High level ab initio calculations for ClF_n⁺ (n = 1-6) ions: Refining the recoupled pair bonding model

Based on detailed, high level ab initio calculations on a number of halogenated compounds of second row, late p-block elements, the SF_n, ClF_n, PF_n, SCl_n, and SF_nCl families, we found that a new type of bond - the recoupled pair bond - accounts for the ability of these elements to form hypervalent, or hypercoordinated, compounds. Hypervalent molecules are formed when it is energetically favorable for the electrons in a lone pair orbital to be recoupled, allowing each of the electrons to form chemical bonds with ligands. In this paper, we characterize the structures and energies of the ground and low-lying excited states of the ClF_n⁺ (n = 1-6) ions, using high level ab initio methods [MRCI, CCSD(T)/RCCSD(T)] with large correlation consistent basis sets. We computed a number of quantities, including ClF_n⁺ structures, bond dissociation energies, and ClF_n ionization energies and compared our results with the available experimental data. Both the bond dissociation energies and the ionization energies oscillate, variations that are readily explained using the recoupled pair bonding model. Comparisons are drawn between the ClF_n⁺ cations and their counterparts in the isoelectronic SF_n series, which possess many similarities. We found two significant differences between the ClF_n⁺ and the SF_n series: (i) the bond dissociation energies of ClF _n⁺ are much weaker than those of the corresponding SF _n species, and (ii) there is no stable ³A₂ state in ClF₂⁺ corresponding to the stable state found in SF₂. An examination of the Mulliken populations at the HF/AVTZ level for ClF_n⁺ and SF_n species predicts that the F atom in the axial (recoupled pair bonding) position is more highly charged than the F atom in the equatorial (covalent bonding) position; there is also less charge transfer to the F atoms in ClF_n⁺ than in SF _n. The positive charge on Cl⁺ makes it more difficult for an F atom to attract electrons from Cl⁺ than from S and correspondingly less favorable to recouple the electrons in the lone pair orbitals in the ClF_n⁺ species.