Investigation of the structure of an insoluble pigment by means of nuclear magnetic resonance

Abstract

The structure of the insoluble pigment, CIPR 57 : 1, the monohydrated calcium salt of 1-(2-sulpho-4-methylphenylazo)-2-hydroxynaphthalen-3-carboxylic acid, has been investigated by high-resolution n.m.r. techniques appropriate to solids. The ¹³C spectra of the pigment and a range of structurally related compounds support the view that these compounds exit as the keto-hydrazo tautomers in the solid state. The selective enrichment of the α- and β-nitrogens with ¹⁵N and the observation of the ¹⁵N high-resolution n.m.r. spectra confirm directly the existence of the keto-hydrazo structure. ¹¹³Cd N.m.r. spectra of pigments obtained by the use of Cd²⁺ in place of Ca²⁺ are used in conjunction with the other n.m.r. results to infer a polymeric sheet-like structure for the pigments in which the divalent ion is co-ordinated by carboxy and sulphonate groups from different pigment molecules.