 is used as the neutral reactant. 

As will be discussed below, the current estimate that kj. 1(3) s for most radiative association reactions may well be incorrect. 

A simple approach to the calculation of both ternary and radiative association reaction rate coefficients for polyatomic systems was given by Herbst (1979 1980a). This approach... 

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