The nature of the chemical bond revisited: An energy-partitioning analysis of nonpolar bonds

The nature of the chemical bond in nonpolar molecules has been investigated by energy-partitioning analysis (EPA) of the ADF program using DFT calculations. The EPA divides the bonding interactions into three major components, that is, the repulsive Pauli term, quasiclassical electrostatic interactions, and orbital interactions. The electrostatic and orbital terms are used to define the nature of the chemical bond. It is shown that nonpolar bonds between main-group elements of the first and higher octal rows of the periodic system, which are prototypical covalent bonds, have large attractive contributions from classical electrostatic interactions, which may even be stronger than the attractive orbital interactions. Fragments of molecules with totally symmetrical electron-density distributions, like the nitrogen atoms in N₂, may strongly attract each other through classical electrostatic forces, which constitute 30.0% of the total attractive interactions. The electrostatic attraction can be enhanced by anisotropic charge distribution of the valence electrons of the atoms that have local areas of (negative) charge concentration. It is shown that the use of atomic partial charges in the analysis of the nature of the interatomic interactions may be misleading because they do not reveal the topography of the electronic charge distribution. Besides dinitrogen, four groups of molecules have been studied. The attractive binding interactions in H_nE-EH_n (E = Li to F; n = 0-3) have between 20.7 (E = F) and 58.4% (E = Be) electrostatic character. The substitution of hydrogen by fluorine does not lead to significant changes in the nature of the binding interactions in F_nE-EF_n (E = Be to O). The electrostatic contributions to the attractive interactions in F _nE-EF_n are between 29.8 (E = O) and 55.3% (E = Be). The fluorine substituents have a significant effect on the Pauli repulsion in the nitrogen and oxygen compounds. This explains why F₂N-NF₂ has a much weaker bond than H₂N-NH₂, whereas the interaction energy in FO-OF is much stronger than in HO-OH. The orbital interactions make larger contributions to the double bonds in HB=BH, H ₂C= CH₂, and HN=NH (between 59.9% in B₂H ₂ and 65.4% in N₂H₂) than to the corresponding single bonds in H_nE-EH_n. The orbital term ΔE _orb (72.4%) makes an even greater contribution to the HC≡CH triple bond. The contribution of AE_orb to the H_nE=EH _n bond increases and the relative contribution of the π bonding decreases as E becomes more electronegative. The π-bonding interactions in HC≡CH amount to 44.4% of the total orbital interactions. The interaction energy in H₃E-EH₃ (E = C to Pb) decreases monotonically as the element E becomes heavier. The electrostatic contributions to the E-E bond increases from E = C (41.4%) to E = Sn (55.1%) but then decreases when E = Pb (51.7%). A true understanding of the strength and trends of the chemical bonds can only be achieved when the Pauli repulsion is considered. In an absolute sense the repulsive ΔE_Pauli term is in most cases the largest term in the EPA.