Fermi Hole Behavior in O–H···O and N–H···S Hydrogen Bonds: Equilibrium Points in the Electrostatic and Total Static Force Fields and the Electron Density Gradient as Reference Positions for the Probe Electron

In this study, the distributions of the Fermi exchange hole density, associated with the three-center interactions O–H···O in the picolinic acid N-oxide molecule and N–H···S in the methimazole supramolecular dimer, were investigated. As reference points for the location of the probe electron, it was proposed to utilize the saddle and sink equilibrium points, which are defined within the vector fields of the per-electronic electrostatic force density, the per-electronic total static force density, and the electron density gradient. The reassignment of the probe electron from the covalent bond region to the hydrogen bond domain causes a sudden transformation of the Fermi exchange hole. Initially confined within the covalent bond, the exchange hole subsequently expands, encompassing both the hydrogen bond acceptor atom and the pair of hydrogen bond donor atoms. This phenomenon underscores the intrinsic three-center character of classical hydrogen bonding. Furthermore, along the bond line of each hydrogen bond, the distribution of the exchange hole density undergoes a smooth yet steady change as the probe electron transitions from one saddle-type equilibrium point to another, following the aforementioned sequence.