MOLECULAR-MOTION AND CONFORMATIONAL INTERCONVERSION OF AZOBENZENES INCRYSTALS 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 ap parent shrinkage of the N=N bond and its temperature dependence were o bserved and are interpreted in terms of an artifact caused by the tors ional vibration of the N-Ph bonds in crystals. In the crystal structur es 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 diff erence in energy of the two conformers as free molecules. The true len gth of the N=N bond in azobenzenes was estimated to be 1.26-1.27 Angst rom.