Modulation of Conformational Preferences of Heteroaromatic Ethers and Amides through Protonation and Ionization: Charge Effect

Multiple approaches reveal the strong effects of a positive charge introduced by protonation or ionization on the conformation of o‐heteroaromatic ethers and amides. The ethers and amides containing an ortho‐N heteroatom are syn‐preferring while those containing an ortho‐O or ortho‐S heteroatom are mostly anti‐preferring. However, for all the monocyclic o‐heteroaromatic ethers and amides, the protonated ones are all anti‐preferring while the ionized ones are all syn‐preferring. Interestingly, although both the protonation and ionization introduce a positive charge, they have such different effects on molecular conformation, very informative for understanding the origin of conformational preferences. Detailed analysis shows that the population of the introduced positive charge dictates the conformational preferences via electrostatic and orbital interactions. Compared to ortho‐heteroatoms, meta‐heteroatoms have weaker effect on conformational preference. Achieved by complete inductive method, the regularity of conformational preferences and switching provides easy ways to modulate conformers (by pH or redox), and makes this kind of ether or amide bond a conformational hinge applicable to design of functional molecules (drugs and materials) and modulation of molecular biological processes.