Jørgen Skibsted

Satellite transitions in MAS NMR spectra of quadrupolar nuclei

Manifolds of spinning sidebands (ssbs) are observed for the satellite transitions of several half-integer quadrupolar nuclei (spin I12) using one-pulse magic-angle spinning (MAS) NMR experiments of crystalline powders. Numerical simulations of the variety of envelope patterns for the ssbs in such spectra show that the quadrupolar coupling parameters can be determined with high accuracy. The theory of the method is described and evaluated using average Hamiltonian theory to first and second order in the secular approximation. The influence of finite RF pulse excitation and quality factor of the MAS probes on the ssb intensities in the spectra is analyzed. Determination of quadrupole coupling constants and asymmetry parameters for the electric field gradient tensors from 17O, 23Na, and 27Al MAS NMR spectra of various solids illustrates the theory and high performance of the method.

A Series of Mixed‐Metal Borohydrides

Mix and match: A novel series of alkali-metal zinc borohydrides, LiZn 2(BH 4) 5 (see picture), NaZn 2(BH 4) 5, and NaZn(BH 4) 3, with fascinating structures are presented. An interpenetrated network structure, containing a [Zn 2(BH 4) 5] -. ion, is observed for the first time for a borohydride. A three-dimensional framework containing a polymeric [{Zn(BH 4) 3} n] n- ion is also identified.

Zeolites by confined space synthesis – characterization of the acid sites in nanosized ZSM-5 by ammonia desorption and 27Al/29Si-MAS NMR spectroscopy

ZSM-5 zeolites prepared by confined space synthesis, i.e., by crystallization inside a mesoporous matrix, are characterized by X-ray powder diffraction (XRPD), ammonia temperature-programmed desorption (TPD), 27Al and 29Si-MAS NMR spectroscopy, and transmission electron microscopy (TEM). It is shown by XRPD that confined space synthesis can be used for the preparation of pure and highly crystalline ZSM-5 with a uniform crystal size distribution governed by the pore size of the inert matrix. Ammonia desorption demonstrates that the small zeolite crystals have the same number of acid sites as large crystals when the Si/Al ratio is similar. The number of acid sites have been determined by ammonia TPD and converted into framework Si/Al ratios. These ratios are higher than the bulk Si/Al ratios but agree with those obtained from 27Al/29Si-MAS NMR. Furthermore, the 27Al MAS NMR spectra show that the synthesis conditions are important in order to prevent the formation of non-framework Al and that the best result is obtained using NaAlO2 as the aluminum source. By proper control of the synthesis conditions, it is possible to produce a ZSM-5 zeolite with a framework Si/Al=50 and without non-framework aluminum. The relative numbers of Si*(OSi)4, HOSi*(OSi)3, and AlOSi*(OSi)3 units have been determined from 29Si-MAS NMR in combination with the Si/Al ratios from 27Al-MAS NMR. These results indicate a higher number of HOSi*(OSi)3 sites for ZSM-5 zeolites of the nm size when compared to those of the μm size. Transmission electron micrographs provide independent support for the formation of small zeolite crystals.

51V MAS NMR spectroscopy: determination of quadrupole and anisotropic shielding tensors, including the relative orientation of their principal-axis systems

Abstract 51 V MAS NMR spectra exhibiting the complete manifold of spinning sidebands (SSBs) from all seven 51 V ( I =7/2) transitions have been observed for NH 4 VO 3 and V 2 O 5 . These spectra are influenced by interactions from both 51 V quadrupole coupling and chemical-shielding anisotropy. A simulation and iterative-fitting program which allows the first determination of quadrupole and anisotropic shielding tensors, including the relative orientation of their principal-axis systems, from MAS NMR spectra is introduced. The 51 V parameters determined from the SSB intensities for NH 4 VO 3 and V 2 O 5 are compared with literature data from static NMR spectra.

29Si MAS NMR studies of portland cement components and effects of microsilica on the hydration reaction

29Si MAS NMR is shown to be a valuable tool for quantitative analysis of synthetic and natural cement minerals and for following the hydration of white Portland cement. Computer deconvolution of the alite/belite 29Si MAS NMR spectrum for white Portland cement allows the C3S and β-C2S content to be obtained with high accuracy. This composition differs significantly from that of a Bogue calculation. In the hydration of white Portland cement addition of microsilica has the effect of accelerating the reaction of C3S. Microsilica itself is consumed during the hydration, and an increased amount of polymer calcium silicate hydrates is formed. Preliminary high-speed 27Al MAS NMR investigations applied to the low aluminate content in white cement show great potential for this technique in studies of cement hydration reactions. An approximate value for the 27Al nuclear quadrupole coupling constant in hydrated white cement is reported.

Direct observation of aluminium guest ions in the silicate phases of cement minerals by 27 Al MAS NMR spectroscopy

The principal mineral phases in Portland cements are the impure forms of the calcium silicates, Ca3SiO5 and Ca2SiO4, known as alite and belite, in which the silicates are modified in composition and crystal structure by incorporation of guest ions such as Mg2+, Al3+ and Fe3+. This work reports the first direct evidence for guestion substitution in alite and belite by the observation of Al substitution employing 27Al magic-angle spinning (MAS) NMR. For both minerals Al3+ is observed to substitute for Si4+ and it is shown that 27Al MAS NMR can be used to quantify the amounts of Al substitution in the silicate phases of ordinary Portland and oilwell cements at levels well below 1 wt.%. The 27Al quadrupole coupling parameters and isotropic chemical shift (δ) have been determined for the unique Al guest-ion site in belite, which exhibits the most deshielded chemical shift (δ= 96.1) yet reported for a tetracoordinated Al bonded to four oxygens.

Quantification of calcium silicate phases in Portland cements by 29Si MAS NMR spectroscopy

29 Si magic-angle spinning (MAS) NMR spectroscopy is shown to be a valuable tool for quantifying two calcium silicates, alite (Ca3SiO5) and belite (Ca2SiO4), in Portland cements. Determination of these minerals involves computer deconvolution of the 29Si MAS NMR spectra combined with elemental analysis of the bulk SiO2 content. The applicability of the method is demonstrated for various types of Portland cements and it is shown that the monoclinic MI and MIII forms of alite can be distinguished by 29Si MAS NMR. Inversion–recovery 29Si spin–lattice relaxation MAS NMR allows quantification of the relative content of alite and belite in an independent way and the results of the deconvolution for two of the cements have been tested and confirmed using this technique. Comparison of the 29Si MAS NMR results for the alite and belite contents with the traditional Bogue calculation demonstrates that this calculation strongly overestimates the belite content at the expense of alite in Portland cements. Improved correlations are observed when the NMR data are compared with the alite and belite quantities determined from a Taylor-modified Bogue calculation.

Correlation between 29Si NMR chemical shifts and mean SiO bond lengths for calcium silicates

Abstract The nine resonances in the hitherto unassigned 29 Si magic-angle-spinning NMR spectrum of tricalcium silicate (C 3 S) are tentatively assigned using a δ versus d SiO correlation between isotropic 29 Si chemical shifts (δ) and mean SiO bond lengths ( d SiO ) observed for the a ′ L -, β-, and γ-dicalcium silicate (C 2 S) polymorphs. This results in a linear correlation, δ(ppm) = − 316.7 d SiO (A) + 445.3 ( R = −0.975, 12 data points), with increasing shielding corresponding to longer d SiO bond distance in calcium monosilicates (Q o ). The results and earlier established δ versus d SiO correlations are discussed in relation to recent ab initio calculations of 29 Si chemical shifts in isolated SiO 4− 4 tetrahedra.

Hydrogen–fluorine exchange in NaBH4–NaBF4

Hydrogen-fluorine exchange in the NaBH4-NaBF4 system is investigated using a range of experimental methods combined with DFT calculations and a possible mechanism for the reactions is proposed. Fluorine substitution is observed using in situ synchrotron radiation powder X-ray diffraction (SR-PXD) as a new Rock salt type compound with idealized composition NaBF2H2 in the temperature range T = 200 to 215 °C. Combined use of solid-state (19)F MAS NMR, FT-IR and DFT calculations supports the formation of a BF2H2(-) complex ion, reproducing the observation of a (19)F chemical shift at -144.2 ppm, which is different from that of NaBF4 at -159.2 ppm, along with the new absorption bands observed in the IR spectra. After further heating, the fluorine substituted compound becomes X-ray amorphous and decomposes to NaF at ~310 °C. This work shows that fluorine-substituted borohydrides tend to decompose to more stable compounds, e.g. NaF and BF3 or amorphous products such as closo-boranes, e.g. Na2B12H12. The NaBH4-NaBF4 composite decomposes at lower temperatures (300 °C) compared to NaBH4 (476 °C), as observed by thermogravimetric analysis. NaBH4-NaBF4 (1:0.5) preserves 30% of the hydrogen storage capacity after three hydrogen release and uptake cycles compared to 8% for NaBH4 as measured using Sieverts method under identical conditions, but more than 50% using prolonged hydrogen absorption time. The reversible hydrogen storage capacity tends to decrease possibly due to the formation of NaF and Na2B12H12. On the other hand, the additive sodium fluoride appears to facilitate hydrogen uptake, prevent foaming, phase segregation and loss of material from the sample container for samples of NaBH4-NaF.

Unusual observation of nitrogen chemical shift anisotropies in tetraalkylammonium halides by 14N MAS NMR spectroscopy.

14N Magic-angle spinning (MAS) NMR spectra for a number of polycrystalline, symmetrical tetraalkylammonium halides with short alkyl chains (C2H(5)- to n-C4H(9)-) have been recorded following a careful setup of the experimental conditions. Analysis of the spectra demonstrates the presence of 14N chemical shift anisotropies (CSAs) on the order of |delta sigma| = 10-30 ppm along with 14N quadrupole coupling constants in the range of 10-70 kHz. The magnitude and sign of the CSAs determined from 14N MAS NMR are confirmed by recording and analysis of the corresponding slow-speed spinning (500-650 Hz) 15N CP/MAS NMR spectra. Most interestingly, it is observed experimentally and demonstrated theoretically and by simulations, that these CSAs are reflected in the spinning sideband (ssb) intensities of the 14N MAS spectra at much higher spinning speeds than can be applied to retrieve the corresponding 15N CSAs from the ssb pattern in the 15N CP/MAS spectra.