An ab initio quantum chemical study of the hydrogen- and "anti"-hydrogen-bonded HF/ClF and HF/Cl₂ dimers

Ab initio electronic structure calculations at the SCF and coupled pair functional (CPF) level have been carried out for the hydrogen- and "anti"-hydrogen-bonded isomers of the ClF/HF and Cl₂/HF complexes. The stability of each isomer with respect to its constituent monomers has been calculated at the appropriate optimized geometry, paying particular attention to correlation effects, zero point energies, and basis set superposition errors. The differences between the latter quantities for a given pair of isomers, all calculated to be ≲0.5 kcal mol^-1, are comparable in magnitude to the difference in stabilities, hence in a quantitative description of bonding in these dimers all these effects must be considered. The dipole moments and the ³⁵Cl and ²H nuclear quadrupole coupling constants for the dimers have also been evaluated and compared with the available experimental data. In agreement with the results of recent molecular beam measurements, in both systems the anti-hydrogen-bonded isomers are found to be the more stable ones.