Structure elucidation of a complex CO₂-based organic framework material by NMR crystallography

A three-dimensional structural model of a complex CO₂-based organic framework made from high molecular weight, self-assembled, flexible and multi-functional oligomeric constituents has been determined de novo by solid-state NMR including DNP-enhanced experiments. The complete assignment of the ¹⁵N, ¹³C and ¹H resonances was obtained from a series of two-dimensional through space and through bond correlation experiments. MM-QM calculations were used to generate different model structures for the material which were then evaluated by comparing multiple experimental and calculated NMR parameters. Both NMR and powder X-ray diffraction were evaluated as tools to determine the packing by crystal modelling, and at the level of structural modelling used here PXRD was found not to be a useful complement. The structure determined reveals a highly optimised H-bonding network that explains the unusual selectivity of the self-assembly process which generates the material. The NMR crystallography approach used here should be applicable for the structure determination of other complex solid materials.