Amide to carboxylic acid hydrogen bonding. The dipyrrinone receptor

Hydrogen bonding between carboxylic acid and amide groups was demonstrated for a series of amides called [n]-semirubins consisting of a dipyrrinone attached to the end of an n-carbon alkanoic acid. Such hydrogen bonding is more effective than the alternative amide to amide or acid to acid types for all of the semirubins studied: n = 1, 3-7, 10 and 20. As determined by ¹H NMR and vapour pressure osmometry, [n]-semirubins, where n = 5-20, are intramolecularly hydrogen bonded in CHCl₃ or CDCl₃; [4]-semirubin is intermolecularly hydrogen bonded as a dimer; [3]-semirubin is a tetramer; and [1]-semirubin is a dimer - all with carboxylic acid to amide hydrogen bonding. The dipyrrinone amide and adjacent pyrrole constitute an efficacious receptor for the carboxylic acid group.