Long-range substituent effects on conformational equilibria

Abstract

Effects of remote substituents on conformational equilibria, though sometimes observed by NMR techniques, are generally assumed to be negligible when conformational behaviour is investigated by the dipole moment method. In this paper a series of 2-aryl-1-chloropropanes was chosen for study by both NMR and dipole moment approaches. The dependence of the conformational equilibrium on para substitution was proven from the variations in the vicinal J HH coupling constants. The dipole moment method, though less sensitive, yielded approximately the same effective conformation. When the rotamer populations obtained from the NMR method were input into the calculation of effective dipole moments, the fit was improved. Input of conformer geometries calculated using molecular mechanics (COSMIC) showed little further improvement. It is concluded that long-range effects can be generally neglected in standard dipole moment work, particularly when the equilibrium is shifted toward one conformation.