Molecular motion and conformational interconversion of azobenzenes in crystals as studied by X-ray diffraction

Crystal structures of (E)-azobenzene (1), (E)-2,2'-dimethylazobenzene (2), (E)-3,3'-dimethylazobenzene (3) and (E)-4,4'-dimethylazobenzene (4) were determined by X-ray diffraction at various temperatures. An apparent shrinkage of the N=N bond and its temperature dependence were observed and are interpreted in terms of an artifact caused by the torsional vibration of the N-Ph bonds in crystals. In the crystal structures of (1), (3) and (4) the dynamic disorder was observed. The disorder is accounted for by the torsional vibration whose amplitude is large enough to give rise to the conformational interconversion. No disorder was observed for a crystal of (2). This is ascribed to the large difference in energy of the two conformers as free molecules. The true length of the N=N bond in azobenzenes was estimated to be 1.26-1.27 Angstrom.