An analysis of the topology of the electron charge density and the reactant-product electronic structure variation along the intrinsic reaction coordinate

In this work the topology of the electron charge density and the variations in the reactant and product electronic structures are analyzed along the Fukui intrinsic reaction coordinate (IRC). The systems studied are the ionic and the Menschutkin S_N2 reactions. This study is performed at ab initio RHF and MP2 levels, and density functional level, employing the B3LYP functional. The basis set in all cases is of split valence type and includes diffuse and polarization functions in nonhydrogen atoms 6-31+G*. As a measure of the variations of reactant and product electronic structures, we calculate at the RHF level, the overlap integral between the total wavefunction and the wavefunction based on the reactant (or product) localized fragment orbitals. This integral can be interpreted, in Hilbert space, as the cosine of the angle between the vector representing the electronic structure of the molecule in each point of the IRC and that of reactant (or product) electronic structure. The calculated molecular properties were analyzed in light of the valence bond approach, and qualitative differences were noted depending on the property studied.