Potential Energy Surfaces of HN(CH)SX:CO2 for X = F, Cl, NC, CN, CCH, and H: N···C Tetrel Bonds and O···S Chalcogen Bonds.

MP2/aug'-cc-pVTZ calculations have been performed in search of complexes, molecules, and transition structures on the HN(CH)SX:CO2 potential energy surfaces, for X = F, Cl, NC, CN, CCH, and H. Complexes stabilized by traditional N···C tetrel bonds and O···S chalcogen bonds exist on all surfaces and are bound relative to the isolated monomers. Molecules stabilized by an N-C covalent bond and an O···S chalcogen bond are found when X = F, Cl, and NC, but only the HN(CH)SF:CO2 molecule is bound. The binding energies of these complexes correlate with the O-S distance but not with the N-C distance. Binding energies of complexes rotated by 90° about the N···C tetrel bond and by 90° about the O···S chalcogen bond provide estimates of these bond energies. Charge-transfer energies across tetrel and chalcogen bonds correlate with the N-C and O-S distances, respectively. As a function of the N-C distance, equation-of-motion coupled cluster singles and doubles spin-spin coupling constants 1tJ(N-C) for complexes and transition structures and 1J(N-C) for molecules describe the evolution of the N···C tetrel bonds in the complexes and transition structures to N-C covalent bonds in the molecules. The O···S chalcogen bond gains some covalency in the transition structures and again in the molecules but does not become a covalent bond.