Chemical reactions, quantum dynamics overview

Unimolecular dissociation is one of the simplest types of irreversible chemical reactions It takes place in a single isolated molecule with an internal energy that exceeds the first dissociation threshold see Fig. 1(a) for a schematic overview. Nevertheless, the underlying atomic-level reaction mechanisms are very complex. Their theoretical description requires all the power of modern quantum chemical methods, statistical physics and nonlinear dynamics, and even then the full rigor can be achieved just for small, mostly triatomic molecules. Experimental studies have to be likewise advanced Up to three laser pulses are combined in a modern experiment to elucidate all details of the dissociation process. 

Abstract. This paper presents an overview of the time-dependent quantum wavepacket approach to chemical reaction dynamics. After a brief review of some early works, the paper gives an up-to-date account of the recent development of computational methodologies in time-dependent quantum dynamics. The presentation of the paper focuses on the development of accurate or numerically exact time-dependent methods and their specific applications to tetraatomic reactions. After summarizing the current state-of-the-art time-dependent wavepacket approach, a perspective on future (development is provided. 

The reactions H + H2 and F + H2 (and their isotopic variants) have been the benchmark systems in the field of chemical reaction dynamics. For them, fully converged three-dimensional (3D) quantum scattering calculations of state-to-state differential cross sections (DCS) have been performed and accurate comparisons with very detailed experimental observables carried out [3-9]. To date, only for one other neutral three-atom system have the exact (i.e., fully converged) 3D quantum scattering calculations of state-to-state DCS on a reliable ab initio potential energy surface (PES) been carried out, namely, for the prototypical reaction Cl + H2 [10], a system chemical kineticists have been interested in since the time of Max Bodenstein (for a historical overview, see the paper by Truhlar in this volume). However, in contrast to H + H2 and F + H2, no experimental dynamical information is available on Cl + H2. Here we highlight the results of the first dynamical investigation of the Cl + H2 and Cl + D2 reactions by the crossed molecular beam... 

The theory of solvent-effects and some of its applications are overviewed. The generalized selfcon-sistent reaction field (SCRF)theory has been used to give a unified approach to quantum chemical calculations of subsystems embedded in a given milieu. The statistical mechanical theory of projected equations of motion has been briefly described. This theory underlies applications of molecular dynamics simulations to the study of solvent and thermal bath effects on carefully defined subsystem of interest. The relationship between different approaches used so far to calculate solvent effects and the general SCRF has been established. Recent work using the continuum approach to model the surrounding media is overviewed. Monte Carlo and molecular dynamics studies of solvent effects on molecular properties and chemical reactions together with simulations of solvent effects on protein structure and dynamics are reviewed. 

An extensive overview of many wave packet (and other) methods can be obtained from the collections of articles in two recent books D. G. Truhlar and B. Simon (eds.), Multiparticle Quantum Scattering with Applications to Nuclear. Atomic, and Molecular Physics , Springer. New York. 1997 R. E. Wyatt and J. Z. H. Zhang (eds.). Dynamics of Molecules and Chemical Reactions , Dekker, New York, 1996. 

The next section gives a brief overview of the main computational techniques currently applied to catalytic problems. These techniques include ab initio electronic structure calculations, (ab initio) molecular dynamics, and Monte Carlo methods. The next three sections are devoted to particular applications of these techniques to catalytic and electrocatalytic issues. We focus on the interaction of CO and hydrogen with metal and alloy surfaces, both from quantum-chemical and statistical-mechanical points of view, as these processes play an important role in fuel-cell catalysis. We also demonstrate the role of the solvent in electrocatalytic bondbreaking reactions, using molecular dynamics simulations as well as extensive electronic structure and ab initio molecular dynamics calculations. Monte Carlo simulations illustrate the importance of lateral interactions, mixing, and surface diffusion in obtaining a correct kinetic description of catalytic processes. Finally, we summarize the main conclusions and give an outlook of the role of computational chemistry in catalysis and electrocatalysis. 

The present Lecture Notes contain the Proceedings of the 1999 Mariapfarr workshop in Theoretical Chemistry. These annual winter workshops, which draw their name from their traditional home , a pleasant resort in the Austrian Alps, are organized by the Computational Chemistry Section of the Austrian Chemical Society. The 1999 event, dedicated to Reaction Dynamics , presented an overview of computational methods developed for the calculation of quantum reaction cross sections and reaction rates. 

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