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.

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Quantum chemical methods have been employed to evaluate the possible configurations of the 1:1 and 1:2 HOSO-formic acid complexes and 1:1:1 HOSO-formic acid-water complexes. The first type of complex involves two H bonds, while the other two types comprise three H bonds in a ring. The complexes are relatively stable, with CBS-QB3 computed binding energies of 14.3 kcal mol(-1) , 23.4 kcal mol(-1) , and 21.1 kcal mol(-1) for the lowest-energy structures of the 1:1, 1:2, and 1:1:1 complexes, respectively. Complex formations induce a large spectral red-shift and an enhancement of the IR intensity for the H-bonded OH stretching modes relative to those in the parent monomers. TDDFT calculations of the low-lying electronic excited states demonstrate that the complexes are photochemically quite stable in the troposphere. Small spectral shifts in comparison to the free HOSO radical suggest that the radical and the complexes would not be easily distinguishable using standard UV/vis absorption spectroscopy. © 2016 Wiley Periodicals, Inc.