An ab initio MO study on the structures and electronic states of hydrogen-bonded O-3-HF and SO2-HF complexes

At, initio molecular orbital (MO) calculations have been carried out for ba se-hydrogen fluoride (HF) complexes (base = O-3 and SO2) in order to elucid ate the structures and energetics of the complexes. The ab initio calculati ons were performed up to the QCISD(T)/6-311++G(d,p) level of theory. In bot h complexes, hydrogen-bonded structures where the hydrogen of HF orients to ward one of the oxygen atoms of bases were obtained as stable forms. The ca lculations showed that cis and trans isomers exist in both complexes. All c alculations for the SO2-HF complex indicated that the cis form is more stab le in energy than the trans form. On the other hand, in Os-HF complexes, th e stable structures are changed by the ab initio levels of theory used, and the energies of the cis and trans forms are close to each other. From the most sophisticated calculations (QCISD(T)/6-311++G(d,p)//QCISD/6-311+G(d) l evel), it was predicted that the complex formation energies for cia SO2-HF, trans SO2-HF, cis O-3-HF, and trans O-3-HF are 6.1, 5.7, 3.4, and 3.6 kcal /mol, respectively, indicating that the binding energy of HF to SO2 is larg er than that of O-3. The harmonic vibrational frequencies calculated for ci a O-3-HF and cis SO2-HF complexes were in good agreement with the experimen tal values measured by Andrews et al. Also, the calculated rotation constan ts for cis SO2-HF agreed with the experiment.