An Assessment of Radial Potential Functions for the Halogen Bond: Pseudo-Diatomic Models for Axially Symmetric Complexes B⋅⋅⋅ClF (B=N2 , CO, PH3 , HCN, and NH3 ).

The radial potential energy is calculated ab initio at the explicitly correlated level of theory CCSD(T)(F12c)/cc-pVTZ-F12 for the five axially symmetric, halogen-bonded complexes B⋅⋅⋅ClF (B=N2 , CO, PH3 , HCN, and NH3 ) as a function of the intermolecular distance r. The PE curves are fitted by the Hulburt-Hirschfelder analytical function under the assumption of a pseudo-diatomic model. The spectroscopic constants ω σ and ω σ x σ , and α σ of the intermolecular stretching mode υ σ are calculated by two closely related approaches. The first involves derivatives of a polynomial fitted to the ab initio calculated points near to re and evaluated at r=re . The second uses the constants of the fitted H-H function. Both procedures are tested on 35 ClF by fitting (a) its RKR-type function and (b) the CCSD(T)(F12c)/cc-pVTZ-F12 version. The complexes OC⋅⋅⋅ClF and H3 P⋅⋅⋅ClF behave differently from the other three. A point of inflection/secondary minimum with a shortened r(C⋅⋅⋅Cl) and an increased r(Cl-F) detected for B=CO, suggests a second isomer with a significant contribution from the valence-bond structure OC+ Cl⋅⋅⋅F- . The shape of the ab initio calculated function for H3 P⋅⋅⋅ClF is different from those involving B=N2 , HCN, or NH3 , a difference attributed to H3 PCl+ ⋅⋅⋅F- character. The ab initio generated curve for H3 P⋅⋅⋅ClF is, nevertheless, satisfactorily fitted by the three-parameter H-H function.