Abstract
AbstractThe equilibrium mixture of the cis and trans isomers of caprylolactam (1‐azacyclononan‐2‐one), was studied by 1H and 13C NMR spectroscopy at various temperatures. The exchange between corresponding carbons in the two isomers in solution was studied by 2D NOE. Assignments were made for all 13C lines of the cis and trans isomers in CDCl3, CD3OD and C5D5N by 2D NMR. The conformations and conformational processes were studied by low‐temperature NMR. The trans isomer was shown to exist in at least two different conformations in solution. In addition to the high barrier interconversion between the cis and trans isomers, two different conformational processes were found for the trans isomer in the temperature range −140 to +60°C and one for the cis isomer. The conformational processes in solution are identified as (1) geminal 1H exchange in the cis and the trans isomers: for the cis isomer with a coalescence temperature of ca. −70°C a free energy of activation of 40 kJ mol−1 and for the trans isomer with a coalescence temperature of ca. +40°C a free energy of activation of 61 kJ mol−1; and (2) a low‐temperature process interconverting two trans conformations with a coalescence temperature of ca. −120°C and a free energy of activation of 29 kJ mol−1, observed in both the 1H and the 13C spectra.
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