Probing local structures of siliceous zeolite frameworks by solid-state NMR and first-principles calculations of ~(29)Si-O-~(29)Si scalar couplings

摘要

Subtle structural details of siliceous zeolites are probed by using two-bond scalar (J) coupling constants to characterize covalently bonded ~(29)Si-O-~(29)Si site pairs and local framework order. Solid-state two-dimensional (2D) ~(29)Si{~(29)Si} NMR measurements and first-principles calculations of ~2J(~(29)Si-O-~(29)Si) couplings shed insights on both the local structures of siliceous zeolites Sigma-2 and ZSM-12, as well as the sensitivity of J couplings for detailed characterization analyses. DFT calculations on a model linear silicate dimer show that ~2J(Si-O-Si) couplings have complicated multiple angular dependencies that make semi-empirical treatments impractical, but which are amenable to cluster approaches for accurate J-coupling calculations in zeolites. DFT calculations of ~2J(~(29)Si-O-~(29)Si) couplings of the siliceous zeolite Sigma-2, whose framework structure is known to high accuracy from single-crystal X-ray diffraction studies, yield excellent agreement between calculated and experimentally measured ~2J(Si-O-Si) couplings. For the siliceous zeolite ZSM-12, calculated ~2J(~(29)Si-O-~(29)Si) couplings based on less-certain powder X-ray diffraction analyses deviate significantly from experimental values, while a refined structure based on ~(29)Si chemical-shift-tensor analyses shows substantially improved agreement. ~(29)Si J-coupling interactions can be used as sensitive probes of local structures of zeolitic frameworks and offer new opportunities for refining and solving complicated structures, in combination with complementary scattering, modeling, and other nuclear spin interactions.