Substituent Effects in the Noncovalent Bonding of SO2 to Molecules Containing a Carbonyl Group. The Dominating Role of the Chalcogen Bond

The SO2 molecule is paired with a number of carbonyl-containing molecules, and the properties of the resulting complexes are calculated by high-level ab initio theory. The global minimum of each pair is held together primarily by a S center dot center dot center dot O chalcogen bond wherein the lone pairs of the carbonyl 0 transfer charge to the pi* antibonding SO orbital, supplemented by smaller contributions from weak CH center dot center dot center dot O H-bonds. The binding energies vary between 4.2 and 8.6 kcal/mol, competitive with even some of the stronger noncovalent forces such as H-bonds and halogen bonds. The geometrical arrangement places the carbonyl 0 atom above the plane of the SO, molecule, consistent with the disposition of the molecular electrostatic potentials of the two monomers. This S center dot center dot center dot O bond differs from the more commonly observed chalcogen bond in both geometry and origin. Substituents exert their influence via inductive effects that change the availability of the carbonyl O lone pairs as well as the intensity of the negative electrostatic potential surrounding this atom.