Conformational analysis by magic-angle spinning NMR spectroscopy for a series of polymorphs of a disperse azobenzene dyestuff

Abstract

Polycrystalline powder samples of an azobenzene disperse dyestuff have been investigated using solid-state and solution-state ¹³C and ¹⁵N NMR spectroscopy in an attempt to understand the effect of polymorphism. Assignment of the ¹³C spectra indicates that it is possible to form an intramolecular hydrogen bond resulting in two conformations co-existing in the crystal structure of two of the polymorphs, as well as in solution, and differing by internal rotation of a side-chain acetamido group. This is clearly evident in the solid-state spectra at ambient temperature and in solution at sufficiently reduced temperatures. Each polymorph contains varying amounts of each conformer as a result of different crystal interactions, and conformer interchange can be shown to occur even in the solid state. This remarkable finding is accounted for by the open nature of the crystal structures, resulting from the steric effects of substituents, as attested by packing coefficients and molecular modelling. The conformational variations are confirmed by ¹⁵N spectra, and isotopic labelling by ¹⁵N allows tensor analysis to be applied which shows that the azo nitrogen's principal shielding components are significantly influenced by the formation of the hydrogen bond in one of the conformations.