A Chemist's Guide to Density Functional Theory-13 Chemical Reactivity-Exploring Potential Energy Surfaces.pdf

A Chemist’s Guide to Density Functional Theory. Second Edition
Wolfram Koch, Max C. Holthausen
Copyright © 2001 Wiley-VCH Verlag GmbH
ISBNs: 3-527-30372-3 (Softcover); 3-527-60004-3 (Electronic)
13 Chemical Reactivity:
Exploring Potential Energy Surfaces
The rates of chemical reactions are in many cases limited because of the presence of energy
barriers between reactants and products, and the barrier heights typically determine the
branching ratio of products. A rigorous understanding of all elementary steps along the
reaction coordinates leading from reactants to products is a prerequisite for the develop-
ment of guidelines for rationalizing or predicting the corresponding chemical transforma-
tions. However, plete characterization of the actual reaction mechanisms, i. e., ex-
plicit information on the structural and energetic details of all intermediates and transition
structures relevant in the course of a particular reaction, by experimental means alone has
been possible in only a very limited number of Quantum chemical calculations
offer in principle plementary source of information. Among the most prominent ap-
plications of modern electronic structure theory is therefore the localization and characteri-
zation of stationary points on those parts of a potential energy surface, which are associated
with a chemical reaction. In particular the ability to directly model the transition structures
connected to the activation barriers is a most appealing feature. However, even for a quali-
tatively correct picture of a reaction path electron correlation effects need to be taken into
account in the framework of conventional ab initio molecular-orbital theory. If quantitative
accuracy is the target, a sophisticated treatment of correlation effects is usually required in
order to ensure a balanced description of minima and transition structures. Hartree-Fock
theory in most instances fails miserably. Typically, this approach leads to