Effects of C(O)-N Bond Rotation on the 13 C, 15 N, and 17 O NMR Chemical Shifts, and Infrared Carbonyl Absorption in a Series of Twisted Amides Shinji Yamada † Department of Materials Science, Faculty of Science, Kanagawa University, Hiratsuka, Kanagawa 259-12, Japan Received September 14, 1995 X 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 CdO 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 CdO 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 CdO 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. Introduction The structure and spectroscopic properties of twisted amides have recently received considerable attention not only in organic chemistry 1-3 but also in biochemistry. 4 The influence of the rotation about the C(O)-N bond on IR, 5 UV, 6 and 1 H, 7 13 C, 8 , 15 N 9 NMR and ESCA 10 spectro- scopic data has been studied, and large differences between those in planar and twisted amides have been observed. The differences are attributable to the reduc- ing of amide resonance 11 throughout the C(O)-N bond rotation. However, the quantitative and systematic relationships between the twist angles and the spectro- scopic data have not always been explored. In particular, there has been no investigation of the 17 O NMR chemical shift dependence of the C(O)-N twist angles, although it has been recognized that the 17 O NMR chemical shift is more sensitive to structural variation than the shifts of 13 C and 15 N. 12,13 The resonance model in amides 11 has been generally accepted to interpret their chemical and physical proper- ties; however, it was challenged 14 on the basis of com- parison of the calculated C, N, O electron populations between planar I and twisted IV amides calculated with Bader's method. 15 In the calculation, the electron popu- lation of N in planar form I is larger than that in twisted form IV, whereas the electron population of C in I is less than that in IV and that of O in I is a little larger than that in IV. 16 These results do not fit the classical resonance model (eq 1); therefore, Wiberg and Rablen proposed a new resonance model (eq 2) 17 instead of the classical one. The new model describes that the domi- nant canonical contributor is highly polarized III, and consequently the nitrogen lone pair can donate electrons to the carbon without needing to further displace much charge density from the carbon to the oxygen. In connection with the calculation and the new model, a number of theoretical studies have been extensively undertaken; 18-26 however, there have been only a few experimental approaches. 10b,25 † Present address: Department of Chemistry, Faculty of Science, Ochanomizu University, Bunkyo-ku, Tokyo 112, Japan. X Abstract published in Advance ACS Abstracts, January 15, 1996. (1) For a review, see: Greenberg, A. In Molecular Structure and Energetics; Liebman, J. F., Greenberg, A., Eds.; VCH: New York, 1988; Vol. 7, pp 139-178. (2) (a) Winkler, F. K.; Dunitz, J. D. J. Mol. Biol. 1971, 59, 169. (b) Wang, A. H.-J.; Paul, I. C. J. Chem. Soc., Chem. Commun. 1972, 43. (c) Blackburn, G. M.; Plackett, J. D. J. Chem. Soc. Perkin Trans. 2 1972, 1366. 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