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A solid azobenzene dyestuff has been studied by a combination of single-crystal X-ray diffraction and 15N CP/MAS NMR. The X-ray work establishes that the crystal structure of one polymorph is disordered. The 15N NMR proves that this is a temporal, rather than a spatial, effect. Three 15N-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.

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Proton and 13C solution‐state spectra were obtained for a substituted triazine and assignments made for the signals using different solvents. Site‐exchange effects were observed in 1H spectra at different temperatures below ambient for a CD3OD solution. The bandshapes were fitted for three‐site exchange and the relevant rate constants extracted. The exchange was attributed to internal rotation of the substituents about the triazine ring. Activation parameters were calculated: for exchange between one pair of sites Δ H‡ = 71 ± 4 kJ mol−1 and Δ S‡ = 33 ± 15 J mol−1 K−1, and for a second pair Δ H‡ = 72 ± 5 kJ mol−1 and Δ S‡ = 39 ± 26 J mol−1 K−1, whereas direct exchange between the remaining pair of sites is negligible. Exchange involving a fourth rotamer affects the spectra at lower temperatures, which assists in a partial assignment of the observed peaks to the rotamers. These results are discussed in relation to molecular modelling information and literature values. Solution‐state and solid‐state 13C spectra were also recorded but the latter are broad, making detailed assignment difficult. Copyright © 2000 John Wiley & Sons, Ltd.