Nuclear spin-spin interactions: Part IX. Relative signs of the geminal nitrogen-proton coupling constants for doubly bonded nitrogen

The relative signs of the two geminal H-C= 15 N coupling constants in 15 N-formaldoxime are determined and are shown to be opposite. Protonation studies conducted on other 15 N-aldoximes and on 15 N-quinoline indicate that these couplings have also opposite signs in the corresponding protonated forms. Solvent effects on these couplings are also described. They are of opposite signs for the two couplings. No clear indication about the absolute signs can be obtained. These results are discussed with respect to the effect of nitrogen lone pair orientation on coupling constants and they are related to structural and electronic properties of the systems.