What is molecular reaction dynamics

Reaction dynamics is the study of the molecular level mechanism of elementary chemical and physical processes. It seeks to understand what actually takes place at that level when a change, chemical or physical, occurs. As an example, when molecular chlorine gas is introduced into a vessel containing bromine vapor, a chemical reaction does take place, and it can be monitored in time by a change in the color. The net chemical change in the vessel is CI2 + Br2 - 2BrCl. The reaction rate is observed to be of second order, that is, the rate of disappearance of CI2 or Br2 is first order in the concentration of each reactant. Yet on the molecular level the elementary reaction 

The study of dynamics allows us to raise additional questions, questions that do not quite make sense when we study the net change, at or near equilibrium, in the bulk. For example, we can wonder if exciting vibrational motion in either or both reactant diatomic molecules will make the CI2 + Br2 four-center reaction proceed more rapidly. In bulk chemical kinetics, when the reactants are intentionally arranged to be in thermal equilibrium, a particular mode cannot be energized preferentially. To learn about a selective role of internal energy in promoting a reaction it is necessary to work under non-equilibrium conditions.  

Section 1.2 provides a case study of the kind of new questions raised by dynamics. In doing so, it also points to where we are going to go in the following chapters. We will, for example, discuss how lasers can act so as to prepare the reactants or, better yet, to access directly the transition region from reactants to products. When the laser is intense enough it can even alter completely the dynamical course. As an example, an intense laser field can be tailored to alter the ratio of products in two dissociation pathways of acetophenone  

Different options for control using lasers feature throughout our road ahead. 

The technical details of tiie experimental and theoretical methods of molecular d5mamics can be intricate but the concepts are simple. An understanding of these concepts - the ability to read the language - is all that is necessary to be able to view the very process of chemical change. This book is a primer of the language for expressing chemical transformations as dynamical events, proceeding in space and time. 

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