Highlights• Classical steric clashes might have the same topological features as bonding interactions.• An AIL can be observed for highly attractive or repulsive interactions.• An AIL might be a result of either an inflow or outflow of density.• Locally accumulated density does not imply an attractive interaction or an inflow of density.• Nature of an interaction can change with molecular environment.
Graphical abstract
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AbstractNine kinds of inter-and intramolecular interactions were investigated by exploring the topology of electron density in the interatomic regions using standard protocols of QTAIM, IQA and NCI techniques as well as in-house developed cross-sections of the electron and deformation density distributions. The first four methods provide the properties of the resultant density distribution in a molecular system whereas the later illustrates the process, inflow or outflow of density from fragments to the interatomic region of an interaction on its formation in a molecular system. We used (i) the QTAIM-defined atomic interaction line, AIL (presence or absence), (ii) IQA-defined interaction energy, attractive to repulsive, (ii) 2 < 0 to 2 >0, or (iii) (r) > 0 to (r) < 0; hence, none of the topological indices used here, either separately or combined, can be used to definitely predict the (de)stabilizing nature of an interaction except highly repulsive ones for which the absence of AIL, interatomic density depletion and outflow of density on interaction formation are observed.
Our
MO-based findings proved a bonding nature of each density bridge
(DB, or a bond path with an associated critical point, CP) on a Bader
molecular graph. A DB pinpoints universal physical and net energy-lowering
processes that might, but do not have to, lead to a chemical bond
formation. Physical processes leading to electron density (ED) concentration
in internuclear regions of three distinctively different homopolar
H,H atom-pairs as well as classical C–C and C–H covalent
bonds were found to be exactly the same. Notably,
properties of individual MOs are internuclear-region specific as they
(i) concentrate, deplete, or do not contribute to ED at a CP and (ii)
delocalize electron-pairs through either in- (positive) or out-of-phase
(negative) interference. Importantly, dominance of a net ED concentration
and positive e
–
-pairs delocalization made by a number of σ-bonding MOs is
a common feature at a CP. This feature was found for the covalently
bonded atoms as well as homopolar H,H atom-pairs investigated. The
latter refer to a DB-free H,H atom-pair of the bay in the twisted
biphenyl (Bph) and DB-linked H,H atom-pairs (i) in cubic Li4H4, where each H atom is involved in three highly repulsive
interactions (over +80 kcal/mol), and (ii) in a weak attractive interaction
when sterically clashing in the planar Bph.
It is shown herein that intuitive and text‐book steric‐clash based interpretation of the higher energy “in‐in” xylene isomer (as arising solely from the repulsive CH⋅⋅⋅HC contact) with respect to the corresponding global‐minimum “out‐out” configuration (where the clashing C−H bonds are tilted out) is misleading. It is demonstrated that the two hydrogen atoms engaged in the CH⋅⋅⋅HC contact in “in‐in” are involved in attractive interaction so they cannot explain the lower stability of this isomer. We have proven, based on the arsenal of modern bonding descriptors (EDDB, HOMA, NICS, FALDI, ETS‐NOCV, DAFH, FAMSEC, IQA), that in order to understand the relative stability of “in‐in” versus “out‐out” xylenes isomers one must consider the changes in the electronic structure encompassing the entire molecules as arising from the cooperative action of hyperconjugation, aromaticity and unintuitive London dispersion plus charge delocalization based intra‐molecular CH⋅⋅⋅HC interactions.
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