Recoupled Pair Bond Research Articles

Bonding in ClFn (n = 1−7) Molecules: Further Insight into the Electronic Structure of Hypervalent Molecules and Recoupled 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 SF(n) species (n = 1-6), the present work explores the ClF(n) (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 ClF(n) 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 ClF ((3)Pi) and ClF(2) ((2)A(1), (2)B(1), (2)A', (4)A(2)). The bond energies for F addition to form ClF(2), ClF(4), and ClF(6) were found to be much lower than those leading to ClF, ClF(3), and ClF(5). The same type of oscillation is observed in SF(n) species. The differences between ClF(n) and SF(n) 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 ClF(n) species demonstrate the improved predictive ability of the recoupled pair bonding model over other models for describing hypervalent bonding.