Gas Phase Reaction Dynamics

Many exciting questions remain in gas phase reaction dynamics. The goal of bond and state selective chemistry is still not fully realized, although this type of control has been observed in the local mode H-I-H20 system [18, 19]. An interesting new approach for chemical control involves preparing reactants in a coherent superposition of states [24], for which some theory already exists [25]. In applying these techniques to complex reactive systems, one hopes that the hard won coherence is not lost before the reaction proceeds. 

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In most of gas phase reaction dynamics, the fundamental reactions of interest are bimolecular reactions. 

The field of gas phase reaction dynamics has been extensively reviewed elsewhere [1, 2 and 3] in considerably greater detail than is appropriate for this chapter. Here, we begin by simnnarizing the key theoretical concepts and experimental teclmiques used in reaction dynamics, followed by a case study , the reaction F + H2 HF + H, which serves as an illustrative example of these ideas. 

Advances in pulse radiolysis studies in the gas phase have been summarized in several review papers. In a comprehensive review by Sauer [4], a review presented by Firestone and Dorfman [5] in 1971 was referred to as the first review on gas-phase pulse radiolysis. Experimental techniques and results obtained were summarized by one of the present authors [6], with emphasis on an important contribution of pulse radiolysis to gas-phase reaction dynamics studies. Examples were chosen by Sauer [7] from the literature prior to 1981 to show the types of species that were investigated in the gas phase using pulse radiolysis technique. Armstrong [8] reviewed experimental data obtained from gas-phase pulse radiolysis together with those from ordinary steady-state radiolysis. Advances in gas-phase pulse radiolysis studies since 1981 were also briefly reviewed by Jonah et al. [9], with emphasis on an important contribution of this technique to free radical reaction studies. One of the present authors reviewed comprehensively the gas-phase collision dynamics studies of low-energy electrons, ions, excited atoms and molecules, and free radicals by means of pulse radiolysis method [1-3]. An important contribution of pulse radiolysis to electron attachment, recombination, and Penning collision studies was also reviewed in Refs. 10-15. 

The complexity of condensed-phase reaction dynamics implies that both from a practical (computational) as well as a conceptual (i.e., insight) point of view, it is desirable to change the approach somewhat compared to gas-phase reaction dynamics. For example, we will see approximate representations of the interaction potentials and the dynamics—the latter, e.g., in the form of so-called stochastic dynamics. 

Crossed molecular beam machines count among the experimental arrangements which allowed a significant breakthrough in reaction dynamics. A recent review by Casavecchia et al. shows how improvements in the crossed molecular beam technique made possible recent progresses in the understanding of gas-phase reaction dynamics [14]. 

A brief survey is given of physicochemical aspects of atomic and molecular processes that are of great importance in reactive plasmas. The processes are composed of the interaction of molecules, in most cases polyatomic molecules, with reactive species such as electrons, ions (both positive and negative), free radicals, and excited atoms and molecules. Topics are chosen from recent studies of some elementary processes in reactive plasmas. Some comments are also given on future problems that call for more work in reactive-plasma research from the viewpoints of physicochemical studies of gas-phase reaction dynamics and kinetics, such as radiation chemistry and photochemistry. 

Laidler, K. J. (1965). Chemical Kinetics, 2nd ed., McGraw-Hill, New York, Chapters 4 and 6. A standard coverage of gas phase reaction dynamics. 

A study directed toward understanding when gas phase dynamics closely resembles the dynamics of the same reaction in solution was performed by Li and Wilson. io In this work, they used a model asymmetric A -t- BC reaction. By using an asymmetric reaction, Li and Wilson were able to test the validity in the solution phase of the Evans—Polanyi rule,3n which has proven to be quite useful in understanding gas phase reaction dynamics. The Evans-Polanyi rule states for a collinear A -t- BC reaction, that if the barrier to reaction is located early in the reaction coordinate, then translational excitation of the reactants is necessary to climb this barrier and vibrational excitation of the products will result. Conversely, a late barrier to reaction requires vibrational excitation of the reactants and results in translational excitation of the products. This rule has been validated numerous times in the gas phase and is an ideal example of how a simple rule can explain the dynamics of a large number of reaction systems. 

The paper by Karplus, Porter and Sharma (KPS) [1] is, from my perspective, the most important early (pre-1970) piece of computational work in gas-phase chemical reaction dynamics. In it, the commonly used quasiclassical trajectory (QCT) method was described for three-dimensional atom-diatom reactive collisions (i.e., A -f BC —> AB + C), and was applied to the H -f H2 reaction to determine cross sections and thermal rate constants. In 35 years of subsequent work on gas-phase reaction dynamics, the QCT method has remained largely the same, and it continues to be a standard tool for studying quantum state-resolved dynamical processes. 

The KPS paper stimulated research in several new directions, and ultimately spawned new fields. Many researchers, including Karplus, got interested in the development of QST of chemical reactions, and this led to accurate quantum descriptions of the H + H2 reaction [8] a decade after the KPS paper. There was also significant interest in the application of QCT methods to gas-phase reactions other than H -f- H2, and in fact this approach is now considered to be a standard research tool for studying gas-phase reaction dynamics of relevance to laser chemistry, combustion chemistry, atmospheric chemistry, and other applications. 

McDonnell L, Heck AJR. 1998. Gas-phase reaction dynamics studied by ion imaging . J. Mass Spectrom. 33(5) 415-428. 

CLASSICAL TRAJECTORY STUDIES OF GAS PHASE REACTION DYNAMICS AND KINETICS USING AB INITIO POTENTIAL ENERGY SURFACES... 

Table 5.3 shows the comparison betv. een the CARS experiment, the present exact theory, and the JSA for the rotationally averaged rate constant at T = 310 K, using the same units as in Table 5.2. As hoped (and expected ), the exact theory agrees quantitatively with the experimental result. Thus, we can truly regard the determination of the D-fH2(v = 1) rate constant as a solved problem in gas phase reaction dynamics. What is more intriguing, perhaps, is that the JSA predicts the...

Although the dynamical problem of the reacting molecular system is of the same complexity as that encountered in gas-phase reaction dynamics, the presence of a surface adds additional processes and phenomena. Such phenomena are, for instance, not only the importance of the structure, including corrugation, steps, and surface anomalities, but also the interaction with the possible excitation processes in the solid, such as phonon (surface vibrations) and electronic excitations. Also for charge transfer and other nonadiabatic electronic processes in the gas phase, the importance of the surface temperature adds additional features to the problem. Aside from this, the various processes of interest occur on different time scales, from fast reactive chemisorption processes on the sub-pico second time scale to the relatively slow diffusion and desorption processes. Thus different theoretical tools are needed in order to describe the variety of processes and the large time span one needs to cover. Also the many-body problem of the solid combined with the few-body gas-phase problem makes it necessary to introduce different methods for treating the dynamics, from classical trajectories and... 

We notice that the molecule can pass either an early barrier located in the entrance channel (where r the gas phase equilibrium distance) or a late barrier in the exit channel where r > If the barrier is late, then vibrational excitation of the incoming molecule enhances the dissociative sticking process if it is early, the dissociation is enhanced by increasing the translational energy of the molecule. This early and late barrier discussion is therefore identical to the one known from gas-phase reaction dynamics [123]. Thus it is natural to try to use similar model potentials in molecule surface interaction to those which have been used in gas-phase dynamics. Such a model potential is the one due to London, Eyring, Polanyi, and Sato, in short, denoted the LEPS potential. 

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