Description of changes in chemical bonding along the pathways of chemical reactions by deformation of the molecular electrostatic potential

Context

The analysis of the changes in the electronic structure along intrinsic reaction coordinate (IRC) paths for model reactions: (i) ethylene + butadiene cycloaddition, (ii) prototype SN2 reaction Cl⁻ + CH3Cl, (iii) HCN/CNH isomerization assisted by water, (iv) CO + HF → C(O)HF was performed, in terms of changes in the deformation density (Δr) and the deformation of MEP (ΔMEP). The main goal was to further examine the utility of the ΔMEP as a descriptor of chemical bonding, and to compare the pictures resulting from Δr and ΔMEP. Both approaches clearly show that the main changes in the electronic structure occur in the TS region. The ΔMEP picture is fully consistent with that based on Δρ for the reactions of the neutral species leading to the neutral products without large charge transfer between the fragments. In the case of reactions with large electron density displacements, the ΔMEP picture is dominated by charge transfer leading to more clear indication of charge shifts than the analysis of Δr.

Methods

All the calculations were performed using the ADF package. The Becke–Perdew exchange–correlation functional was used with the Grimme’s dispersion correction (D3 version) with Becke-Johnson damping. The Slater TZP basis sets defined within the ADF program were applied. For the analysed reactions, the stationary points were determined and verified by frequency calculations, and the IRC was determined. Further analysis was performed for the structures of reactants, TS, products, and the points corresponding to the minimum and maximum of the reaction force. For each point, two fragments, A and B, corresponding to the reactants were considered. The deformation density was calculated as the difference between the electron density of the system AB and the sum of densities of A and B, \(\Delta \rho \left(r\right)= {\rho }^{AB}\left(r\right)-{\rho }^{A}\left(r\right){-\rho }^{B}\left(r\right),\) with the same fragment definition as in the ETS-NOCV method. Correspondingly, deformation in MEP was determined as \(\Delta V\left(r\right)={V}^{AB}\left(r\right)- {V}^{A}\left(r\right)- {V}^{B}\left(r\right)\).