Effect of the orientation of substituents on the chemical shifts of13C. IV—13C NMR spectra ofN,N-diisopropylamides and -thioamides
作者:Hans Fritz、Paul Hug、Hanspeter Sauter、Tammo Winkler、Sven-Olov Lawesson、Bjarne Skindhoj Pedersen、Sten Scheibye
DOI:10.1002/mrc.1270160111
日期:1981.5
AbstractN,N‐diisopropylamides and ‐thioamides show hindered rotation around the NCH bonds, and the presence of mixtures of conformational isomers can be demonstrated at temperatures below 273 K in solution. 1H and 13C NMR spectra of these conformers are measured and assigned. The 13C data serve to study through‐space effects on 13C chemical shifts, which strongly depend on the conformations of the isopropyl groups. For amides, a through‐space shielding of the N‐methine carbons is found to exist only for conformers in which the methine hydrogen atom is spatially close to the oxygen atom. Chemical shift differences between amides and thioamides can be rationalized in terms of through‐bond and through‐space contributions, and serve for a better understanding of the shift differences in N,N‐dialkylamides and ‐thioamides.