Structure, Stability, and Spectroscopic Properties of H‑Bonded Complexes of HOSO and CH₃SO with H₂O

Quantum chemical calculations have been carried out to investigate the structure and stability of 1:1 and 1:2 HOSO–water and CH₃SO–water complexes. All of the geometries have been optimized at the DFT and at the CCSD levels of theory using 6-311++G­(2df,2pd) and aug-cc-pVDZ basis sets, respectively. The energetics of the hydrogen-bonded complexes are reported at G4 and CBS-QB3 levels of theory. A general characteristic future of the minimum-energy structure complexes is cyclic double H bonding for 1:1 complexes and cyclic triple H bonding for 1:2 complexes. Calculations predict relative large binding energies of 8.2 and 16.8 kcal mol^–1 for 1:1 and 1:2 HOSO–water complexes, respectively, at the CBS-QB3 level of theory. CH₃SO–water complexes have somewhat lower stability; the binding energy of 3.8 kcal mol^–1 for the 1:1 CH₃SO–water complex increases to 9.5 kcal mol^–1 for the 1:2 complex. The calculated shifts in vibrational frequencies due to complex formation show that the frequencies and intensities of H-bonded OH stretching regions are most affected by complex formation. The large frequency shift is mainly oriented to these OH bonds involved in H-bonding interactions. Vertical electronic excitation energies and the corresponding oscillator strengths have been calculated for the representative radical–water complexes using the TDDFT method and aug-cc-pVTZ basis set. No significant excitation energy difference was observed between the low-lying electronic states of either HOSO within the HOSO–water complexes or CH₃SO within the CH₃SO–water 1:1 complexes