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首页> 美国政府科技报告 >High-Resolution Solid-State exp 13 C NMR Study of the Templating Cations in Tetrapropylammonium Fluoride Silicalite: The Effect of Steric Interactions
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High-Resolution Solid-State exp 13 C NMR Study of the Templating Cations in Tetrapropylammonium Fluoride Silicalite: The Effect of Steric Interactions

机译：高分辨率固态exp 13 C NmR研究四氟丙基氟化铝硅酸盐中的模板阳离子：空间相互作用的影响

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We have examined the high resolution solid state exp 13 C NMR spectrum of this TPA fluoride silicalite at 50.3 MHz, using proton cross-polarization (CP), magic angle spinning (MAS), and heteronuclear dipolar decoupling techniques. The beta -methylene carbons fail to manifest the magnetic inequivalence expected as a consequence of hydrogen bonding with fluoride ion in only two of the four limbs. We deduce that the strength of the H-bond between F exp - and the beta -CH sub 2 hydrogens is too weak to affect significantly the chemical shift of the beta - exp 13 C. The methyl group exp 13 C resonance, on the other hand, is split into a doublet with chemical shift difference of approx. = 1.1 ppM. Such a splitting also occurs in regular silicalite, and has previously been reported for ZSM-5 zeolite preparations. It has been suggested that this splitting in ZSM-5 zeolite was due to slightly different chemical environments for the methyl groups in the two sets of channels, and the inference was made that the CPMAS exp 13 C NMR spectrum is sensitive to weak interactions between organic molecules and the zeolite framework. On the basis of the spectrum of TPA-F silicalite, we contend that the methyl group resonance splitting is not due to interactions with the silica framework. Rather it is due to the strong steric perturbation which arises from the close approach of the ends of the propyl limbs of adjacent TPA groups in the straight channels of the crystal structure. The results of the crystal structure determination yielded a value of the end C sub 9 -C sub 9 separation in this channel of 2.72 A, compared to a separation C sub 3 -C sub 6 of 3.09 A for the end carbons in the undulating channel. However, the imprecision of the atomic coordinates in this difficult structure determination made the authors uncertain as to whether or not the distances were significantly different. Our NMR results confirm that these distances are significantly different. (ERA citation 08:036466)

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