Spectroscopic studies on ranitidine—its structure and the influence of temperature and pH

Abstract

Spectroscopic studies on ranitidine (1) show that the nitroethene dialkylamine group in both the base and hydrochloride exists virtually exclusively in the enediamine tautomeric form and a low barrier to rotation about the carbon–carbon double bond can be demonstrated. The energy barrier to this rotation is partly influenced by intramolecular hydrogen bonding between the nitro and amino groups. In strongly acid solution, C-protonation with charge delocalisation occurs on the enediamine with consequent loss of conjugation. Two polymorphic forms of the hydrochloride have been identified by i.r. spectroscopy.