On the nature of halogen bonded complexes between carbonyl bases and chlorotrifluoromethane

The structures and intermolecular interactions in the halogen bonded complexes of six carbonyl bases with chlorotrifluoromethane have been studied by ab initio MP2 and CCSD(T) as well as density functional methods: B3LYP, B3LYP-D3 and wB97XD. The CCSD(T)/aug-cc-pVTZ calculated interaction energies of these complexes range between -1.22 and -2.62 kcal mol(-1). The interaction energies are related to the ionization potential and proton affinity of the carbonyl bases. The intermolecular interaction results in a contraction of the C-Cl bond and an elongation of the C-F bonds accompanied by a relevant frequency shift of the corresponding stretching vibrations. The origin of the variation of the C-Cl and C-F bond lengths is discussed on the basis of an NBO analysis. It is suggested that the formation of these complexes can be considered as a two-step mechanism, involving the intermolecular charge transfer from the lone-pair orbitals on the oxygen atom to the antibonding sigma*(C-Cl) orbital followed by a decrease of the intramolecular charge transfer from the lone-pair orbitals on the fluorine atoms to the antibonding sigma*(C-Cl) orbital. A symmetry-adapted perturbation theory (SAPT) decomposition of the interaction energies has been performed.