Nuclear spin-spin interactions: Part X Absolute signs of geminal and vicinal nitrogen-proton coupling constants Stereochemistry and medium effects on N, H couplings

The absolute signs of the geminal and vicinal N, H coupling constants in ¹⁵N-oximes and ¹⁵N-quinoline have been determined by ¹⁵N-¹H and ¹H-¹H spin-tickling experiments. The reduced geminal N, H couplings are positive (large coupling) or negative (small coupling); the reduced vicinal N, H couplings are positive. The same signs hold for the protonated forms of the various compounds. The magnitude of the vicinal N, H couplings depends on the oxime configuration. The effect of aprotic solvent dielectric constant on the geminal N, H couplings is opposite to the effect observed for geminal H, H couplings. Protonation (or quaternization) of the nitrogen atom has remarkable effects on N, H couplings; protonation leads to an increase/decrease/increase of the bonded/geminal/vicinal reduced N, H couplings respectively. A similar alternation of the protonation effects is observed for P, H coupling constants. It is suggested that these effects might be useful for determining the signs of N, H and P, H coupling constants. As a consequence of the present results, the geminal H, H coupling in formaldoxime is positive. © 1969 Taylor and Francis Group, LLC.