Conformational structure of N-methyl-8-octanelactam and poly(N-methyl-8-octanelactam) as studied by NMR, infrared and Raman spectroscopy and by theoretical calculations

It was proved by H-1 and C-13 NMR spectra that in N-methyl-8-octanelactam in CDCl3 solution, the amide bond assumes only the cis form. Minor inversion about the plane of the amide bond occurs with an activation enthalpy Delta H# = 44.7 kJ/mol. Molecular mechanics and quantum mechanical calculations yielded four favoured conformational structures for the isolated molecule. By vibrational spectra, one of two distinguishable structures strongly predominates. From H-1 NMR spectra, this predominant conformation was identified with one of the calculated stable forms. By C-13 NMR evidence, poly(N-methyl-8-octanelactam) in tetrachloroethane solution contains equal populations of the cis and trans amide forms; the structure of this polymer in the rubbery solid state seems to be similar to its structure in solution.