Comparison of CH•••O, SH•••O, Chalcogen, and Tetrel Bonds Formed by Neutral and Cationic Sulfur-Containing Compounds

The ability of neutral and charged S-compounds to form different sorts of noncovalent bonds is examined by ab initio calculations. Neutrals are represented by CH3SH and fluoro-substituted FSCH3; cations are (CH3)(3)S+, CH3SH2+, and FHSCH3+. Each is paired with N-methylacetamide (NMA) whose O atom serves as a common electron donor. Charged species engage in much stronger noncovalent bonds than do the neutral molecules, by as much as an order of magnitude. The strongest noncovalent bond for any system is a O center dot center dot center dot SF chalcogen bond wherein the O lies directly opposite a S-F covalent bond, amounting to as much as 39 kcal/mol. Second in binding energy is the SH center dot center dot center dot O H-bond, which can be as large as 34 kcal/mol. Somewhat weaker is the O center dot center dot center dot SC chalcogen bond, followed by the CH center dot center dot center dot O H-bond and finally the O center dot center dot center dot C tetrel bond, which has the appearance CH group that participates in a CH center dot center dot center dot O H-bond shifts its NMR signal downfield by an amount strength of the H-bond. This situation is clearly distinguishable from that in a O center dot center dot center dot S chalcogen or methyl protons are shifted upfield. of a trifurcated H-bond. Any roughly proportional to the SH center dot center dot center dot O H-bond wherein the methyl protons are shifted upfield.