Polyatomic bimolecular reactions

Bowman, J. M., and G. C. Schatz. 1995. Theoretical Studies of Polyatomic Bimolecular Reaction Dynamics. Annu. Rev. Phys. Chem. 46,169. 

Bowman JM, Schatz GC (1995) Theoretical studies of polyatomic bimolecular reaction dynamics. Annu Rev Phys Chem 46 169... 

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although confonnational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thennally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, k = A. The quantity /cg T//i from transition state theory provides... 

Undoubtedly, the technique most suited to tackle polyatomic multichannel reactions is the crossed molecular beam (CMB) scattering technique with mass spectrometric detection and time-of-flight (TOF) analysis. This technique, based on universal electron-impact (El) ionization coupled with a quadrupole mass filter for mass selection, has been central in the investigation of the dynamics of bimolecular reactions during the past 35 years.1,9-11 El ionization affords, in principle, a universal detection method for all possible reaction products of even a complex reaction exhibiting multiple reaction pathways. Although the technique is not usually able to provide state-resolved information, especially on a polyatomic... 

Occasionally, the rates of bimolecular reactions are observed to exhibit negative temperature dependencies, i.e., their rates decrease with increasing temperature. This counterintuitive situation can be explained via the transition state theory for reactions with no activation energy harriers that is, preexponential terms can exhibit negative temperature dependencies for polyatomic reactions as a consequence of partition function considerations (see, for example, Table 5.2 in Moore and Pearson, 1981). However, another plausible explanation involves the formation of a bound intermediate complex (Fontijn and Zellner, 1983 Mozurkewich and Benson, 1984). To... 

Other bimolecular reactions of complex systems, such as those of benzene and iodine and acid-base reactions, have also been studied. Currently, we are examining the inelastic and reactive collision of halogen atoms with polyatomics (e.g., CH3I). Other groups at the National Institutes of Science and Technology and at the University of Southern California have studied a new class of reactions O + CH4 — [CH3OH] — CH3 + OH and H + ON2 — HO + N2 or HN + NO. 

In this section, we review the comparitively small field of energy disposal in the elementary bimolecular reactions of diatomic molecules with other diatomic or polyatomic species. Because of the smaller number of reactions studied, they are summarised in a single table (Table 9). 

There have been many attempts made to calculate the preexponential factors of bimolecular reactions from molecular constants based on the considerations of the transition-state theory. Such efforts depend on a number of educated guesses as to the vibrational properties and structure of the transition-state complex, an assumption about the transmission coefficient for the reaction, and the assumption of the validity of the normal coordinate treatment for computing the thermodynamic properties of polyatomic molecules. 

For bimolecular reactions, we can easily compare collision theory with absolute reaction rate theory, using the results of the preceding section. Consider the bimolecular reaction between two polyatomic molecules A and B to yield a complex... 

A useful comparison between the predictions of simple collision theory and experiment can be made, since if the activation energy is determined, the experimental frequency factor can be directly compared with that predicted by Eq. (2-33). The hard-sphere diameter can be estimated from transport properties, although the choice of this parameter is somewhat arbitrary. In Table 2-1 a comparison between theory and experiment is presented for several well-studied bimolecular reactions (cf. Benson [10] for a more complete compilation). The tabulated steric factor is that value which makes the experimental and theoretical values coincide. In view of the assumptions involved, many of the steric factors are surprisingly close to unity. However, marked deviations in the form of unreasonably small steric factors do occur, especially if polyatomic molecules are involved. This often indicates that quantum-mechanical effects may be important or that a different classical theory may be required. 

Further examples for ion imaging of more complex processes, like photofragmentation of polyatomic molecules or bimolecular reactions, are discussed in Parts 4 and 5, including state-of-the-art experiments in which the reagents were pre-aligned by external fields. 

Ion imaging is now, some ten years after its introduction, a well-established tool for the study of molecular reaction dynamics. The most recent years have seen some remarkable progress in the field, in particular in the areas of velocity resolution and photoelectron imaging, complex photodissociations of polyatomic molecules exhibiting many competing reaction pathways, and reaction product imaging of bimolecular reactions. 

Here, the nonrandom excitation of C2H4F is described by the dynamics of the F - - C2H4 bimolecular reaction. To simulate chemical activation, proper initial conditions must be chosen for the reactants and for their relative properties. The procedure for choosing initial conditions for the reactant s relative properties is given below in the discussion of bimolecular reactions. The quasi-classical method may be used to select initial conditions for molecular reactants. The energy for a symmetric-top polyatomic molecule in a specific vibrational-rotational state may be approximated by the harmonic oscilla-tor/rigid rotor model... 

Polyatomic molecules have large zero-point energies and in classical mechanics simulations of bimolecular reactions this energy may be accessible for surmounting the potential energy barrier for reaction. " " In the absence of quantum-mechanical tunneling, the threshold for a bimolecular reaction is the vibrationally adiabatic barrier " " with zero-point energy in the vibrational... 

Most of the methodology of classical trajectory calculations has been developed for A + BC bimolecular reactions, for which extensive reviews exist.Much of this methodology is transferable to unimolecular studies of polyatomic molecules. However, several modifications pertaining to the selection of initial conditions, the analysis of final results, and the numerical integration of the classical equations of motion are required. These modifictions can be illustrated by considering the reaction given in equation (4). 

Flowever, in order to deliver on its promise and maximize its impact on the broader field of chemistry, the methodology of reaction dynamics must be extended toward more complex reactions involving polyatomic molecules and radicals for which even the primary products may not be known. There certainly have been examples of this notably the crossed molecular beams work by Lee [59] on the reactions of O atoms with a series of hydrocarbons. In such cases the spectroscopy of the products is often too complicated to investigate using laser-based techniques, but the recent marriage of intense syncluotron radiation light sources with state-of-the-art scattering instruments holds considerable promise for the elucidation of the bimolecular and photodissociation dynamics of these more complex species. 

Pressure eflFects are observed on the dissociation of diatomic molecules and small polyatomic species such as CHO and HNO since the decomposition occurs in a bimolecular process. In reaction (7)... 

These generalities are subject to many qualifications—the bimolecular value of A, for example, applies to the interaction of two atoms. For the interaction of more complex structures it may be much smaller. This is sometimes expressed by a steric factor multiplied by the frequency factor. Thus if an atom and polyatomic molecule interact, the value of A will be reduced by a factor of betwen 10" -10" if two polyatomic molecules are involved, the steric factor may be as small as For reactions in solution,... 

In contrast to the bimolecular recombination of polyatomic radicals (equation (A3.4.34)) there is no long-lived intermediate AB since there are no extra intramolecular vibrational degrees of freedom to accommodate the excess energy. Therefore, the formation of the bond and the deactivation through collision with the inert collision partner M have to occur simultaneously (within 10-100 fs). The rate law for trimolecular recombination reactions of the type in equation (A3.4.47) is given by... 

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