ReactionDynamicsIntroNotes - Introduction to reaction...

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Introduction to reaction dynamics By now you should be well and truly used to the concepts involved in reaction kinetics on a macroscopic scale, the kinds of techniques used to measure rate constants and determine reaction orders, and the use of the steady-state approximation for determining rate laws once the sequence of elementary steps involved in a reaction is known. What you will probably not yet be familiar with is the idea that we can go much deeper than this and study these elementary reactions in much greater detail. Reaction dynamics looks at chemical reactions on the scale of single reactive collisions between molecules, and involves carrying out experiments and calculations with the aim of understanding chemical reactions at the most fundamental level possible. This is one area of chemistry in which the link between theory and experiment is particularly strong, with the bridge between the two embodied by the reaction potential energy surface . A potential energy surface (or PES) is a multidimensional surface which gives the potential energy of a chemical system as a function of the nuclear coordinates of all the atoms involved in the reaction. Potential energy surfaces can be calculated using quantum mechanics, at least for simple systems. If you can remember a little classical mechanics, you will recall that the force in a particular direction is given by F x =-dV/dx. The first derivative of the potential energy surface at a given point (i.e. a given nuclear configuration) therefore gives the forces acting on all the atoms at that configuration. Since the forces acting on an atom determine its motion, the potential energy surface for a reaction determines its course, including what (if any) products will be formed, in what direction(s) they are scattered, what their geometries and energies will be, how fast the reaction will proceed, and so on. Calculation of these potential energy surfaces is part of the theoreticians job. The second part of the theoreticians job is to provide the link between these potential energy surfaces and the dynamics that would be observed for a reaction that proceeds over one of these surfaces. In practice, there are two ways to do this, quantum scattering calculations or quasi-classical trajectory (QCT) calculations. Quantum scattering theory is rather beyond the scope of this introduction to reaction dynamics, but a qualitative description of QCT calculations may be useful. As we said above, the potential energy surface for a reaction defines all the forces acting on each atom at any geometry of the reacting system. If we start from a particular point x 1 on the surface (where x is a vector containing all the nuclear coordinates) we can take the first derivative at that point (i.e. the gradient
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This note was uploaded on 05/09/2010 for the course CHE 3051 taught by Professor Sitimachmud during the Spring '08 term at National Central University.

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ReactionDynamicsIntroNotes - Introduction to reaction...

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