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

Abstract

We demonstrate by selective saturation deuteron NMR experiments on a deuterated crystal of bis-(4-chlorophenyl)-sulphone (BCPS, the `butterfly molecule’) that, in the crystalline state, the two phenyl rings of this molecule are flipping through 180° . This process is thermally activated, the kinetic parameters are ΔE = (71+-5) kJmol- 1 and k0 = 1015.56+-0.8 s- 1. Our spectra also indicate a slow magnetization transfer, on a time scale of 50 s at room temperature, between deuterons located on different wings of the molecule. Flips of the molecule as a whole about a crystal and molecular twofold axis would account for this magnetization transfer. An alternative explanation is spin diffusion. To discriminate between these two possibilities we develop and apply a new criterion. It exploits the fact that the sign of quadrupolar order transferred between two I = 1 spin ensembles with quadrupole splittings of opposite sign depends on whether the quadrupolar order transfer occurs via chemical exchange or via spin diffusion. This criterion thus allows one, in a single experiment, to discriminate between chemical exchange and spin diffusion in a yes/no fashion. We therefore call it binary quadrupolar order criterion. Its application to BCPS yields 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 time scale of 50 s at room temperature