Phenomenological description of the transition state, and the bond breaking and bond forming processes of selected elementary chemical reactions: an information-theoretic study

Theoretic-information measures of the Shannon type are employed to describe the course of the simplest hydrogen abstraction and the identity S(N)2 exchange chemical reactions. For these elementary chemical processes, the transition state is detected and the bond breaking/forming regions are revealed. A plausibility argument of the former is provided and verified numerically. It is shown that the information entropy profiles posses much more chemically meaningful structure than the profile of the total energy for these chemical reactions. Our results support the concept of a continuum of transient of Zewail and Polanyi for the transition state rather than a single state, which is also in agreement with reaction force analyses. This is performed by following the intrinsic reaction coordinate (IRC) path calculated at the MP2 level of theory from which Shannon entropies in position and momentum spaces at the QCISD(T)/6-311++G(3df,2p) level are determined. Several selected descriptors of the density are utilized to support the observations, such as the molecular electrostatic potential, the hardness, the dipole moment along with geometrical parameters.