ROTATION BARRIERS IN CRYSTALS FROM DIFFRACTION STUDIES - 2,2',4,4',6,6'-HEXA-TERT-BUTYLAZOBENZENE

X-ray crystal studies yield similar anisotropic displacement parameters (ADPs) for the two chemically equivalent para tert-butyl groups in 2,2'4,4'.6.6'-hexa-tert-butylazobenzene (C36H58N2) at 100 K. As the temperature rises, however, the ADPs increase more rapidly for one para group than for the other. Both para substituents have consistently larger libration amplitudes than those in the more crowded ortho positions. We have analyzed the ADPs with an internal-motion model which gives libration amplitudes for the tert-butyl groups at 100, 115, 128, 173, 200, 223 and 295 K. The results are consistent with rotational barriers for the six different groups of about 7 to more than 50 kJ mol-1; at higher temperatures one of the para groups is best described by a twofold-disorder model. The low apparent barrier for this group and the disappearance of the disorder on cooling both suggest that the disorder is dynamic. Potential-energy calculations are employed to propose models for the hindering potential and for further disorder at room temperature. These calculations support the relative magnitudes of the barriers and the degree of disorder inferred from the X-ray diffraction data.