Energy barriers to internal rotation: Hyperconjugation and electrostatic description

Analysis of internal rotation around single bonds was aimed at answering the following question: Is the hyperconjugation always a driving force for molecular shape and conformational preferences? For hydroxydiazenium N-oxide, one of the molecules investigated here, the answer appeared to be negative. As a consequence, there arose another question: What are the forces that hinder internal rotations around single bonds? To provide the answer to the latter, the individual repulsive and attractive terms to the potential energy changes accompanying internal rotation were calculated. Density functional theory was applied as it allows for the separate determination of the Coulomb repulsion, exchange, and correlation to the electron-electron interaction. Calculations were performed for ethane, biphenyl, 2,2'-difluorobiphenyl, formic acid, and hydroxydiazenium N-oxide. It has been found that both attractive and repulsive interactions are diminished near the barrier top but the attraction attenuates to a higher degree than the repulsion does. This holds true even for the molecules that are sterically crowded near the top. It has been shown that the exchange and correlation contributions are lower by factors of 10 and 100, respectively, than the electrostatic interactions.