Rotational barriers adjacent to the CN double bond in methyl and ethylimines

Abstract

The rotational barrier about the C-C bond adjacent to the CN group in methyl and ethylimines was evaluated by MO calculations at the RHF level and by considering electron-correlation effects. Structural regularities (unequal lengths for in plane and out-of-plane CH bonds, changes in C-C bond lengths or NC-C angles) were used as a test for hyperconjugative interactions. For trans situations (alkyl group facing the nitrogen lone pair) n/π ∗ CH 3 , hyperconjugation is effective only for the eclipsed conformations (CH or CC bonds eclipsing the double bond), and leads to stabilization. For cis situations (alkyl group facing the NH bond) π ∗ CN/π CH 3 hyperconjugation is observed only when an alkyl group bisects the double bond and destabilizes this conformation. In both cases, the expected interaction (< 0.5 kcal mol −1) is too small to account for the barrier heights. These are mainly controlled by bond-bond exchange repulsions. In most cases, direct or reverse methyl tilts do not result from hyperconjugative interactions as is commonly affirmed.