Introduction

In the earlier lectures on chemical kinetics, we have studied the empirical aspects in detail. We now begin the theoretical explanations of chemical rate processes. This is one of the most challenging areas in chemistry. Molecular structure is fairly well understood through the advances in spectroscopy and quantum chemistry. With the knowledge of the same molecular structures and intermolecular forces, chemical equilibrium (thermodynamics) can be predicted quite accurately. An understanding the rates of processes microscopically requires answers to several interesting questions. Some of the main questions are as follows. How close should the molecules approach one another for the reaction to occur? Does the reaction "occur" only after reaching this separation or does it start well before the distance of closest separation is reached? Do all the reactants react identically or is there a distribution over several possibilities? Do all "encounters" between the reactants produce the products? How closely can we monitor the progress of the reaction theoretically (as well as experimentally)? Over the last four decades, the time resolution of this monitoring has improved dramatically from microseconds to femtoseconds.

While most of the above questions relate to gas phase processes, solution reactions have the additional contributions from the participation of the solvent. We shall take up these issues one by one in modest details. We first begin with the gas phase collision theory. This is followed by the transition state theory (lecture 33). Potential energy surfaces are introduced in lecture 34 and the trajectories over potential energy surfaces will be considered in lecture 35.