A. Kovács, C. Esterhuysen, G. Frenking

The Nature of the Chemical Bond Revisited: An Energy Partitioning Analysis

of Nonpolar Bonds.

Chemistry - A European Journal, 11 (2005) 1813-1825

The nature of the chemical bond in nonpolar molecules has been investigated with the help of the energy partitioning analysis (EPA) of the ADF program using DFT calculations. The EPA divides the bonding interactions into three major components, i.e. the repulsive Pauli term, quasi-classical electrostatic interactions and orbital interactions. The latter two 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 by classical electrostatic interactions, which may even be stronger than the attractive orbital interactions. Fragments with totally symmetrical electron density distributions, like the nitrogen atoms in N₂, may strongly attract each other through classical electrostatic forces, which contribute 30.0% of the total attractive interactions. The electrostatic attraction can become enhanced by anisotropic charge distribution of the valence electrons of the atoms that have local areas of (negative) charge concentration. It is shown that atomic partial charges may be misleading for analyzing the nature of the interatomic interactions, 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) possess 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 are between 29.8% (E = O) and 55.3% (E = Be). The fluorine substituents have a large effect on the Pauli repulsion in the nitrogen and oxygen compounds. This explains why F₂N-NF₂ has a much weaker bond than H₂N-NH₂ while the interaction energy in FO-OF is much stronger than in HO-OH. The double bonds in the molecules HB=BH, H₂C=CH₂, HN=NH have larger contributions from orbital interactions (between 59.9% in B₂H₂ and 65.4% in N₂H₂) than the respective single bonds in H_nE-EH_n. An even higher degree of the orbital term DE_orb (72.4%) is found in the HC≡CH triple bond. The contribution of DE_orb in H_nE=EH_n increases and the relative contribution of the π bonding decreases when 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 becomes less when E = Pb (51.7%). A true understanding of the strength and trends of the chemical bonding can only be achieved when the Pauli repulsion is considered. The repulsive DE_Pauli term is in most cases the largest term in the EPA in an absolute sense.