An Assessment of Radial Potential Functions for the Halogen Bond: Pseudo‐Diatomic Models for Axially Symmetric Complexes B⋅⋅⋅ClF (B=N 2 , CO, PH 3 , HCN, and NH 3 )

Abstract 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=N 2 , CO, PH 3 , HCN, and NH 3 ) 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 constantsω σandω σ x σ , and α σof 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 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 H 3 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 H 3 P⋅⋅⋅ClF is different from those involving B=N 2 , HCN, or NH 3 , a difference attributed to H 3 PCl + ⋅⋅⋅F − character. The ab initio generated curve for H 3 P⋅⋅⋅ClF is, nevertheless, satisfactorily fitted by the three‐parameter H−H function.