Bond Paths Are Not Chemical Bonds

Abstract

This account takes to task papers that criticize the definition of a bond path as a criterion for the bonding between the atoms it links by mistakenly identifying it with a chemical bond. It is argued that the notion of a chemical bond is too restrictive to account for the physics underlying the broad spectrum of interactions between atoms and molecules that determine the properties of matter. A bond path on the other hand, as well as being accessible to experimental verification and subject to the theorems of quantum mechanics, is applicable to any and all of the interactions that account for the properties of matter. It is shown that one may define a bond path operator as a Dirac obserVable, making the bond path the measurable expectation Value of a quantum mechanical operator. Particular attention is given to van der Waals interactions that traditionally are assumed to represent attractive interactions that are distinct from chemical bonding. They are assumed by some to act in concert with Pauli repulsions to account for the existence of condensed states of molecules. It is such dichotomies of interpretation that are resolved by the experimental detection of bond paths and the delineation of their properties in molecular crystals. Specific criticisms of the stabilization afforded by the presence of bond paths derived from spectroscopic measurements performed on dideuteriophenanthrene are shown to be physically unsound. The concept of a bond path as a "bridge of density" linking bonded atoms was introduced by London in 1928 following the definition of the electron density by Schrödinger in 1926. These papers marked the beginning of the theory of atoms in molecules linked by bond paths.

Analysis of CF···FC Interactions on Cyclohexane and Naphthalene Frameworks

“…The corresponding H-H interactions in the parent biphenyl have led to intense debates in the literature in order to characterize them either as repulsive or attractive. 53,54 There are also debates in the literature about the nature of phenanthrene H-H interactions. 55 By changing the hydrogen atoms for fluorine atoms, one may expect increasingly repulsive interactions.…”

EL: the new aromaticity measure based on one-electron density function

“…In particular the properties of the electron density function in the so-called bond critical point (BCP, the (3, -1) saddle point on electron density curvature being a minimum in the direction of the atomic interaction line and a maximum in two directions perpendicular to it) seem to be valuable information for chemists, since it was proven in many papers that the chemical bonding can be characterized and classified on the basis of electron density characteristics measured in BCPs [24,[39][40][41][42]. It was emphasized in many papers dealing with QTAIM applications that the presence of a bond path (BP, the line linking points of maximum electron density along the direction of the bond) linking a pair of atoms and the BCP corresponding to it fulfills the sufficient and necessary condition requiring that the atoms be bonded to one another [43,44]. The above statement can be used as the universal criterion for the presence of the chemical bond, no matter what kind of bond it is.…”

Perspective: Found in translation: Quantum chemical tools for grasping non-covalent interactions

“…The bond paths are better thought of as representing interacting atoms rather than any actual chemical bond. 172 QTAIM identifies all atomic interactions as "local pairings of the densities of opposite spin electrons" 173 which, in fact, covers all non-covalent interactions. However, when the density is computed from DFT, which only roughly accounts for dispersion, it becomes unclear whether the van der Waals interactions remain correctly depicted by QTAIM.…”