CH4-O2, NH3-O2, H2O-O2 and HF-O2 triplet complexes. Ab initio studies and comparisons. From van der Waals to hydrogen bonding

Using coupled cluster singles, doubles and perturbative triples CCSD(T) methods with augmented correlation consistent basis sets up to the 5Z level, dissociation energies and structural parameters were obtained for CH4-O-2, NH3-O-2, H2O-O-2 and HF-O-2 triplet complexes. Most stable for CH4-O-2 is a structure with three hydrogens facing O-2 in T-shape (dissociation energy D-e = 168 cm(-1)), for NH3-O-2 a structure with two hydrogens facing O-2 in X shape (D-e = 196 cm(-1)), and for H2O-O-2 (D-e = 222 cm(-1)) and HF-O-2 (D-e = 301 cm(-1)) a hydrogen bonded structure. Energies, geometries, vibrational frequencies, infrared intensities and dipole/quadrupole moments of the four complexes were compared. While such properties change gradually from CH4-O-2 to H2O-O-2, they are much more pronounced for HF-O-2. The transition from van der Waals to hydrogen bonding was followed. There are significant changes in the O-2 frequencies for hydrogen-bonded structures. Due to increased vibrational intensities such complexes may contribute to the greenhouse effect.