Radial potential energy functions of linear halogen-bonded complexes YX…ClF, (YX = FB, OC, SC, N2) 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 N2) 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 was 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.