Hindered rotation in thioamides—II

Abstract

AbstractThe NMR spectra for a series of amides and thioamides of type magnified image where R = CH3, C6H5 and C6H5CH2, were investigated from −60° to 150°C. The Arrhenius activation energies, Ea, for the normal amide equilibration were determined by the W½ method and the values range from 15.4 to 20.9 Kcal/mole.The three thioamides, compounds (2), (4) and (6) yield spectra showing a second, low temperature, equilibration. This exchange was first studied by Siddall and co‐workers and they interpreted the spectral changes in terms of population changes of the conformational isomers about the NCH bonds. They assumed strong interaction of nitrogen alkyl groups which gave rise to three low energy dl pairs of isomers. Their results, however, as well as those obtained for similar compounds reported herein, show only two sets of patterns at low temperatures. These spectra suggest a first order, A⇄B, type equilibration where PA≠PB. An alternative assignment based upon the following classification of steric effects of S, R, R′ and R″, magnified image is suggested; (1) a strong interaction of the sulfur atom with the adjacent alkyl group on the nitrogen atom, R′, (2) a somewhat weaker interaction of R with the adjacent nitrogen alkyl group R″ and (3) a still weaker interaction of the two nitrogen alkyl groups, R′ and R″. This leads to four isomers, two dl pairs, C1T4, C6T3 and C1T6, C6T3, which are expected to have a lower energy than the rest and the first pair is preferred.The W½ method of measuring exchange rates was extended to cover A⇄B equilibrations, where PA≠PB. The results indicate an accuracy that is not significantly lower than that obtained by the more elaborate total line shape analysis.