Noncovalent Interactions of π Systems with Sulfur: The Atomic Chameleon of Molecular Recognition.

The relative strength of noncovalent interactions between a thioether sulfur atom and various π systems in designed top pan molecular balances was determined by NMR spectroscopy. Compared to its oxygen counterpart, the sulfur atom displays a remarkable ability to interact with almost equal facility over the entire range of π systems studied, with the simple alkene emerging as the most powerful partner. With the exception of the O⋅⋅⋅heteroarene interaction, all noncovalent interactions of sulfur with π systems are favoured over oxygen.

Surfing π†clouds for noncovalent interactions: arenes versus alkenes.

A comparative study of molecular balances by NMR spectroscopy indicates that noncovalent functional-group interactions with an arene dominate over those with an alkene, and that a π-facial intramolecular hydrogen bond from a hydroxy group to an arene is favored by approximately 1.2 kJ mol(-1). The strongest interaction observed in this study was with the cyano group. Analysis of the series of groups CH2CH3, CH=CH2, C≡CH, and C≡N shows a correlation between conformational free-energy differences and the calculated charge on the C(α) atom of these substituents, which is indicative of the electrostatic nature of their π interactions. Changes in the free-energy differences of conformers show a linear dependence on the solvent hydrogen bond acceptor parameter ß.