Marek Pruski

Titanium catalyzed solid-state transformations in LiAlH4 during high-energy ball-milling

Abstract Mechanical processing of polycrystalline LiAlH 4 in the presence of titanium- and iron-based catalysts induces the transformation of LiAlH 4 into Li 3 AlH 6 , Al and H 2 at room temperature. Several catalysts were tested and it was established that their activity gradually decreases from TiCl 4 to Fe in the series TiCl 4 >Al 3 Ti≫Al 22 Fe 3 Ti 8 >Al 3 Fe>Fe. The high catalytic activity of TiCl 4 has been attributed to microcrystalline intermetallic Al 3 Ti, which rapidly forms in situ from TiCl 4 and LiAlH 4 during mechanical processing and then acts as a heterogeneous dehydrogenation catalyst.

Quantitative reliability of aromaticity and related measurements on coals by 13C n.m.r. A debate

While solid state 13C n.m.r. has made a major contribution to the characterization of coal and other insoluble carbonaceous materials over the past decade, there has been considerable uncertainty concerning the quantitative reliability of the technique. This debate addresses this important topic and comprises six contributions from authors who are recognized experts in n.m.r. characterization of solid fuels. The principal issue discussed is the accuracy of aromaticity measurements on coals by cross-polarization — magic-angle spinning (CP/MAS) 13C n.m.r., together with additional problems posed by high field measurements and spectral editing, and with some discussion of emerging techniques. There is a consensus that significant errors can arise in CP/MAS 13C n.m.r. measurements of aromaticity due to the unfortunate spin-dynamics of coals, which typically result in only ≈50% of the carbon being observed for bituminous coals. There is clear discrimination against aromatic carbon, but differences of opinion exist over the magnitude of the errors (from 2 to 15 mole carbon %) and whether high field (⩾ 50 MHz) measurements are as accurate as those of low field (< 25 MHz) because either sideband suppression or extremely high speed MAS has to be employed to eliminate sidebands. From the evidence presented, it is suggested that a combination of low field, single pulse excitation with long relaxation delays and the use of a suitable reagent to quench paramagnetic centres is the most satisfactory, albeit time consuming, recipe for obtaining reasonably reliable results on unknown samples. An inter-laboratory exercise is being organized by Argonne National Laboratory to check the precision and to further investigate quantitative reliability of 13C n.m.r. measurements on coals from their Premium Coal Sample Bank.

Indirectly detected through-bond chemical shift correlation NMR spectroscopy in solids under fast MAS: studies of organic-inorganic hybrid materials.

Indirectly detected, through-bond NMR correlation spectra between (13)C and (1)H nuclei are reported for the first time in solid state. The capabilities of the new method are demonstrated using naturally abundant organic-inorganic mesoporous hybrid materials. The time performance is significantly better, almost by a factor of 10, than in the corresponding (13)C detected experiment. The proposed scheme represents a new analytical tool for studying other solid-state systems and the basis for the development of more advanced 2D and 3D correlation methods.

Natural abundance 17O DNP two-dimensional and surface-enhanced NMR spectroscopy

Due to its extremely low natural abundance and quadrupolar nature, the (17)O nuclide is very rarely used for spectroscopic investigation of solids by NMR without isotope enrichment. Additionally, the applicability of dynamic nuclear polarization (DNP), which leads to sensitivity enhancements of 2 orders of magnitude, to (17)O is wrought with challenges due to the lack of spin diffusion and low polarization transfer efficiency from (1)H. Here, we demonstrate new DNP-based measurements that extend (17)O solid-state NMR beyond its current capabilities. The use of the PRESTO technique instead of conventional (1)H-(17)O cross-polarization greatly improves the sensitivity and enables the facile measurement of undistorted line shapes and two-dimensional (1)H-(17)O HETCOR NMR spectra as well as accurate internuclear distance measurements at natural abundance. This was applied for distinguishing hydrogen-bonded and lone (17)O sites on the surface of silica gel; the one-dimensional spectrum of which could not be used to extract such detail. Lastly, this greatly enhanced sensitivity has enabled, for the first time, the detection of surface hydroxyl sites on mesoporous silica at natural abundance, thereby extending the concept of DNP surface-enhanced NMR spectroscopy to the (17)O nuclide.

Study of Intermolecular Interactions in the Corrole Matrix by Solid‐State NMR under 100 kHz MAS and Theoretical Calculations

Recent progress in solid-state (SS)NMR spectroscopic methods based on fast magic angle spinning (MAS) has enabled new opportunities for the structural study of small quantities (< 5 mg) of natural abundance samples. Utilizing throughspace and through-bond polarization transfer, indirect detection of low-g nuclei, and suitable homoand heteronuclear decoupling, oneand two-dimensional (1D and 2D) spectra of such samples can be measured with excellent sensitivity and resolution. However, determination of the short-range intermolecular order often remains elusive. Such analyses can be well-served by studying heteronuclear correlations that take advantage of the large chemical shift range of most low-g nuclei (for example, C or N). Indeed, heteronuclear correlation (HETCOR) NMR spectroscopy and measurements of internuclear distances, often in concert with theoretical calculations, have provided structural details of complex hydrogen-bonded systems in chemistry and biology, blended materials, and host–guest pairs. Still, intermolecular polarization transfers to low-g nuclei are often hampered by low sensitivity. A promising solution to this challenge is offered by homonuclear H–H 2D correlation methods, such as double-quantum (DQ)MAS or spin-diffusion (NOESYlike) experiments, provided that sufficient resolution is achieved in both dimensions. One of the possible approaches is the use of H CRAMPS decoupling in concert with fast MAS to boost resolution in these experiments. The recent development of ultrafast MAS (at 100 kHz and more) provides access to appropriate H resolution without RF decoupling. Herein, we report the first application of H 2D SSNMR measurements under MAS at 100 kHz, which are used in combination with indirectly detected H{C} and H{N} HETCOR experiments and theoretical calculations to scrutinize the interactions within a host–guest (HG) system consisting of 5,10,15-tris(pentafluorophenyl)corrole 1, and toluene (Scheme 1).

Optimization of data acquisition and processing in Carr–Purcell–Meiboom–Gill multiple quantum magic angle spinning nuclear magnetic resonance

Data acquisition using the Carr–Purcell–Meiboom–Gill (CPMG) train of π pulses has been recently explored in multiple quantum magic angle spinning (MQMAS) nuclear magnetic resonance of half-integer quadrupolar nuclei [T. Vosegaard, F. H. Larsen, H. J. Jakobsen, P. D. Ellis, and N. C. Nielsen, J. Am. Chem. Soc. 119, 9055 (1997)]. Significant increase of sensitivity can be obtained by using this technique at the expense of spectral definition, as the spectrum transforms into a manifold of narrow sidebands. A detailed analysis of the CPMG method, with emphasis on the MQMAS experiment, is presented. The MQ-QCPMG-MAS approach is adapted to spin-5/2 nuclei. Several numerical methods of data treatment are shown that allow for improvement of the definition of the sideband envelope, reconstruction of the standard line shape, and improvement of S/N ratio via optimized filtering. Theoretical and experimental estimates of signal enhancement through proper acquisition and processing of MQ-QCPMG-MAS data are given.

Directly and indirectly detected through-bond heteronuclear correlation solid-state NMR spectroscopy under fast MAS

Two-dimensional through-bond (1)H{(13)C} solid-state NMR experiments utilizing fast magic angle spinning (MAS) and homonuclear multipulse (1)H decoupling are presented. Remarkable efficiency of polarization transfer can be achieved at MAS rates exceeding 40 kHz, which is instrumental in these measurements. Schemes utilizing direct and indirect detection of heteronuclei are compared in terms of resolution and sensitivity. A simple procedure for optimization of (1)H homonuclear decoupling sequences under these conditions is proposed. The capabilities of these techniques were confirmed on two naturally abundant solids, tripeptide N-formyl-L-methionyl-L-leucyl-L-phenylalanine (f-MLF-OH) and brown coal.

Analysis of sensitivity enhancement by dynamic nuclear polarization in solid-state NMR: a case study of functionalized mesoporous materials

We systematically studied the enhancement factor (per scan) and the sensitivity enhancement (per unit time) in (13)C and (29)Si cross-polarization magic angle spinning (CP-MAS) NMR boosted by dynamic nuclear polarization (DNP) of functionalized mesoporous silica nanoparticles (MSNs). Specifically, we separated contributions due to: (i) microwave irradiation, (ii) quenching by paramagnetic effects, (iii) the presence of frozen solvent, (iv) the temperature, as well as changes in (v) relaxation and (vi) cross-polarization behaviour. No line-broadening effects were observed for MSNs when lowering the temperature from 300 to 100 K. Notwithstanding a significant signal reduction due to quenching by TOTAPOL radicals, DNP-CP-MAS at 100 K provided global sensitivity enhancements of 23 and 45 for (13)C and (29)Si, respectively, relative to standard CP-MAS measurements at room temperature. The effects of DNP were also ascertained by comparing with state-of-the-art two-dimensional heteronuclear (1)H{(13)C} and (29)Si{(1)H} correlation spectra, using, respectively, indirect detection or Carr-Purcell-Meiboom-Gill (CPMG) refocusing to boost signal acquisition. This study highlights opportunities for further improvements through the development of high-field DNP, better polarizing agents, and improved capabilities for low-temperature MAS.

Probing Quadrupolar Nuclei by Solid-State NMR Spectroscopy: Recent Advances

Solid-state nuclear magnetic resonance (NMR) of quadrupolar nuclei has recently undergone remarkable development of capabilities for obtaining structural and dynamic information at the molecular level. This review summarizes the key achievements attained during the last couple of decades in solid-state NMR of both integer spin and half-integer spin quadrupolar nuclei. We provide a concise description of the first- and second-order quadrupolar interactions, and their effect on the static and magic angle spinning (MAS) spectra. Methods are explained for efficient excitation of single- and multiple-quantum coherences, and acquisition of spectra under low- and high-resolution conditions. Most of all, we present a coherent, comparative description of the high-resolution methods for half-integer quadrupolar nuclei, including double rotation (DOR), dynamic angle spinning (DAS), multiple-quantum magic angle spinning (MQMAS), and satellite transition magic angle spinning (STMAS). Also highlighted are methods for processing and analysis of the spectra. Finally, we review methods for probing the heteronuclear and homonuclear correlations between the quadrupolar nuclei and their quadrupolar or spin-1/2 neighbors.

29Si NMR in solid state with CPMG acquisition under MAS.

A remarkable enhancement of sensitivity can be often achieved in 29Si solid-state NMR by applying the well-known Carr-Purcell-Meiboom-Gill (CPMG) train of rotor-synchronized pi pulses during the detection of silicon magnetization. Here, several one- and two-dimensional (1D and 2D) techniques are used to demonstrate the capabilities of this approach. Examples include 1D 29Si{X} CPMAS spectra and 2D 29Si{X} HETCOR spectra of mesoporous silicas, zeolites and minerals, where X=1H or 27Al. Data processing methods, experimental strategies and sensitivity limits are discussed and illustrated by experiments. The mechanisms of transverse dephasing of 29Si nuclei in solids are analyzed. Fast magic angle spinning, at rates between 25 and 40 kHz, is instrumental in achieving the highest sensitivity gain in some of these experiments. In the case of 29Si-29Si double-quantum techniques, CPMG detection can be exploited to measure homonuclear J-couplings.