Description de l'effet Overhauser dans les systèmes en échange: application à l'analyse conformationnelle d'aldéhydes hétérocycliques

Abstract

The equations of the nuclear Overhauser effect (n.O.e.) in exchanging systems predict relative increases for proton signals where the life times for exchange are of the same order of magnitude as the relaxation times. This method allows one to obtain structural informations. Hence, for a temperature sufficiently below that of the coalescence, it allows one to infer a conformational preference by an examination of the relative increase in intensity with respect to spatial position of exchange sites. We have hereby determined the conformations of formyl-2 bromo-4 thiophene and acetyl-2 dibromo-4,5 thiophene. The compounds show at ambient temperature a single rotamer OS-cis. Formyl-2 bromo-4 furan exists at −90 °C in an equilibrium between the rotamers OO-cis and OO-trans. In this case, the results of different irradiations show OO-cis to be the preferred rotamer. Moreover, this method is able to give indications of the life time on different sites. We also give an evaluation of the limits of the method.