Reaction complex collisional stabilization

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. 

Although collision-stabilized reaction complexes take part in chain propagation, the complex spectra of ions observed for ethylene and acetylene suggest that this mechanism undoubtedly must compete with consecutive reactions of species produced by unimolecular dissociation of the complexes and by collisional dissociation of other ions. ... 

Reactants AB+ + CD are considered to associate to form a weakly bonded intermediate complex, AB+ CD, the ground vibrational state of which has a barrier to the formation of the more strongly bound form, ABCD+. The reactants, of course, have access to both of these isomeric forms, although the presence of the barrier will affect the rate of unimolecular isomerization between them. Note that the minimum energy barrier may not be accessed in a particular interaction of AB+ with CD since the dynamics, i.e. initial trajectories and the detailed nature of the potential surface, control the reaction coordinate followed. Even in the absence (left hand dashed line in Figure 1) of a formal barrier (i.e. of a local potential maximum), the intermediate will resonate between the conformations having AB+ CD or ABCD+ character. These complexes only have the possibilities of unimolecular decomposition back to AB+ + CD or collisional stabilization. In the stabilization process,... 

Notes The data clearly illustrate that at the low pressures in the ICR binary channels dominate, but that the intermediate complex is sufficiently long lived to be collisionally stabilized in the higher pressure SIFT experiments. The cyclic isomer, c-C,H(, was unreactive with these reactant neutrals. bAn isomerization reaction to yield c-C,H( also occurs.515-1... 

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 ... 

Transition state theory suggests that reactions (11-13) and (11-14) stem from the same process, illustrated in Fig. 11-29 (Westmoreland et al., 1986). Reactants associate to a metastable state, HO2, and temporarily store 205 kJ/mol of energy of the H-O2 bond in mostly vibrational modes. In hydroperoxy radical formation, the third body M collisionally stabilizes the HO2 complex into the stable HO2 potential energy well. Failing stabilization, HO2 may decompose back to H and O2, while at higher temperatures, the energy barrier... 

While isolated laboratory experiments involving reactions which may be important in flames provide information under controlled conditions, these conditions may be somewhat removed from those which are found in flames. Clearly our reaction rate determinations should be extended to higher temperatures, which may cause the association reactions seen to have lower rates, due to dissociation of the ion/molecule reaction complexes formed. Also, studies at higher pressures should be performed, where there is increased opportunity for collisional stabilization of the collision complexes. As data from work in our laboratories and those of others accumulate, they can be used to refine computational models such as those already reported (10), in order to more fully test the proposed ionic soot formation mechanism. 

One example in which collisional stabilization of the initial complex, reaction (5), competes with reaction via an exothermic channel, reaction (4), is for the reaction... 

The general phenomena of chain branching and of collisional stabilization of reaction complexes at higher pressure already discussed for acetylene were observed in the ethylene system as well. For this system, the main reaction sequence initiated by the parent molecular ion is production of the secondary ion C3H5 , which in turn leads to the production of 5119, ... 

We can estimate the rate constant for the collisional stabilization process, Reaction (7), by assuming that only those hard-core collisions that result from the long-range polarization force between the ion and the neutral molecule are important and that orbiting collisions have unit effectiveness for stabilizing the complex. Thus the rate constant of this reaction is estimated to be 1.1 X 10 cm mole sec from the expression... 

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. 

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