o-Terphenyl: flips of the end rings in the crystal phase

We demonstrate by selective saturation deuteron NMR experiments on a crystal of selectively deuterated o-terphenyl (OTP) that both end rings I and II attached to the central ring undergo thermally activated flip motions. In crystals of OTP, the end rings I and II are not symmetry related, their dynamics can be different and, in fact, is different, although mutual steric hindrance of the rings suggests a strong correlation. We measured the rates k _I and k _II of the flips of both end rings I and II as a function of temperature. We find that, independent of the temperature, k _I exceeds k _II by roughly a factor of 100. This result excludes correlated flips of rings I and II in the sense that a flip of one ring necessarily entails a flip of the other. The activation energies E_aI and E_aII of the two flip processes turn out to be equal (80.5 kJ mol^?1) within experimental errors. This is taken as a hint that the flips are, after all, related to each other. A mechanism is proposed of how this is possible under the constraint k _I » k _II.