| Abstract | Combining quantum-chemical calculations and ultrahigh-field NMR measurements of 29Si chemical shielding (CS) tensors has provided a powerful approach for probing the fine details of zeolite crystal structures. In previous work, the quantum-chemical calculations have been performed on ‘molecular fragments’ extracted from the zeolite crystal structure using Hartree–Fock methods (as implemented inGaussian). Using recently acquired ultrahigh-field 29SiNMRdata for the pure silica zeolite ITQ-4,we report the results of calculations using recently developed quantum-chemical calculationmethods for periodic crystalline solids (as implemented in CAmbridge Serial Total Energy Package (CASTEP) and compare these calculations to those calculated with Gaussian. Furthermore, in the context of NMR crystallography of zeolites,we report the completion of the NMR crystallography of the zeolite ITQ-4,whichwaspreviously solved fromNMR data.We compare three options for the ‘refinement’ of zeolite crystal structures from ‘NMR-solved’ structures: (i) a simple target-distance based geometry optimization, (ii) refinement of atomic coordinates inwhich the differences between experimental and calculated 29Si CS tensors are minimized, and (iii) refinement of atomic coordinates tominimize the total energyof the latticeusingCASTEPquantum-chemical calculations.All three refinement approaches give structures that are in remarkably good agreement with the single-crystal X -ray diffraction structure of ITQ-4. |
|---|
