Unsaturated macrocyclic compounds. Part LXXXIII. A quantitative study of the conformational mobility of [18]annulene

Abstract

[18]Annulene undergoes an isodynamic conformational process by which the inner and outer protons exchange their chemical (magnetic) environment and become equivalent above +41° in the n.m.r. spectrum (60 MHz). Three isodynamic conformers are implied in this process.

A quantitative study of this conformtional mobility, based on complete n.m.r. line shape analysis, is presented. The kinetic parameters deduced (enthalpy and entropy of activation) are discussed; a mechanism with the possible transition state structures is proposed.

Thermochemical considerations making use of the enthalpy of activation observed for the conformational mobility of [18]annulene lead to the conclusion that the resonance energy in this aromatic molecule cannot be greater than 19 kcal mol^–1.