12 - Characterization of Chemical Reactions

Abstract

Quantum mechanics is an important tool to understand at the theoretical level the electronic structure of chemical compounds and the mechanism, thermodynamics, and kinetics of chemical reactions. It also provides reactivity parameters to understand a reaction process and helps in the characterization of chemical reactions. Reactivity, selectivity, and site activation are classical concepts in chemistry which can be quantitatively represented in terms of static global and local density response functions. These aspects, related to the characterization of chemical reactions, are discussed in this chapter. The chapter starts with functional representation of potential energy surfaces (PES) by molecular mechanics and their use for dynamical calculations and prediction of activation energies. It is followed by the use of isodesmic reaction methods for determining quantum mechanically the heat of formation and effect of substitution of functional groups on stabilization energy of chemical reaction. Two minima on the PES may have more than one reaction path connecting them, corresponding to different transition structures through which the reaction passes. IRC methods have been described to determine such intrinsic reaction paths. Mathematical formulation of global chemical reactivity indices (electronegativity, chemical hardness, chemical and softness, Fukui functions, etc.) and local reactivity indices (local philicity, local softness, etc.) and their interpretation in terms of “Frontier orbitals” has been given with illustrative examples.