Rapid solid-state 29Si NMR chemical shift refinements of pure silica zeolite frameworks with an improved structure-to-shift correlation

The chemical shifts measured in solid-state 29Si NMR spectroscopy of pure silica zeolites are highly sensitive to the local geometry surrounding the corresponding silicon atoms and therefore hold promise for playing an important role in the structure determination of zeolite frameworks. Through an analysis of the large set of experimental 29Si NMR spectra of pure silica zeolites available in the Database of Zeolite Structures, including the recently added extra-large pore zeolite ZEO-5 which has several uniquely strained silicon geometries, a new structure-to-shift correlation equation was developed that allows for rapid and accurate calculations of isotropic 29Si chemical shifts from Si-O distances and Si-O-Si bond angles. This structure-to-shift relationship is supported by a comprehensive set of quantum chemical calculations of 29Si chemical shieldings performed on 200 different zeolite frameworks. With this ability to rapidly and accurately calculate 29Si chemical shifts, it was possible to carry out rapid structure refinements of zeolite frameworks in which small adjustments to the silicon and oxygen atomic coordinates were made until the differences between calculated and experimental chemical shifts were minimized. Chemical shift structure refinements for 51 zeolite frameworks were carried out and presented here. Based on the set of zeolites for which single-crystal X-ray diffraction structures are available, it is estimated that this chemical shift refinement strategy yields structures with silicon and oxygen atomic coordinates that differ by an average of only about 0.06 Å compared to the single crystal structures.