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Issue 11, 2009
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Probing local structures of siliceous zeolite frameworks by solid-state NMR and first-principles calculations of 29Si–O–29Si scalar couplings

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Abstract

Subtle structural details of siliceous zeolites are probed by using two-bond scalar (J) coupling constants to characterize covalently bonded 29Si–O–29Si site pairs and local framework order. Solid-state two-dimensional (2D) 29Si{29Si} NMR measurements and first-principles calculations of 2J(29Si–O–29Si) 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(29Si–O–29Si) 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(29Si–O–29Si) couplings based on less-certain powder X-ray diffraction analyses deviate significantly from experimental values, while a refined structure based on 29Si chemical-shift-tensor analyses shows substantially improved agreement. 29SiJ-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.

Graphical abstract: Probing local structures of siliceous zeolite frameworks by solid-state NMR and first-principles calculations of 29Si–O–29Si scalar couplings

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The article was received on 03 Sep 2008, accepted on 08 Dec 2008 and first published on 28 Jan 2009


Article type: Paper
DOI: 10.1039/B815361B
Citation: Phys. Chem. Chem. Phys., 2009,11, 1825-1837
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    Probing local structures of siliceous zeolite frameworks by solid-state NMR and first-principles calculations of 29Si–O–29Si scalar couplings

    S. Cadars, D. H. Brouwer and B. F. Chmelka, Phys. Chem. Chem. Phys., 2009, 11, 1825
    DOI: 10.1039/B815361B

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