Complexes of O=C=S with Nitrogen Bases: Chalcogen Bonds, Tetrel Bonds, and Other Secondary Interactions

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate chalcogen-bond formation through the σ-hole at S and tetrel-bond formation through the π-hole at C in complexes of OCS with a series of nitrogen bases. The binding energies of chalcogen- and tetrel-bonded complexes with the sp-hybridized bases correlate exponentially with the N-S and N-C distances, respectively. The presence of secondary interactions between an N-H or C-H group of an sp² -hybridized base and OCS in chalcogen-bonded complexes decreases the correlation between binding energies and the N-S distance. These secondary interactions are stronger in the tetrel-bonded complexes with the sp² bases, particularly in the isomers of OCS:imidazole and OCS : N₂ H₂ , where they may be described as distorted N-H⋅⋅⋅O or N-H⋅⋅⋅S hydrogen bonds. Charge-transfer interactions are consistent with the nature of the primary and secondary interactions in these complexes. The in-plane OCS bending frequencies are blue-shift in the chalcogen-bonded complexes, and red-shifted in the tetrel-bonded complexes. EOM-CCSD spin-spin coupling constants ^1c J(N4-S) across chalcogen bonds have absolute values less than 9.0 Hz, while the two-bond coupling constants ^2c J(N4-C) do not exceed 4.0 Hz. These are greater in absolute value that the one-bond coupling constants ^1t J(N4-C) across tetrel bonds that are less than 0.5 Hz at much shorter N-C distances.