A comparative study of isoelectronic and isogyric reactions: Molecular orbital calculations of diatomic hydrides and halides

MP4/6-311G(2df,2pd)//MP2/6-31G** and MPn/6-31G**//ClD/6-31G** (n = 2-4) calculations have been carried out for HeF⁺, NeF⁺, ArF⁺, ArH⁺, HCl, HCl⁺, ClF, Cl₂, HF, HF⁺, F₂, H₂, and H₂⁺. Structures, vibrational frequencies, ionization potentials, reaction enthalpies, and triplet vs singlet energies (for HeF⁺, NeF⁺, and ArF⁺) have been examined. The reactions considered include hydrogenolysis and bond dissociation reactions. Isoelectronic comparisons have been made to assist in the study of reaction enthalpies. It is found that the MP4/6-311G(2df,2pd) enthalpies of reaction reproduce the experimental enthalpies well for all but nonisogyric reactions. MP4/6-31G** results reproduce the higher level computational results and the experimental results consistently well only for isogyric reactions that involve isoelectronic pairs of reactants and products. There is also considerable disagreement in the ionization potentials and singlet-triplet splittings obtained with the two basis sets. The MP4/6-311G(2df,2pd) and experimental ionization potentials differ by ≤0.3 eV. Calculated vibrational frequencies and bond lengths are in good accord with experiment.