Theoretical Study of Formic Acid−Sulfur Dioxide Dimers

We report the first theoretical study of noncovalent and covalent interactions in formic acid (FA)−SO2 complexes. Using ab initio and DFT model chemistries, five stable noncovalent complexes were identified, as well as a covalent adduct, formic sulfurous anhydride HOSO2CHO. syn-FA is predicted to form two nonplanar bidentate complexes with SO2: the more stable one contains a normal hydrogen bond donated by OH, and the less stable one contains a blue-shifted hydrogen bond donated by CH. Both are stabilized by charge transfer from FA to SO2. anti-FA forms three planar complexes of nearly equal energy containing OH-to-SO2 hydrogen bonds. Formic sulfurous anhydride forms via an endothermic concerted cycloaddition. Natural bond orbital analysis showed that the bidentate SO2−FA complexes are stabilized by n → π* donation from FA to SO2, and back-donation from SO2 n and π* orbitals into FA σOH* or σCH* orbitals. The bidentate formic acid−SO2 complex that contains an O−H···O hydrogen bond is more stable than the ...