Transition from Noncovalent Interaction to Covalent Bond Based on One-Electron Potential, Quantum Chemical Topology, and Molecular Face Theory

Noncovalent interactions and covalent bonds can be distinguished via quantum chemical topology analysis and molecular face theory, which are based on the potential acting on one electron in a molecule or molecular system (PAEM). A covalent bond forms when a PAEM bond critical point (BCP) occurs on the line connecting two atoms and when their molecular faces contact or fuse together, whereas a noncovalent interaction occurs between two adjacent atoms or chemical species when their molecular faces remain separate. The force acting on one electron within a molecule, which starts at infinity and ends at the BCP, forms nonoverlapping boundary surfaces that partition a molecule into distinct atomic regions. This is demonstrated with the following example reactions: H + H → H₂, H + <i>X</i> → H<i>X</i> (<i>X</i> = F, Cl, Br, I), O (¹<i>D</i>) + H₂ → H₂O, and S (¹<i>D</i>) + H₂ → H₂S. The exploration of the physical quantities at PAEM critical points, such as the eigenvalues of the Hessian matrix, ellipticity, and electron interflow frequency, reveals their changing trends during the transition from a noncovalent interaction to a covalent bond or vice versa. These changes can help predict chemical bond formation or breakage, providing insight into chemical bonding.