The strength of a chemical bond

Abstract

We discuss the physical criteria that have been proposed as a measure of the strength of a chemical bond. Critical examination of the strengths and weaknesses of the various quantities shows that the force constant is the most general measure for determining the strength of a chemical bond in molecules. The force constant of a bond AB is a direct indicator of the interatomic forces between the fragments A and B at the equilibrium distance, which is independent from the energies and electronic structures of the free species A and B. This is a great advantage compared to the use of energy differences for determining the bond strength, which are subject to the choice of the reference system that is often not uniquely defined. This is particularly evident when considering the strength of a chemical bond in strongly bonded but metastable species such as [O2]2+. The change in the electronic structure of the reference system obscures the intrinsic bond strength at equilibrium. Energy changes along the course of chemical reactions are elementary quantities in chemistry; they provide crucial information for the kinetics and thermodynamics of processors. However, they are not suitable as a measure of bond strength. The definition of local vibrational constants by Konkoli and Cremer eliminates the coupling of the stretching vibration with other vibrational modes, which makes it possible to use force constants as direct measure for bond strength also in larger molecules.

The Orbital Origins of Chemical Bonding in Ge−Sb−Te Phase‐Change Materials**

“…While COBI (measuring the bond order) is extracted from the wavefunction, pFCs (measuring the bond stiffness) are derived perturbatively from atomic displacements. While the bond energy (as a measure of its “strength”) and the corresponding force constant often run parallel to each other, [43] there is no obvious theoretical reason for them to be connected. For example, the bond dissociation energy of CO is higher than for N 2 but the opposite is true for the corresponding force constants.…”

CO bonding in hexa‐ and pentacoordinate carboxy‐neuroglobin: A quantum mechanics/molecular mechanics and local vibrational mode study

“…In their landmark paper, Zou and Cremer 138 proved that the local stretching force constant k a n (AB) reflects the intrinsic strength of the bond/interaction between two atoms A and B being described by an internal coordinate q n . In essence, LMA has advanced as a powerful analytical tool, extensively applied to a broad range of chemical systems from simple molecular systems to systems in solution 139,140 and proteins 102,141 accounting for both covalent bonds 79,88,131,138,142–152 and non‐covalent interactions 89,150,153–166 including hydrogen bonds 167–177 . Recently, a whole new scope of chemical systems were unlocked with the extension of LVM theory to periodic systems 178 …”

“…Comprehensive computational studies on transition metal carbonyl complexes, for example, performed by Frenking and co‐workers 65–70 and other groups 71–77 have applied the DC model to explore the stabilizing role of CO ligands for both mono and poly‐nuclear metal‐carbonyl complexes. Some limitation of the DC and other Lewis type bonding model were recently pointed out 78–80 . Two other popular strategies to describe M‐L bond strengths are based on (1) M‐L bond dissociation energies (BDE)s 76,81–84 and (2) M‐L bond lengths.…”

“…It reflects the energy needed for bond breaking, but also contains energy contributions due to geometry relaxation and electron density reorganization in the dissociation fragments. Therefore, the BDE or related energy decomposition schemes 85 are not suitable measures of intrinsic chemical bond strength and their use has led in some cases to misinterpretation of bond strength 79,86–89 . Also the M‐L bond length is not a qualified bond strength descriptor.…”

Orbitale als Ausgangspunkt der Chemischen Bindung in Ge−Sb−Te‐Phasenwechselmaterialien**

“…Während COBI (zur Ermittlung der Bindungsordnung) aus der Wellenfunktion extrahiert wird, werden die pFCs (zur Ermittlung der Bindungssteifigkeit) störungstheoretisch mittels Atomverschiebungen hergeleitet. Obwohl die Bindungsenergie (als Maß für die “Stärke” einer Bindung) und die entsprechende Kraftkonstante oft parallel zueinander verlaufen, [43] gibt es keinen echten theoretischen Grund für eine solche Verknüpfung. So ist beispielsweise die Bindungsdissoziationsenergie von CO größer als die von N 2 , aber für die entsprechenden Kraftkonstanten gilt das genaue Gegenteil.…”