Conformations and conformational processes of pelargolactam

Pelargolactam (1-azacyclodecan-2-one) was studied by 1H and 13C NMR spectroscopy in solution between 25 and −145°C. Equilibrium solutions of pelargolactam at 25°C consist mainly of the trans isomer together with 5–10% of the cis isomer. Solid pelargolactam consists of a mixture of cis and trans isomers, the cis form dissolving more rapidly than the trans form at −95°C. At temperatures above 90°C the 13C NMR resonances for the cis and trans amides coalesce to one averaged set. A conformational process with a coalescence temperature of −30 to −50°C and a free energy of activation of ΔG≠ = 46 kJ mol−1 observed in the 1H spectrum of the trans isomer is interpreted as geminal exchange in the methylene groups. A lower barrier process was also observed, with a coalescence temperature of ca −115°C and ΔG≠ = 32 kJ mol−1. Only the latter process was observed in the 13C spectrum, and below −120°C all 13C lines split in two in the proportion 1:2. This process is interpreted as the interconversion between two trans conformers. Four conformational candidates were considered and their relative energies calculated by molecular mechanics.