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. Author manuscript; available in PMC: 2020 Aug 27.
Published in final edited form as: Chemistry. 2019 Aug 5;25(48):11356–11364. doi: 10.1002/chem.201902382

Figure 5.

Figure 5.

Natural bond orbital (NBO) analysis[25] of geometry-optimized structures of Piv-flp-OMe and For-flp-OMe, with a trans amide bond, endo ring pucker, and PPII/β conformation. By this analysis, the conformation in the pivaloyl derivative is stabilized by 0.52 kcal mol–1 by the n→π* interaction, while in the formyl derivative the conformation is only stabilized by 0.12 kcal mol–1. These energy differences are reflected in the differing extents of orbital overlap and the divergent Oi…Ci+1 intercarbonyl distances (Piv: d = 2.92 Å; For: d = 3.17 Å). Additional computational analysis as a function of acyl capping group and conformation is included in the Supporting Information.