Molecules in Motion: a Study in the Synergy of X-ray Diffraction and Solid State NMR

The Fourier transform of the X-ray diffraction pattern of a crystal is the electron density of the crystal averaged over space and time. In the determination of the structure of a crystal this electron density is modeled by a set of spheres of electron density representing the atoms together with a »smearing function«, the atomic displacement parameters, representing the motion (translation and libration) of those atoms. Studies of the deuterium and 13C NMR of these solids show that these XRD parameters give a less than full picture of the motional behavior in crystals. In particular XRD parameters do not give information about infrequent activated processes in the crystal or about some motional behavior related to phase changes. A number of examples of the synergy of XRD and solid state NMR will be given. These examples include phenyl ring flips in phenoxypenicillin salts, whole molecule rotations in ansatitanocene halides, guest molecule tumbling, spins and flips in thiourea clathrates cycloheptane and cyclooctane guests, cation flips and spins in thiourea pyridinium halides, wobbles, spins and phase changes in the deoxycholic acid ferrocene clathrate and motion, phase changes and chiral selectivity in the deoxycholic acid clathrates with R and S camphor, with reference to how solid state NMR gives information about the mechanism of the phase change.