Structural phase transition in polyphenyls. X. Potential barrier heights in crystalline polyphenyls and in gaseous biphenyl determined uniquely from diffraction data

The analysis from X-ray or neutron diffraction data, of the librational motion around the long molecular axis of the p-terphenyl central ring permits resolution of its disorder: the phenyl rotation angle on either side of the average molecular plane is ¢ = _+ 13.3 c. This disorder is associated with a double-well potential between two twisted conformations, and the overall libration on each site, at the bottoms of the double well, has the mean-square amplitude (02) = 52.5 deg 2 at room temperature. The analysis also permits separation in (02) of the mean-square amplitude of the torsional g mode (0~) = 35 deg 2 from that of the external mode (0~) = 17.5deg 2. Thus it becomes possible to scale the parameters of a simple model describing inter- and intramolecular interactions in the whole family of polyphenyls. It is shown that the intramolecular potential between two adjacent phenyl rings cannot be described by a simple sinusoidal function but exhibits a steeper gradient near the planar conformation. This double potential well model accounts for disorder and libration in crystalline p-terphenyl and p-quaterphenyl. It gives