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Effect of the orientation of substituents on the chemical shifts of 13C. IV—13C NMR spectra of N,N-diisopropylamides and -thioamides

✍ Scribed by Hans Fritz; Paul Hug; Hanspeter Sauter; Tammo Winkler; Sven-Olov Lawesson; Bjarne Skindhoj Pedersen; Sten Scheibye

Publisher: John Wiley and Sons
Year: 1981
Tongue: English
Weight: 834 KB
Volume: 16
Category: Article
ISSN: 0749-1581
DOI: 10.1002/mrc.1270160111

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✦ Synopsis

Abstract

N,N‐diisopropylamides and ‐thioamides show hindered rotation around the NCH bonds, and the presence of mixtures of conformational isomers can be demonstrated at temperatures below 273 K in solution. ^1^H and ^13^C NMR spectra of these conformers are measured and assigned. The ^13^C data serve to study through‐space effects on ^13^C 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.

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