Complexation of n SO2 molecules (n = 1, 2, 3) with formaldehyde and thioformaldehyde.

Ab initio and density functional theory calculations are used to examine complexes formed between H2CO and H2CS with 1, 2, and 3 molecules of SO2. The nature of the interactions is probed by a variety of means, including electrostatic potentials, natural bond orbital, atoms in molecules, energy decomposition, and electron density redistribution maps. The dimers are relatively strongly bound, with interaction energies exceeding 5 kcal/mol. The structures are cyclic, containing both a O/S⋯S chalcogen bond and a CH⋯O H-bond. Addition of a second SO2 molecule leads to a variety of heterotrimer structures, most of which resemble the original dimer, where the second SO2 molecule engages in a chalcogen bond with the first SO2, and a C⋯O attraction with the H2CX. Some cooperativity is apparent in the trimers and tetramers, with an attractive three-body interaction energy and shortened intermolecular distances.