The nature of the chemical bond revisited.: An energy partitioning analysis of diatomic molecules E2 (E=N-Bi, F-I), CO and BF

The nature of the chemical bonds in the diatomic molecules E-2 (E=N-Bi, F-I), CO and BF has been studied with an energy partitioning analysis using gradient-corrected density functional theory calculations. The results make it possible to estimate quantitatively the strength of covalent and electrostatic attractions and the Pauli repulsion between the atoms. The data suggest that some traditional explanations regarding the strength of the molecules should be modified. The energy partitioning analysis shows that the chemical bonds in the group 15 diatomic molecules have significant electrostatic character, which increases from 30.1% in N-2 to 58.3% in Bi-2. The contribution of the electrostatic attraction to the binding interactions in Sb-2 and Bi-2 is larger than the covalent bonding. The strength of the pi bonding in the triply bonded dinitrogen is less than that of the sigma bonding. The calculations indicate that DeltaE(pi) is between 32.2% (Bi-2) and 40.0% (P-2) of the total orbital interaction energy (DeltaE(orb)). The much stronger bond of N-2, as compared with the heavier group 15 E-2 homologues, is not caused by a particularly strong contribution by the pi bonding, but rather by the relatively large sigma interactions. The comparison of N-2 with isoelectronic CO shows that the electrostatic character in the heteroatomic molecule is slightly smaller (28.8%) than in the homoatomic molecule. The contribution of the pi bonding in CO is larger (49.2%) than in N-2 (34.3%). The reason why CO has a stronger bond than N-2 is the significantly weaker Pauli repulsion in CO. The electrostatic character of the bonding in BF is slightly larger (32.0%) than in CO and N-2. BF has much weaker pi-bonding contributions that provide only 11.2% of the covalent interactions, which is why BF has a much weaker bond than CO and N-2. The chemical bonds in the dihalogen molecules have much higher covalent than electrostatic character. The DeltaE(orb) term contributes between 74.4% (Br-2) and 79.7% (F-2) to the total attractive interactions. The relatively weak bond in F-2 comes from the rather large Pauli repulsion.