Unified Picture of Interatomic Interactions, Structures, and Chemical Reactions by Means of Electrostatic and Kinetic Force Density Fields: Appel?s Salt and Its Ion Pairs

This paper presents a binding approach based on the electrostatic and kinetic force density fields and proposed to elucidate interatomic interactions, structures, and chemical reactions. Among other elements, it consists in the understanding of (i) the arrangement of pseudoatomic zero-flux surfaces (ZFSs), (ii) the mutual compression of force-field pseudoatoms of the same type, (iii) the penetration of a bonded atom into neighboring force field pseudoatoms, (iv) the distortion of the force fields, and (v) the occurrence of a binding path connecting two force-field attractors as the inherent consequences of the emergence and existence of any many-electron multinuclear system. Herein, the interatomic charge transfer and the reciprocal quantum-chemical response accompanying the covalent and noncovalent bond formation were studied using the abovementioned penetration, exemplified by Appel's salt. Such consideration gave rise to the concept of force-induced "push-pull" assembling of interacting (pseudo)atoms. In this regard, various halogen, chalcogen, and tetrel noncovalent interactions, as well as polar covalent bonds, were examined. Furthermore, the notion of the force-based binding structure was proposed instead of the bonding one, which defines the mutual connectivity of force-field pseudoatoms of the same type via binding paths. The thus-defined chemical structure proved to be more sensitive to variations in the configuration space of nuclear coordinates. It was found that the three-center chalcogen bonding Cl-center dot center dot center dot S-S and tetrel bonding Cl-center dot center dot center dot C center dot center dot center dot Cl- in the crystal studied resemble the initial and transition states of bimolecular nucleophilic substitution reactions, respectively. Their central atoms act as electron contributors with a different degree of the transferred electron density sharing. Being extracted from the crystal structure, the corresponding ion pairs undergo chemical rearrangement: Forming the covalent bonds Cl-S and Cl-C leads to the collapse of the pseudoatomic ZFSs and the alignment of the bond and binding paths in the internuclear regions.