Effects of C(O)−N Bond Rotation on the13C,15N, and17O NMR Chemical Shifts, and Infrared Carbonyl Absorption in a Series of Twisted Amides

A series of the C(O)−N twisted amides, 3-acyl-4-alkyl-1,3-thiazolidine-2-thiones 1a − e, was synthesized, and the structures were elucidated by X-ray crystallographic analysis. The relationship between the C(O)−N twist angles τ, the 13 C, 15 N, and 17 O NMR chemical shifts, and the infrared absorption of carbonyl groups were investigated in order to provide insight into the changes in charge distribution dependence on the C(O)−N twist angle. Furthermore, the relationship of the ν C O and the 15 N chemical shift was also investigated. Because the spectral data reflect considerable substituent effects, the 13 C and 17 O chemical shifts and ν C O were compared with those of corresponding N, N -dimethylamides 2a − c, and the 15 N chemical shifts were compared with those of corresponding N -methyl-1,3-thiazolidine-2-thiones 3a − c . As the twist angle increased, the Δδ 13 C and Δδ 17 O increased, whereas, the Δδ 15 N decreased. Furthermore, the Δν C O increased with increasing τ and decreased with increasing Δδ 15 N. The relationship of the results to the classical amide resonance model and recently proposed model is also discussed.