Radial Potential Energy Functions of Linear Halogen-Bonded Complexes YX···ClF (YX = FB, OC, SC, N 2 ) and the Effects of Substituting X by Second-Row Analogues: Mulliken Inner and Outer Complexes.

Energies of linear, halogen-bonded complexes in the isoelectronic series YX···ClF (YX = FB, OC, or N 2 ) are calculated at several levels of theory as a function of the intermolecular distance r (X···Cl) to yield radial potential energy functions. When YX = OC, a secondary minimum is observed corresponding to lengthened and shortened distances r (ClF) and r (CCl), respectively, relative to the primary minimum, suggesting a significant contribution from the Mulliken inner complex structure [O═C-Cl] + ···F - . A conventional weak, halogen-bond complex OC···ClF occurs at the primary minimum. For YX = FB, the primary minimum corresponds to the inner complex [F═B-Cl] + ···F - , while the outer complex FB···ClF is at the secondary minimum. The effects on the potential energy function of systematic substitution of Y and X by second-row congeners and of reversing the order of X and Y are also investigated. Symmetry-adapted perturbation theory and natural population analyses are applied to further understand the nature of the various halogen-bond interactions.