Article révisé par les pairs
Résumé : A conformational free-energy différence of -2 kcal/mole for a C4CH3 group in the N,N-dimethyl-4-methylpiperidinium salt has been derived from application of Eliel's NMR method. The attribution of the N-methyl absorptions was based on the high stereospecific dependence of the long range coupling constants 4J. Long range 4-alkyl substituent effects on the chemical shifts of the N-methyl groups were also considered. The free energy differences between the equatorial and axial conformers of 4-hydroxy-4-chloro- and 4-acetoxypiperidine have been obtained by studies on the C4-proton band-width. The ΔG° values of the dree derivatives are respectively -0·55, -0·34 and -0·56 kcal/mole while the ΔG° values for the corresponding piperidinium compounds are +0·03, +0·67 and +0·43 kcal/mole. An electrostatic attraction between the electronegative 4-substituent, carrying a partial negative charge, and the positive nitrogen atom can justify the observed variation of δG°. © 1971.