Nature and stability of the chemical bond in H₃C-XH_n (XH_n = CH₃, NH₂, OH, F, Cl, Br, I)

We have quantum chemically analyzed the trends in bond dissociation enthalpy (BDE) of H₃C-XH_n single bonds (XH_n = CH₃, NH₂, OH, F, Cl, Br, I) along three different dissociation pathways at ZORA-BLYP-D3(BJ)/TZ2P: (i) homolytic dissociation into H₃C^∙ + ^∙XH_n, (ii) heterolytic dissociation into H₃C⁺ + ⁻XH_n, and (iii) heterolytic dissociation into H₃C⁻ + ⁺XH_n. The associated BDEs for the three pathways differ not only quantitatively but, in some cases, also in terms of opposite trends along the C-X series. Based on activation strain analyses and quantitative molecular orbital theory, we explain how these differences are caused by the profoundly different electronic structures of, and thus bonding mechanisms between, the resulting fragments in the three different dissociation pathways. We demonstrate that the nature and strength of a chemical bond are only fully defined when considering both (i) the molecule in which the bond exists and (ii) the fragments from which it forms or into which it dissociates.