Conformational equilibria in some cyclic imines: NH and CH stretching vibrations and the axial lone pair

Abstract

The fundamental NH and CH stretching vibrations of a number of cyclic imines have been examined in dilute CC14 solution. A trans orientation of the N lone pair orbital and one or more hydrogen atoms on adjacent carbons lowers the relevant vCH ("Bohlmann" bands) and raises vNH reflecting an increase in the s-character of the CH bond(s), consistent with a partial delocalization of the lone pair electrons into the CN bond. Conformations in which this interaction occurs are thermodynamically favored, and the ΔH values for the lone pair axial–equatorial equilibrium in piperidine, pyrrolidine, and indolene are estimated to be 0.4, 0.2, and 0.1 kcal/mole, respectively, in dilute CCl4 solution.The effects of hetero ring size, N-substitution, α-methyl substitution, and the solvent environment are investigated.