Utilisation de l'effet overhauser des mesures de relaxation en RMN 1H, et des deplacements chimiques en RMN 13C dans l'etude de la conformation des amides—I : Rappels theoriques sur l'effet overhauser dans les systemes en echange. Application a l'etude conformationnelle d'amides aroamtiques (acetylpyrroles) et d'amides α-β insatures (N-acylindolines et N-acyltetrahydroquinoleines)

Abstract

The life time τ of preferred rotamers and the relaxation times T 1 of protons affected by the exchange process are both required in the study of a conformational equilibrium by the nuclear Overhauser effect (NOE) method. It is shown by a careful investigation of N-acetylpyrroles that the relaxation times are more sensitive to a change in correlations times τ c than the NOE. Accordingly, NOE is the method of choice in conformational studies. However, in N-acyl indolines and tetrahydroquinolines at room temperature, exchange is slow in terms of chemical shift (decoalescend conformers) but is in the intermediate range in terms or relaxation (τ < T 1). Therefore, a low temperature is needed to assess the structure of the rotamers by NOE. 1H data led to unequivocal assignments for the 13C NMR spectra of these compounds. This enables us to establish that reliable information is provided neither by 1H nor 13C NMR, as far as the changes in the resonance frequencies of a carbon or a hydrogen in the two rotamers are concerned. It is concluded that attempts to use carbonyl field effects in conformational studies ( 1H or 13C NMR) must be treated with caution.