An Assessment of Radial Potential Functions for the Halogen Bond: Pseudo-Diatomic Models for Axially Symmetric...

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 $$ and $$ , and $$ of the intermolecular stretching mode $$ are calculated by two closely related approaches. The first involves derivatives of a polynomial fitted to the ab initio calculated points near to r_e and evaluated at r=r_e . 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.