Conformations and Conformational Processes of Undecylolactam

Abstract

The conformation of undecylolactam (1-azacyclododecan-2-one) was investigated by 1H and 13C NMR spectroscopy in solutions between 25 and -140°C. Only the trans isomer is present. At low temperature, the solution was shown to contain two species, different with respect to the ring conformation. Two conformational processes were observed. The higher energy process was visible in the 1H but not in the 13C spectra, with a coalescence temperature of ca. -75°C and a free energy of activation of ΔG = 37 kJ mol−1. This process was identified as geminal exchange of methylene hydrogens. A lower energy process was visible in both the 1H and 13C NMR spectra with coalescence temperatures of ca. -87°C and a free energy of activation of ΔG = 35 kJ mol−1. Below -90°C all 13C resonances are split into a double set of lines in the proportion of about 1:2. Changes also take place in the 1H spectrum, but the resonances are broad and overlapping. This conformational process was identified as interconversion between the two trans forms of undecylolactam. Conformational candidates for the two trans conformers in solution are proposed.