The lactams C=O-NH[CH2]N-2 with a ring size N = 8 or smaller have a cis peptide conformation. For N = 9, caprylolactam, the crystalline form is transoid with a twist of the peptide bond by 30 degrees.1 Both cis and trans forms of caprylolactam occur in solution as established by IR2 and NMR3. We have shown with standard computational methods that there are cis and trans conformation of caprylolactam with very similar energy. The trans form is very similar to that four in the crystal. The IR study of N-H stretching shows two additional conformations, one close to cis and one close to trans. A low temperature NMR study3 shows one additional conformation of the cis type in both 1H and 13C NMR. We computationally examine possible explanations for these observations including (a) alternative conformation(s) as postulated, (b) dimerization of the cis form and (c) water in the CCl4. A preliminary investigation finds no conformations near the cis form that are metastable. 1 J. D. Dunitz, F. K. Winkler Acta Cryst B31 251-263 (1975) 2 H. E. Hallam, C. M. Jones J. Mol. Struct 1, 413-423 (1968) 3. G. Borgen, Magn. Res Chem. 29, 805-812 (1991)

Other conformations of caprylolactam: A computational study

251st National Meeting (2016)

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