13C DNMR studies of codeine and sinomenine (derivatives of N-Me morphinan) indicated that N-inversion-C-N rotation (NIR) is unusually fast for these substituted piperidines when compared with other N-Me piperidines. Since only broadening, but no signal splitting, was reached at low temperatures and the difference of chemical shifts (Δδ) for individual conformers with the equatorially and axially oriented N-Me substituent was unavailable, the limits of the NIR barrier for these amines were determined by line shape analysis using Δδ values provided by ab initio calculations. On the basis of the comparison of experimentally determined 13C NMR chemical shifts for tropane conformers with the ones calculated at different theory levels for this N-Me piperidine, the B3LYP/6-31G(p)/GIAO level was chosen as a sufficiently accurate method for calculations of Δδ. By this new "semiempirical" procedure of line shape analysis the NIR barrier for the studied morphinans lies within a 25-27 kJ mol-1 (6.0-6.5 kcal mol-1) range. A low NIR barrier for morphine alkaloids is supposed to be an important factor in the activation of morphine receptor.