LOW-MELTING POINT ORGANIC CRYSTAL STRUCTURES BY HIGH RESOLUTION NEUTRON POWDER DIFFRACTION Richard M Ibberson, ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 OQX, UK
High resolution neutron powder diffraction is a particularly suitable technique for structure solution and refinement of low-melting point organic crystals. Whereas materials that are gas or liquid at ambient temperature pose practical difficulties both in growing single crystals and also in preparing small volume samples for X-ray studies that exhibit a good powder average, low-temperature sample preparation and data collection for neutron powder diffraction experiments is relatively straightforward. These practical advantages and high resolution instrumentation are a powerful combination that enable the precise and accurate determination of moderately complex molecular crystal structures to be performed.
This paper discusses recent examples of ab-initio crystal-structure determination and refinement of model methyl-group containing compounds, and the use of such structural data in the calculation of methyl-group rotation potentials for comparison with potentials derived from complementary measurements of tunnelling and librational transitions. Examples discussed will include:
(a) dimethylacetylene, C2(CH3)2; m.p. 241K
(b) dimethylsulphide, S(CH3)2; m.p. 175K
(c) methylfluoride, CH3F; m.p.131K, b.p. 195K.