Phenyl ring flips in crystals of bis-(4-chlorophenyl)-sulphone and a binary criterion for distinguishing chemical exchange from spin diffusion in deuteron exchange nuclear magnetic resonance

We demonstrate by selective saturation deuteron NMR experiments on a deuter ated crystal of bis-(4-chlorophenyl)-sulphone (BCPS, the 'butterfly molecul e') that, in the crystalline state, the two phenyl rings of this molecule a re flipping through 180 degrees. This process is thermally activated, the k inetic parameters are Delta E = (71 +/- 5) kJ mol(-1) and k(0) = 10(15.5+/- 0.8) s(-1). Our spectra also indicate a slow magnetization transfer, on a t ime scale of 50 s at room temperature, between deuterons located on differe nt wings of the molecule. Flips of the molecule as a whole about a crystal and molecular twofold axis would account for this magnetization transfer. A n alternative explanation is spin diffusion. To discriminate between these two possibilities we develop and apply a new criterion. It exploits the fac t that the sign of quadrupolar order transferred between two I = 1 spin ens embles with quadrupole splittings of opposite sign depends on whether the q uadrupolar order transfer occurs via chemical exchange or via spin diffusio n. This criterion thus allows one, in a single experiment, to discriminate between chemical exchange and spin diffusion in a yes/no fashion. We theref ore call it binary quadrupolar order criterion. Its application to BCPS yie lds the result that the observed slow magnetization transfer is due to spin diffusion and that the BCPS molecules are not flipping as a whole on a tim e scale of 50 s at room temperature.