ANALYSIS OF A SOLID-STATE CONFORMATIONAL REARRANGEMENT USING N-15 NMRAND X-RAY CRYSTALLOGRAPHY

A solid azobenzene dyestuff has been studied by a combination of singl e-crystal X-ray diffraction and N-15 CP/MAS NMR. The X-ray work establ ishes that the crystal structure of one polymorph is disordered. The N -15 NMR proves that this is a temporal, rather than a spatial, effect. Three N-15-enriched isotopomers (together with a second polymorph) we re examined by NMR, and assignments established by rotational resonanc e. The shielding tensor components were determined, and the angles of change between their orientations were established by a two-dimensiona l exchange experiment, the theory of which is discussed. Exchange rate s were measured both by selective polarization inversion and by band s hape analysis. Thermodynamic parameters for the barrier to exchange ar e presented. It is demonstrated, by the combination of X-ray and NMR w ork, that the process is essentially a crankshaft motion of the azo li nkage.