1H and 13C NMR spectra, 13C NMR relaxation times and molecular modelling studies of the narcotic agonist–antagonist α‐( − )‐N‐propynylnormetazocine

The analysis of high-frequency H-1 and C-13 NMR spectra and both H-1-H-1 homonuclear and H-1-C-13 heteronuclear chemical shift correlation experiments indicate that N-propynylnormetazocine (NPMH+) is configurationally heterogeneous in solution, with the N-equatorial isomer being more populated than the N-axial isomer. The experimental proton-proton coupling constants measured for both diastereoisomers were in agreement with the calculated values for a chair conformation of the piperidine ring. AM1 and MM2 theoretical calculations both indicated the chair conformation as that preferred by the piperidine ring, and the N-equatorial isomer as being energetically favoured with respect to the N-axial form. The best fit of the C-13 T1 relaxation time data and the related motional analysis of NPMH+ was carried out using the anisotropic model of reorientation with superimposed internal motion. The results showed an increase of the rotational anisotropy of the molecule on passing from the gas phase to the solution state.