Mayer's Chemical Energy Component Analysis as a tool to identify the factors determining the shapes of potential surfaces of chemical reactions

Abstract

Energy partitioning is a way to convert the information obtained in numerical quantum chemistry to chemically interpretable, qualitative or semiquantitative information. Such methods may be useful in studying how interactions develop in chemical reactions. In this work we test how one can gain meaningful new information about a chemically reactive system using the mono- and diatomic energy terms calculated with two energy-partitioning schemes developed by István Mayer: The Chemical Energy Component Analysis, CECA, and the scheme named E2. In our test reactions a H-atom is transferred from an HR molecule to a methyl radical, CH3 + H'R → CH3H' + R, with R=H, CH¬3, C(CH¬3)3 and OH. The diatomic energy component associated with the forming bond is zero in the reactant limit and gradually becomes attractive when one moves on the minimum energy path toward the product limit; that of the breaking bond simultaneously changes from attractive to zero. Their sum displays a maximum which appears to be a contributor to the potental barrier. The dominant term in the increase/decrease of the diatomic energy components is exchange, which characterizes the strength of covalent interactions. Its change indicates that the build-up of one covalent interaction does not completely cover the energy neded to break the other. Energy component analysis identified a continuous repulsion between the atoms from/to which the H-atom is transferred, which is also a major contributor to the potential barrier. The origin of this interaction is the repulsion involving overlap densities. Overlap repulsion is also the main contributor to the steric repulsion involving the spectator atoms. Energy component analysis performed on wave functions calculated with different basis sets yields the same semiquantitative information. The CECA method is a promising source of information for studying the change of the nature of interactions during chemical reactions, and can help identifying general rules. The diatomic energy components derived with the E2 scheme are close in magnitude to bond dissociation energies and change smoothly with molecular geometry, but they cannot be decomposed to contributions like overlap and exchange.