Analysis of a Solid-State Conformational Rearrangement Using ¹⁵N NMR and X-ray Crystallography

A solid azobenzene dyestuff has been studied by a combination of single-crystal X-ray diffraction and 15 N CP/MAS NMR. The X-ray work establishes that the crystal structure of one polymorph is disordered. The 15 N NMR proves that this is a temporal, rather than a spatial, effect. Three 15 N-enriched isotopomers (together with a second polymorph) were examined by NMR, and assignments established by rotational resonance. The shielding tensor components were determined, and the angles of change between their orientations were established by a two-dimensional exchange experiment, the theory of which is discussed. Exchange rates were measured both by selective polarization inversion and by band shape analysis. Thermodynamic parameters for the barrier to exchange are presented. It is demonstrated, by the combination of X-ray and NMR work, that the process is essentially a crankshaft motion of the azo linkage.