A test of ab initio-generated, radial intermolecular potential energy functions for five axially-symmetric,...

The radial potential energy functions for five axially symmetric, hydrogen-bonded complexes BHF (B = N₂, CO, PH₃, HCN and NH₃) have been calculated ab initio at the explicitly correlated level of theory CCSD(T)(F12c)/cc-pVTZ-F12 as a function of the hydrogen-bond distance r(ZH), where Z is the hydrogen-bond acceptor atom of B. The remaining geometric parameters were optimised at each point in the calculation. The functions so generated were then used to estimate the spectroscopic constants ω_σ, ω_σx_σ,α_σ, and the v_σ = 1 ← 0 transition wavenumber associated with the intermolecular stretching mode ν_σ of each BHF by using two equivalent approaches. Both involved the assumption that the vibrational modes of the B and HF molecules were sufficiently stiff relative to the intermolecular stretching mode that BHF could be treated in a pseudo-diatomic approximation. One approach used derivatives of the potential evaluated at the distance r = r_e while the other used the potential constants obtained by non-linear regression fits of three analytical functions (Morse, Rydberg and Hulburt-Hirschfelder) to the ab initio calculated points. The two approaches would give exactly the same results if the functions were a perfect fit. The H-H function gave the best fit. The determined spectroscopic constants were found to be in reasonable agreement with the limited number available by experiment. The efficacy of the approach was tested for the diatomic molecule H³⁵Cl by taking advantage of both an accurate RKR-type potential and an accurate set of spectroscopic constants. It was also established that the relationship D_e = k_σ/(2a²) between two measures (D_e and k_σ, both calculated ab initio) of the strength of the hydrogen bond in the BHF complexes (and required if the H-H function were an accurate representation of the BHF potential functions) holds to an excellent level of approximation, and supports the conclusion that this function is appropriate to represent the hydrogen bond in the complexes investigated.