Transition states fundamental dynamical

The original microscopic rate theory is the transition state theory (TST) [10-12]. This theory is based on two fundamental assumptions about the system dynamics. (1) There is a transition state dividing surface that separates the short-time intrastate dynamics from the long-time interstate dynamics. (2) Once the reactant gains sufficient energy in its reaction coordinate and crosses the transition state the system will lose energy and become deactivated product. That is, the reaction dynamics is activated crossing of the barrier, and every activated state will successfully react to fonn product. 

Kinetics on the level of individual molecules is often referred to as reaction dynamics. Subtle details are taken into account, such as the effect of the orientation of molecules in a collision that may result in a reaction, and the distribution of energy over a molecule s various degrees of freedom. This is the fundamental level of study needed if we want to link reactivity to quantum mechanics, which is really what rules the game at this fundamental level. This is the domain of molecular beam experiments, laser spectroscopy, ah initio theoretical chemistry and transition state theory. It is at this level that we can learn what determines whether a chemical reaction is feasible. 

Usually we talk about reactions in solution, but recently techniques have been developed to follow reactions that occur in a vacuum when a stream of reactant A and a stream of reactant B cross each other in a defined direction, as with molecular beams. From the direction in which the products are ejected and their energies, much fundamental information can be deduced about the details of the molecular processes. Lasers, which emit light-energy in a highly focused beam, are sometimes used to put energy into one of the reactants in a defined way. Such a technique reveals less about the nature of the transition state than about what is called the dynamics of the process—how molecules collide so as to react, and how the products carry away the energy of the overall reaction. The development and application of such techniques were recognized by a Nobel Prize in 1986 to Dudley Herschbach, Yuan Lee, and John Polanyi. 

The well-known Born-Oppenheimer approximation (BOA) assumes all couplings Kpa between the PES are identically zero. In this case, the dynamics is described simply as nuclear motion on a single adiabatic PES and is the fundamental basis for most traditional descriptions of chemistry, e.g., transition state theory (TST). Because the nuclear system remains on a single adiabatic PES, this is also often referred to as the adiabatic approximation. 

Reaction dynamics on the femtosecond time scale are now studied in all phases of matter, including physical, chemical, and biological systems (see Fig. 1). Perhaps the most important concepts to have emerged from studies over the past 20 years are the five we summarize in Fig. 2. These concepts are fundamental to the elementary processes of chemistry—bond breaking and bond making—and are central to the nature of the dynamics of the chemical bond, specifically intramolecular vibrational-energy redistribution, reaction rates, and transition states. 

The dynamics of a chemical process can change considerably in going from the gas phase to the liquid phase. One fundamental reason for such differences is that liquids are able to solvate chemical species. For example, solvation might stabilize the transition state in a chemical reaction to a greater extent than it stabilizes the reactants, thereby accelerating the reaction rate. Of course, solvation itself is a dynamic process, which has important implications for chemical processes in solution. If the lifetime of a transition state is shorter than the inherent dynamic time scale of the solvent, for instance, solvation will not be able to stabilize the transition state to the fullest possible extent. The above example illustrates the importance of gaining a molecular-level understanding of the dynamics of solvents. 

Dynamics on two-basin potential energy surfaces has been extensively explored in the context of chemical reactions over the past several decades [1-14]. Transition state theories (TST), first developed by Eyring [3] and Evans [4] and by Wigner [5] in the 1930s, have had great success in elucidating absolute reaction rates of chemical reactions. All the various forms of (classical) TST are based on two fundamental assumptions ... 

A close look at the place of time within chemistry raises questions about that science s fundamental conceptual and explanatory entities. Put very simply, what is chemistry about A conventional narrative depicts chemistry, in its youth a science of substances, as reaching maturity when it metamorphosed into a science of molecules. The development of transition-state theory certainly conforms to and reinforces that narrative because the theory s successes can be ascribed to its "reduction of the dynamics problem to the consideration of a single structure" (Truhlar et al., 1983, p. 2665). Yet questions have been raised recently as to whether molecular explanations are adequate to account for all chemical phenomena (Woolley, 1978 Weininger, 1984), and the view that substances are still the primary subject matter of chemistry has by no means disappeared (van Brakel, 1997). I suggest that chemists can call on a variety of explanatory entities that are intermediate between the molecule and the substance, and these entities need not have the permanence of either molecules or substances. 

The fundamental assumption of transition state theory is that of direct dynamics, i.e., that all trajectories which cross the dividing surface do so only once (1,3,5). If this is true then a trajectory will be on the product side of the dividing surface at t - only if it begins at t = 0 (on the dividing surface) headed in the product direction, i.e., with positive momentum normal to the dividing surface. 

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