Solid-state NMR, up to Volume 32, Issue 4, Pages 99-136 (December 2007)

Solid-state NMR, up to Volume 32, Issue 4, Pages 99-136 (December 2007)

Solid State Nuclear Magnetic Resonance
Volume 32, Issue 4, December 2007, Pages 99-108

doi:10.1016/j.ssnmr.2007.09.003
A practical comparison of MQMAS techniques

Johanna Kanellopoulos, Dieter Freude, and Arno Kentgens
Received 30 August 2007. Available online 5 October 2007.

Abstract

A systematic experimental evaluation of several approaches to multiple-quantum MAS NMR was performed for spin-5/2 nuclei using 27Al NMR of the aluminosilicate andalusite and the porous aluminum phosphate AlPO4-14 as model. Experiments were conducted in the fields of 9.4 and 17.6 T using magic-angle spinning frequencies up to 30 kHz and rf-field strengths of 250 and 120 kHz. Numerical SIMPSON optimizations of the NMR parameters were performed alongside the experimental evaluations. Both theory and experiment show that the optimization is most critical for the species in the sample that have the largest quadrupolar coupling constant. For 5QMAS experiments it could be confirmed that the highest available rf-field strength and rotation frequency are favorable for the efficiency of the experiments. For 3QMAS experiments of sites with moderate quadrupolar coupling constants optimum results were obtained at less stringent conditions. The comparison of a FAM II-modification and DFS gave the expected improvement by a factor of about two with respect to a rectangular pulse. No significant difference between these techniques concerning the signal-to-noise ratios was obtained. An actual improvement of the isotropic resolution by a factor of about two was obtained going from 3QMAS to 5QMAS. In addition the resolution of the spectra increases by a factor of about two going from 9.4 to 17.6 T.

Keywords: Multiple-quantum; MQMAS NMR; Quadrupolar nuclei; 27Al

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Solid State Nuclear Magnetic Resonance
Volume 32, Issue 4, December 2007, Pages 109-117

doi:10.1016/j.ssnmr.2007.10.002
A multiple-field 23Na NMR study of sodium species in porous carbons

Jair C.C. Freitas, Miguel A. Schettino Jr., Francisco G. Emmerich, Alan Wong and Mark E. Smith

Abstract

The sodium environments in porous carbon materials prepared from NaOH activation of a char were investigated by means of multiple-field solid-state 23Na NMR measurements, carried out at magnetic fields of 4.7, 8.45 and 14.1 T, with single-pulse excitation and magic angle spinning (MAS). The recorded spectra showed a relatively featureless resonance with linewidth and peak shift strongly dependent on the magnetic field strength and on the hydration level of the samples. The existence of second-order quadrupolar effects was inferred, although the structural disorder and the mobile character associated with the Na environment precluded the direct observation of typical quadrupolar features in the MAS NMR spectra. The analysis of the spectra collected at multiple magnetic fields yielded the values of -2.8 ppm for the isotropic chemical shift and 1.8 MHz for the quadrupole coupling constant, which were interpreted as due to Na+ ions bonded to oxygenated groups at the edges of the graphene planes within the carbon pore network.

Keywords: Activated carbon; Nuclear magnetic resonance; Chemical structure; 23Na NMR; Multiple magnetic fields

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Solid State Nuclear Magnetic Resonance
Volume 32, Issue 3, November 2007, Pages 78-88

doi:10.1016/j.ssnmr.2007.09.001

New inserts and low temperature—Two strategies to overcome the bottleneck in MAS NMR on wet gels

R. König, G.and E. Kemnitz
Received 7 June 2007; revised 1 August 2007. Available online 25 September 2007.

Abstract

MAS NMR experiments on wet and dry aluminium alkoxide fluoride gels are presented here for the first time. For the MAS studies on wet jelly like gels special inserts were developed which fit perfectly a 4 mm rotor and allow rotation frequencies up to 12 kHz. Six different insert materials were tested concerning their suitability including different organic polymers ((poly(methyl methacrylate) (PMMA), poly(ether ether ketone) (PEEK), poly(vinylchloride) (PVC) and poly(tetrafluorethylene) (PTFE)), glass as well as pure quartz (SiO2). Alternatively, low-temperature MAS NMR experiments taken at 150 K with the wet gel frozen directly in the rotor led to comparable results.27Al, 19F, 13C, 1H were used as nuclear spin probes to identify local structures in the wet as well as in the dry xerogels. Both wet and dry gels consist of a network structure of AlF6-xOiPrx species (x: 0–3). The main structural units of the dried gel are already preformed in the jelly like gel and change only little at aging and drying processes.

Keywords: MAS NMR; Wet fluoride gels; Inserts; Low temperature

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Solid State Nuclear Magnetic Resonance
Volume 32, Issue 3, November 2007, Pages 89-98

doi:10.1016/j.ssnmr.2007.09.002
Site discrimination in the crystalline borophosphate Na5B2P3O13 using advanced solid-state NMR techniques

Wenzel Strojek, Constanze Miriam Fehse, Hellmut Eckert, Bastian Ewald and Rüdiger Kniep
Received 20 August 2007. Available online 28 September 2007.

Abstract

A comprehensive solid-state NMR investigation on crystalline Na5B2P3O13 is presented. Triple-quantum magic angle spinning (TQMAS) and rotational echo double resonance (REDOR) studies are used for accurate determinations of the 11B, 23Na and 31P interaction parameters. Based on these results and complementary quantum mechnical calculations, plausible site assignments can be derived. Generally, the results show that detailed, quantitative information about structures in borophosphate compunds can be obtained by investigating both the local site environments characterized by chemical shift and quadrupolar interaction parameters and the correlated dipolar interactions to atoms in the second coordination sphere.

Keywords: Dipolar coupling; Rotational echo double resonance; Borophosphates

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Solid State Nuclear Magnetic Resonance
Volume 32, Issue 2, October 2007, Pages 34-43

doi:10.1016/j.ssnmr.2007.07.001
93Nb and 17O NMR chemical shifts of niobiophosphate compounds

A. Flambard, L. Montagne, L. Delevoye and S. Steuernagel
Received 13 April 2007. Available online 10 July 2007.

Abstract

Niobiophosphate compounds with a large range of niobium and oxygen environments were studied with 93Nb and 17O solid-state NMR. 93Nb isotropic chemical shift of pure niobate Nb(ONb)6, pure phosphate Nb(OP)6 and mixed phosphate–niobate Nb(OP)x(ONb)(6-x) (1less-than-or-equals, slantxless-than-or-equals, slant5) sites were measured at a high magnetic field (18.8 T). 93Nb chemical shifts were found to be sensitive to the variation of local charge on Nb, but not to the local bond geometry (i.e. crystallographic site and edge or corner connectivity). A systematic shift to high field of the 93Nb chemical shift is measured when x increases. Then, 17O NMR spectra of a series of enriched samples provided the chemical shift and quadrupolar parameters for 4 types of oxygen environment (P–O–P, P–O–Na, P–O–Nb and Nb–O–Nb). Finally, Nb–O–Nb sites were characterized by a large 17O chemical shift anisotropy.

Keywords: 93Nb NMR; 17O NMR; Phosphate; Niobate; Niobiophosphate