A localized picture of back bonding in CH3−nXn (X = F, Cl and Br; n = 1, 2 or 3) radical and cation systems

The electronic structure of back bonding in CH3−nXn cation and radicals (X = F, Cl and Br; n = 1, 2 or 3) has been studied using the B3LYP approach. The Natural Bond Orbital (NBO) approach presents some limitations, particularly, when there are multi-identical C-X bonds in di- or tri-halogenated cation species, because it does not show back bonding in all C-X bonds simultaneously, especially in cations, although experimental and theoretical methods such as Quantum Theory of Atoms In Molecules (QTAIM) parameters confirm the existence of back bonding for multi-identical C-X bonds in the di- and tri-halogenated species. We will show that NBO is not a proper tool for such systems. Hence, Natural Localized Molecular Orbital (NLMO) procedure is suggested in this paper to remove this trouble, qualitatively. In some radical species, it is found that the carbon atom can establish two or three π back bondings with either halogen atoms in the same configuration, which resembles delocalized electronic structure. Finally, NLMO can detect π back bonding in both cation and radical species.