Bonding in CIFn (n=1-7) Molecules: Further Insight into the Electronic Structure of Hypervalent Molecules and Recounted Pair Bonds

As a result of new studies into the nature of hypervalent molecules, we identified a new type of bond called a recoupled pair bond. Hypervalency or hypercoordination was shown to arise by decoupling a pair of valence electrons, each of which becomes available to participate in a new bond. Energy must be expended to decouple an electron pair, so the first recoupled pair bond is weaker than the analogous covalent bond. However, the second bond, which involves a singly occupied antibonding orbital in the hypervalent fragment, is stronger than the analogous covalent bond. Following an initial study of SFn species (n = 1-6), the present work explores the CIFn (n = 1-7) series to further examine the explanatory usefulness of the recoupled pair bonding model. Optimized structures and energies of the ground and low-lying excited states of the CIFn molecules were determined by employing high level ab initio calculations [MRCI, CCSD(T)] with correlation consistent basis sets. Low-lying states that are due to recoupled pair bonding are found in CIF ((3)Pi) and CIF2 ((2)A(1), B-2(1), (2)A', (4)A(2)). The bond energies for F addition to form CIF2, CIF4, and CIF6 were found to be much lower than those leading to CIF, CIF3, and CIF5. The same type of oscillation is observed in SFn species. The differences between CIFn and SFn reflect the fact that the 3s(2) and 3p(2) electron pairs are more strongly bound in Cl than in S. This behavior and other trends observed in the CIFn species demonstrate the improved predictive ability of the recoupled pair bonding model over other models for describing hypervalent bonding.