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 Cα 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 β. Where do their affinities lie? In studies using molecular balances, noncovalent interactions with an arene dominated over those with an alkene, and a π‐facial hydrogen bond from a hydroxy group to an arene was favored by about 1.2 kJ mol−1. Data for Y=CH2CH3, CHCH2, CCH, and CN groups indicated a long‐range electrostatic C⋅⋅⋅π interaction between the Y substituent and the π system. The strongest interaction observed was with the cyano group.