A nitrogen-15 NMR study of hydrogen bonding in 1-alkyl-4-imino-1,4-dihydro-3-quinolinecarboxylic acids and related compounds.

The title compounds contain groups (amine, amide, imine, carboxylic acid) that are capable of forming intramolecular hydrogen bonds involving a six-membered ring. In compounds where the two interacting functional groups are imine and carboxylic acid, the imine is protonated to give a zwitterion; where the two groups are imine and amide, the amide remains intact and forms a hydrogen bond to the imine nitrogen. The former is confirmed by the iminium 15N signal, which shows the coupling of 1J(15N,1H) -85 to -86.