Bond paths between distant atoms do not necessarily indicate dominant interactions

Abstract

The goal of the article is to revive discussion on the interpretation of bond paths linking distant atoms, particularly tracing weak interactions in dimers. According to the Pendás' concept of privileged exchange channel, a bond path is formed between this pair of competing atoms, which is associated with larger value of the exchange energy. We point out that, due to the short-range nature of the exchange energy, bond paths linking distant atoms clearly become doubtful indicators of dominant intermolecular interactions, particularly if some other characteristics (geometric, spectroscopic, based on electrostatic parameters, etc.) indicate other intermolecular interactions as dominant. Several such cases are thoroughly investigated. We show that electrostatic parameters are much more reliable indicators of dominant intermolecular interactions than bond paths. Then, we pay attention that the presence of ("unexpected", i.e., not necessarily indicating dominant intermolecular interactions) bond paths between pairs of atoms featuring highly expanded charge distributions can be easily explained by visual exploration of isodensity contour plots. As always pointing in the direction of the steepest increase, the gradient vector of the electron density favors areas of its high values gaining higher exchange energy, yet being blind to highly electron deficient areas nearby, which, however, can quite often be involved in dominant intermolecular interactions as strongly suggested by many other bonding analysis. We also suggest that an interatomic component of Hellmann-Feynman force would most likely be the most reliable indicator of attractive or repulsive character of individual interatomic interaction.

On the Uselessness of Bond Paths Linking Distant Atoms and on the Violation of the Concept of Privileged Exchange Channels

“…According to the orthodox QTAIM, this would mean sudden formation or breaking of bonds. As no one expects sudden formations or breakings of bonds during molecular vibrations or, what is more, changes in the theoretical method only, bond path cannot mean chemical bond ,,. Moreover, topology of the electron density distribution is mainly governed by atomic densities, whereas the electron density build‐up, which is characteristic for covalent bonds, is rather modest ,.…”

“…As an explanation of this seeming puzzle, it will be shown, what the main purpose of this article is, that in general the bond path between distant atoms has nothing to do with the dominant interatomic interactions. These, instead, are suggested by other than topological parameters, mainly being of electrostatic nature …”

Metal–Halogen Bonding Seen through the Eyes of Vibrational Spectroscopy

“…Considering the problem of finding bond paths connecting electron-rich regions instead of reflecting dominant interactions [53,54], the topological electron density analysis was complemented by electron difference densities. To distinguish an atom-atom interaction from a dispersive/electrostatic interaction involving multiple atoms, termed here nonspecific (NS), we calculated and analyzed reduced density gradient (RDG) iso-surfaces of 0.45 a.u., colored according to the value of the second largest eigenvalue of the electron density Hessian, also termed non-covalent index (NCI) [104].…”

“…Unexpectedly, a bond path connecting C-X• • • X-C is also observed even though the distance between the halogens are greater than the sum of their van der Waals radii. There may be a very weak dispersive interaction between these halogens, but the X• • • X bond path may also be a consequence of the increased electron density at the π region of the halogens stabilized by the C-H• • • X-C HB [53]. Either way, this interaction plays little to no role for the stabilization of the dimers.…”

“…However, the sole existence of a bond path and a BCP does not necessarily imply the existence of a chemical bond [52]. For distant atoms, bond paths may favor a connection between electron-rich regions instead of reflecting dominant interactions, making the interpretation of bond paths as bond indicators and the use of the density at the BCP as bond strength measure problematic [52][53][54]. The same holds for bond strength measures derived from dissociation energies (BDE)s [55][56][57][58][59][60] and molecular geometries.…”