Relationship between Rotational Barriers and Charge Shifts.

Conjugation and hyperconjugation are closely related stereoelectronic effects that are usually discussed in a qualitative fashion. It is possible to make it more quantitative. The natural bond orbital method provides a means of estimating the energetic consequence. They may also be studied by a charge shift analysis. Rotational barriers are often appropriately discussed in terms of such effects where the rotational transition state has broken the interaction between donor and acceptor atoms or groups. A comparison of charge distribution between the ground state and the transition state will show how much charge transfer occurs and if any other atoms are involved. This has been studied for substituted vinyl methyl ethers, substituted -dimethylvinylamines, -substituted anilines, and some esters and amides. Linear relationships were found for the rotational barriers vs the charge shifts. For some compounds, the σ- and π-components of the charge were obtained and showed that competition for σ-charge can lead to changes in the π-charge. Cases where there cannot be charge transfer are also discussed.