We report that spatial (<1 nm) proximity between different molecules in solid bulk materials and, for the first time, different moieties on the surface of a catalyst, can be established without isotope enrichment by means of homonuclear CHHC solid-state nuclear magnetic resonance experiment. This 13C–13C correlation measurement, which hitherto was not possible for natural-abundance solids, was enabled by the use of dynamic nuclear polarization. Importantly, it allows the study of long-range correlations in a variety of materials with high resolution.

The preparation and unambiguous characterization of isolated Brønsted-acidic silanol species on silica–alumina catalysts presents a key challenge in the rational design of solid acid catalysts. In this report, atomic layer deposition (ALD) and liquid-phase preparation (chemical liquid deposition, CLD) are used to install the SiOx sites on Al2O3 catalysts using the same Si source (tetraethylorthosilicate, TEOS). The ALD-derived and CLD-derived SiOx sites are probed with dynamic nuclear polarization (DNP)-enhanced 29Si–29Si double-quantum/single-quantum (DQ/SQ) correlation NMR spectroscopy. The investigation reveals conclusively that the SiOx/Al2O3 material prepared by ALD and CLD, followed by calcination under an O2 stream, contains fully spatially isolated Si species, in contrast with those resulting from the calcination under static air, which is widely accepted as a postgrafting treatment for CLD. Insight into the formation mechanism of these sites is obtained via in situ monitoring of the TEOS + γ-Al2O3 reaction in an environmental diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell. Upon calcination, the DRIFTS spectra of SiOx/Al2O3 reveal a signature unambiguously assignable to isolated Brønsted-acidic silanol species. Surprisingly, the results of this study indicate that the method of preparing SiOx/Al2O3 catalysts is less important to the final structure of the silanol sites than the post-treatment conditions. This finding should greatly simplify the methods for synthesizing site-isolated, Brønsted-acidic SiOx/Al2O3 catalysts.

The characterization of nanometer-scale interactions between carbon-containing substrates and alumina surfaces is of paramount importance to industrial and academic catalysis applications, but it is also very challenging. Here, we demonstrate that dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP SENS) allows the unambiguous description of the coordination geometries and conformations of the substrates at the alumina surface through high-resolution measurements of 13C–27Al distances. We apply this new technique to elucidate the molecular-level geometry of 13C-enriched methionine and natural abundance poly(vinyl alcohol) adsorbed on γ-Al2O3-supported Pd catalysts, and we support these results with element-specific X-ray absorption near-edge measurements. This work clearly demonstrates a surprising bimodal coordination of methionine at the Pd–Al2O3 interface.

Brønsted acidic sites on catalyst surfaces are observed by 17O dynamic nuclear-polarization-enhanced NMR spectroscopy, which hyperpolarizes nuclei by microwave-induced saturation of electrons. In their Communication on page 9165 ff., M. Pruski et al. report the measurement of O−H distances with sub-picometer precision. The length of the O−H bond is directly related to the Brønsted acidity of the sites and is also a reporter of the formation of intermolecular hydrogen-bonding interactions.

Brønsted-saure aktive Zentren auf Katalysatoroberflächen werden mit dynamischer 17O-NMR-Spektroskopie mit Kernpolarisationsverstärkung untersucht. Bei dieser Methode werden die Kerne durch Mikrowellen-induzierte Elektronensättigung hyperpolarisiert. M. Pruski et al. berichten in ihrer Zuschrift auf S. 9293 ff. über die Messung von O-H-Abständen mit Subpikometergenauigkeit. Die Länge der O-H-Bindung hängt direkt mit der Brønsted-Säurestärke zusammen und ist auch ein Indiz für die Bildung intramolekularer Wasserstoffbrücken.

AbstractHeterogeneous Brønsted acid catalysts are tremendously important in industry, particularly in catalytic cracking processes. Here we show that these Brønsted acid sites can be directly observed at natural abundance by 17O DNP surface-enhanced NMR spectroscopy (SENS). We additionally show that the O−H bond length in these catalysts can be measured with sub-picometer precision, to enable a direct structural gauge of the lability of protons in a given material, which is correlated with the pH of the zero point of charge of the material. Experiments performed on materials impregnated with pyridine also allow for the direct detection of intermolecular hydrogen bonding interactions through the lengthening of O−H bonds.

AbstractHeterogeneous Brønsted acid catalysts are tremendously important in industry, particularly in catalytic cracking processes. Here we show that these Brønsted acid sites can be directly observed at natural abundance by 17O DNP surface-enhanced NMR spectroscopy (SENS). We additionally show that the O−H bond length in these catalysts can be measured with sub-picometer precision, to enable a direct structural gauge of the lability of protons in a given material, which is correlated with the pH of the zero point of charge of the material. Experiments performed on materials impregnated with pyridine also allow for the direct detection of intermolecular hydrogen bonding interactions through the lengthening of O−H bonds.

Dynamic nuclear polarization (DNP) is used to enhance the (ultra)wideline 207Pb solid-state NMR spectra of lead compounds of relevance in the preservation of cultural heritage objects. The DNP SSNMR experiments enabled, for the first time, the detection of the basic lead carbonate phase of the lead white pigment by 207Pb SSNMR spectroscopy. Variable-temperature experiments revealed that the short T′2 relaxation time of the basic lead carbonate phase hinders the acquisition of the NMR signal at room temperature. We additionally observe that the DNP enhancement is twice as large for lead palmitate (a lead soap, which is a degradation product implicated in the visible deterioration of lead-based oil paintings), than it is for the basic lead carbonate. This enhancement has allowed us to detect the formation of a lead soap in an aged paint film by 207Pb SSNMR spectroscopy; which may aid in the detection of deterioration products in smaller samples removed from works of art.

Solid-state NMR spectroscopy, both conventional and dynamic nuclear polarization (DNP)-enhanced, was employed to study the spatial distribution of organic functional groups attached to the surface of mesoporous silica nanoparticles via co-condensation and grafting. The most revealing information was provided by DNP-enhanced two-dimensional 29Si–29Si correlation measurements, which unambiguously showed that post-synthesis grafting leads to a more homogeneous dispersion of propyl and mercaptopropyl functionalities than co-condensation. During the anhydrous grafting process, the organosilane precursors do not self-condense and are unlikely to bond to the silica surface in close proximity (less than 4 Å) due to the limited availability of suitably arranged hydroxyl groups.

•DNP-enhanced 2D 1H-X HETCOR spectra of surface species were measured using proton-free solvents.•The approach eliminates solvent-derived correlation signals.•In hydrogen-rich materials, the use of proton-less solvents improves the DNP performance.We demonstrate that dynamic nuclear polarization (DNP)-enhanced 1H-X heteronuclear correlation (HETCOR) measurements of hydrogen-rich surface species are better accomplished by using proton-free solvents. This approach notably prevents HETCOR spectra from being obfuscated by the solvent-derived signals otherwise present in DNP measurements. Additionally, in the hydrogen-rich materials studied here, which included functionalized mesoporous silica nanoparticles and metal organic frameworks, the use of proton-free solvents afforded higher sensitivity gains than the commonly used solvents containing protons. We also explored the possibility of using a solvent-free sample formulation and the feasibility of indirect detection in DNP-enhanced HETCOR experiments.Download high-res image (186KB)Download full-size image

Lignocellulosic biomass is a promising sustainable feedstock for the production of biofuels, biomaterials, and biospecialty chemicals. However, efficient utilization of biomass has been limited by our poor understanding of its molecular structure. Here, we report a dynamic nuclear polarization (DNP)-enhanced solid-state (SS)NMR study of the molecular structure of biomass, both pre- and postcatalytic treatment. This technique enables the measurement of 2D homonuclear 13C–13C correlation SSNMR spectra under natural abundance, yielding, for the first time, an atomic-level picture of the structure of raw and catalytically treated biomass samples. We foresee that further such experiments could be used to determine structure–function relationships and facilitate the development of more efficient, and chemically targeted, biomass-conversion technologies.

DNP-NMR spectroscopy has been applied to enhance the signal for organic molecules adsorbed on γ-Al2O3-supported Pd nanoparticle catalysts. By offering >2500-fold time savings, the technique enabled the observation of 13C–13C cross-peaks for low coverage species, which were assigned to products from oxidative degradation of methionine adsorbed on the nanoparticle surface.

14N ultra-wideline (UW), 1H{15N} indirectly-detected HETCOR (idHETCOR) and 15N dynamic nuclear polarization (DNP) solid-state NMR (SSNMR) experiments, in combination with plane-wave density functional theory (DFT) calculations of 14N EFG tensors, were utilized to characterize a series of nitrogen-containing active pharmaceutical ingredients (APIs), including HCl salts of scopolamine, alprenolol, isoprenaline, acebutolol, dibucaine, nicardipine, and ranitidine. A case study applying these methods for the differentiation of polymorphs of bupivacaine HCl is also presented. All experiments were conducted upon samples with naturally-abundant nitrogen isotopes. For most of the APIs, it was possible to acquire frequency-stepped UW 14N SSNMR spectra of stationary samples, which display powder patterns corresponding to pseudo-tetrahedral (i.e., RR′R′′NH+ and RR′NH2+) or other (i.e., RNH2 and RNO2) nitrogen environments. Directly-excited 14N NMR spectra were acquired using the WURST-CPMG pulse sequence, which incorporates WURST (wideband, uniform rate, and smooth truncation) pulses and a CPMG (Carr-Purcell Meiboom-Gill) refocusing protocol. In certain cases, spectra were acquired using 1H → 14N broadband cross-polarization, via the BRAIN-CP (broadband adiabatic inversion – cross polarization) pulse sequence. These spectra provide 14N electric field gradient (EFG) tensor parameters and orientations that are particularly sensitive to variations in local structure and intermolecular hydrogen-bonding interactions. The 1H{15N} idHETCOR spectra, acquired under conditions of fast magic-angle spinning (MAS), used CP transfers to provide 1H–15N chemical shift correlations for all nitrogen environments, except for two sites in acebutolol and nicardipine. One of these two sites (RR′NH2+ in acebutolol) was successfully detected using the DNP-enhanced 15N{1H} CP/MAS measurement, and one (RNO2 in nicardipine) remained elusive due to the absence of nearby protons. This exploratory study suggests that this combination of techniques has great potential for the characterization of solid APIs and numerous other organic, biological, and inorganic systems.

Ultrawideline dynamic nuclear polarization (DNP)-enhanced 195Pt solid-state NMR (SSNMR) spectroscopy and theoretical calculations are used to determine the coordination of atomic Pt species supported within the pores of metal–organic frameworks (MOFs). The 195Pt SSNMR spectra, with breadths reaching 10 000 ppm, were obtained by combining DNP with broadbanded cross-polarization and CPMG acquisition. Although the DNP enhancements in static samples are lower than those typically observed under magic-angle spinning conditions, the presented measurements would be very challenging using the conventional SSNMR methods. The DNP-enhanced ultrawideline NMR spectra served to separate signals from cis- and trans-coordinated atomic Pt2+ species supported on the UiO-66-NH2 MOF. Additionally, the data revealed a dominance of kinetic effects in the formation of Pt2+ complexes and the thermodynamic effects in their reduction to nanoparticles. A single cis-coordinated Pt2+ complex was confirmed in MOF-253.

Dynamic nuclear polarization (DNP)-enhanced solid-state nuclear magnetic resonance (SSNMR) spectroscopy is increasingly being used as a tool for the atomic-level characterization of surface sites. DNP surface-enhanced SSNMR spectroscopy of materials has, however, been limited to studying relatively receptive nuclei, and the particularly rare 17O nuclide, which is of great interest for materials science, has not been utilized. We demonstrate that advanced 17O SSNMR experiments can be performed on surface species at natural isotopic abundance using DNP. We use 17O DNP surface-enhanced 2D SSNMR to measure 17O{1H} HETCOR spectra as well as dipolar oscillations on a series of thermally treated mesoporous silica nanoparticle samples having different pore diameters. These experiments allow for a nonintrusive and unambiguous characterization of hydrogen bonding and dynamics at the surface of the material; no other single experiment can give such details about the interactions at the surface. Our data show that, upon drying, strongly hydrogen-bonded surface silanols, whose motions are greatly restricted by the interaction when compared to lone silanols, are selectively dehydroxylated.

Due to its extremely low natural abundance and quadrupolar nature, the 17O 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 17O is wrought with challenges due to the lack of spin diffusion and low polarization transfer efficiency from 1H. Here, we demonstrate new DNP-based measurements that extend 17O solid-state NMR beyond its current capabilities. The use of the PRESTO technique instead of conventional 1H–17O cross-polarization greatly improves the sensitivity and enables the facile measurement of undistorted line shapes and two-dimensional 1H–17O HETCOR NMR spectra as well as accurate internuclear distance measurements at natural abundance. This was applied for distinguishing hydrogen-bonded and lone 17O 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 17O nuclide.

The hydroboration of aldehydes and ketones using a silica-supported zirconium catalyst is reported. Reaction of Zr(NMe2)4 and mesoporous silica nanoparticles (MSN) provides the catalytic material Zr(NMe2)n@MSN. Exhaustive characterization of Zr(NMe2)n@MSN with solid-state (SS)NMR and infrared spectroscopy, as well as through reactivity studies, suggests its surface structure is primarily ≡SiOZr(NMe2)3. The presence of these nitrogen-containing zirconium sites is supported by 15N NMR spectroscopy, including natural abundance 15N NMR measurements using dynamic nuclear polarization (DNP) SSNMR. The Zr(NMe2)n@MSN material reacts with pinacolborane (HBpin) to provide Me2NBpin and the material ZrH/Bpin@MSN that is composed of interacting surface-bonded zirconium hydride and surface-bonded borane ≡SiOBpin moieties in an approximately 1:1 ratio, as well as zirconium sites coordinated by dimethylamine. The ZrH/Bpin@MSN is characterized by 1H/2H and 11B SSNMR and infrared spectroscopy and through its reactivity with D2. The zirconium hydride material or the zirconium amide precursor Zr(NMe2)n@MSN catalyzes the selective hydroboration of aldehydes and ketones with HBpin in the presence of functional groups that are often reduced under hydroboration conditions or are sensitive to metal hydrides, including olefins, alkynes, nitro groups, halides, and ethers. Remarkably, this catalytic material may be recycled without loss of activity at least eight times, and air-exposed materials are catalytically active. Thus, these supported zirconium centers are robust catalytic sites for carbonyl reduction and that surface-supported, catalytically reactive zirconium hydride may be generated from zirconium-amide or zirconium alkoxide sites.Keywords: carbonyl hydroboration; interfacial catalysis; mesoporous silica; single-site catalysts; solid-state NMR; zirconium hydride

We show both experimentally and numerically on a series of model systems that in experiments involving transfer of magnetization from 1H to the quadrupolar nuclei under magic-angle-spinning (MAS), the PRESTO technique consistently outperforms traditionally used cross polarization (CP), affording more quantitative intensities, improved lineshapes, better overall sensitivity, and straightforward optimization. This advantage derives from the fact that PRESTO circumvents the convoluted and uncooperative spin dynamics during the CP transfer under MAS, by replacing the spin-locking of quadrupolar nuclei with a single central transition selective 90° pulse and using a symmetry-based recoupling sequence in the 1H channel. This is of particular importance in the context of dynamic nuclear polarization (DNP) NMR of quadrupolar nuclei, where the efficient transfer of enhanced 1H polarization is desired to obtain the highest sensitivity.

•Reloading as-synthesized SBA-15 with additional surfactant protects internal surfaces.•Reloaded materials can be selectively grafted on the exterior.•SSNMR and porosimetry were highly effective to interrogate surface functionalization.•Stability of the silyl layer to hot ethanol was found to be remarkably high.A method has been developed that permits the highly selective functionalization of the interior and exterior surfaces of the ubiquitous mesoporous material, SBA-15. The key step is reloading the as-synthesized material with structure-directing agent, Pluronic® P123, prior to selective functionalization of the external surface with a silylating agent. This new approach represents a significant improvement over literature procedures. Results from physisorption analyses as well as solid-state NMR permit a detailed, quantitative assessment of functionalized SBA-15. This work also provides insight into the stability of the silyl layer during extraction procedures – an issue often neglected in other studies but of significant importance as decomposition of this layer could result in the introduction of new silanols and reduce the effectiveness of any selective grafting procedure.

Two-dimensional 1H{13C} heteronuclear correlation solid-state NMR spectra of naturally abundant solid materials are presented, acquired using the 0.75-mm magic angle spinning (MAS) probe at spinning rates up to 100 kHz. In spite of the miniscule sample volume (290 nL), high-quality HSQC-type spectra of bulk samples as well as surface-bound molecules can be obtained within hours of experimental time. The experiments are compared with those carried out at 40 kHz MAS using a 1.6-mm probe, which offered higher overall sensitivity due to a larger rotor volume. The benefits of ultrafast MAS in such experiments include superior resolution in 1H dimension without resorting to 1H–1H homonuclear RF decoupling, easy optimization, and applicability to mass-limited samples. The HMQC spectra of surface-bound species can be also acquired under 100 kHz MAS, although the dephasing of transverse magnetization has significant effect on the efficiency transfer under MAS alone.

Two-dimensional indirectly detected through-space and through-bond 1H{15N} solid-state NMR experiments utilizing fast magic angle spinning (MAS) and homonuclear multipulse 1H decoupling are evaluated. Remarkable efficiency of polarization transfer can be achieved at a MAS rate of 40 kHz by both cross-polarization and INEPT, which makes these methods applicable for routine characterizations of natural abundance solids. The first measurement of 2D 1H{15N} HETCOR spectrum of natural abundance surface species is also reported.

The solid-state thermolysis of ammonia borane (NH3BH3, AB) was explored using state-of-the-art 15N solid-state NMR spectroscopy, including 2D indirectly detected 1H{15N} heteronuclear correlation and dynamic nuclear polarization (DNP)-enhanced 15N{1H} cross-polarization experiments as well as 11B NMR. The complementary use of 15N and 11B NMR experiments, supported by density functional theory calculations of the chemical shift tensors, provided insights into the dehydrogenation mechanism of AB—insights that have not been available by 11B NMR alone. Specifically, highly branched polyaminoborane derivatives were shown to form from AB via oligomerization in the “head-to-tail” manner, which then transform directly into hexagonal boron nitride analog through the dehydrocyclization reaction, bypassing the formation of polyiminoborane.

We report direct hydrogenation of MgB2 in a planetary ball mill. Magnesium borohydride, Mg(BH4)2, and various polyhedral borane anion salts have been synthesized at pressures between 50 and 350 bar H2 without the need for subsequent isothermal hydrogenation at elevated temperature and pressure. The obtained products release ∼4 wt% H2 below 390 °C, and a major portion of Mg(BH4)2 transforms back to MgB2 at around 300 °C, demonstrating the possibility of reversible hydrogen storage in an Mg(BH4)2–MgB2 system.

We systematically studied the enhancement factor (per scan) and the sensitivity enhancement (per unit time) in 13C and 29Si 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 13C and 29Si, 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 1H{13C} and 29Si{1H} 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.

We show that dynamic nuclear polarization (DNP) can be used to enhance NMR signals of 13C and 29Si nuclei located in mesoporous organic/inorganic hybrid materials, at several hundreds of nanometers from stable radicals (TOTAPOL) trapped in the surrounding frozen disordered water. The approach is demonstrated using mesoporous silica nanoparticles (MSN), functionalized with 3-(N-phenylureido)propyl (PUP) groups, filled with the surfactant cetyltrimethylammonium bromide (CTAB). The DNP-enhanced proton magnetization is transported into the mesopores via 1H–1H spin diffusion and transferred to rare spins by cross-polarization, yielding signal enhancements εon/off of around 8. When the CTAB molecules are extracted, so that the radicals can enter the mesopores, the enhancements increase to εon/off ≈ 30 for both nuclei. A quantitative analysis of the signal enhancements in MSN with and without surfactant is based on a one-dimensional proton spin diffusion model. The effect of solvent deuteration is also investigated.

The mechanism of thermochemical dehydrogenation of the 1:3 mixture of Li3AlH6 and NH3BH3 (AB) has been studied by the extensive use of solid-state NMR spectroscopy and theoretical calculations. The activation energy for the dehydrogenation is estimated to be 110 kJ mol–1, which is lower than for pristine AB (184 kJ mol–1). The major hydrogen release from the mixture occurs at 60 and 72 °C, which compares favorably with pristine AB and related hydrogen storage materials, such as lithium amidoborane (LiNH2BH3, LiAB). The NMR studies suggest that Li3AlH6 improves the dehydrogenation kinetics of AB by forming an intermediate compound (LiAB)x(AB)1–x. A part of AB in the mixture transforms into LiAB to form this intermediate, which accelerates the subsequent formation of branched polyaminoborane species and further release of hydrogen. The detailed reaction mechanism, in particular the role of lithium, revealed in the present study highlights new opportunities for using ammonia borane and its derivatives as hydrogen storage materials.

Mechanochemical transformations occurring during ball milling of sodium amide (NaNH2) with magnesium hydride (MgH2) taken in 2:3 and 2:1 molar ratios have been investigated using X-ray powder diffraction (XRD) and solid-state nuclear magnetic resonance (SSNMR) techniques. For the 2NaNH2–3MgH2 system the mechanochemical reaction proceeds via the formation of MgNH as an intermediate, whereas magnesium nitride (Mg3N2), sodium hydride (NaH) and hydrogen (∼5 wt%) form as the final products. The overall solid state reaction for this system is 2NaNH2 + 3MgH2 → Mg3N2 + 2NaH + 4H2. However, the mechanochemical transformation of the 2NaNH2–MgH2 system proceeds through the reaction: 2NaNH2 + MgH2 → Mg(NH2)2 + 2NaH, without any hydrogen release. Comparison of the mechanochemical transformations with the previously studied thermochemical transformations reveals that the two approaches lead to the same final products via different reaction pathways.Highlights► We investigate mechanochemically induced transformations in mixtures of NaNH2 and MgH2. ► ∼5 wt% of H2 is released from 2:3 mixture, no H2 is released from 2:1 mixture. ► Mechanochemical and thermochemical reactions in 2:3 mixture yield identical products via different pathways.

Surface silanol groups in mesoporous silica MCM-41 particles were successfully silylated with trimethylsilyl trifluoromethanesulfonate (TMSOTf). Characterization of modified mesoporous silica materials was conducted using X-ray diffraction, infrared spectroscopy, nitrogen absorption, elemental analysis, and solid-state NMR spectroscopy. In particular, extensive use of 1H, 13C, and 29Si solid-state NMR provided unique insights into the silylation process and served as a key guiding tool for the synthetic effort. Treatment of as-synthesized MCM-41 with TMSOTf was found to selectively and efficiently passivate the external surface of particles without assistance of a base, whereas modification by other silylating reagents, including trimethylchlorosilane (TMCS), N,O-bis(trimethylsilyl)acetamide (BSA), and triethoxymethylsilane (MeSi(OEt)3), yielded lower coverage and/or resulted in partial silylation of the internal surface. The 29Si and 1H solid-state NMR spectra gave accurate concentrations of silicon sites and densities of trimethylsilyl (TMS) groups on the external and internal surfaces of MCM-41. The 1H and 13C NMR spectra revealed the definite structures and concentrations of all organic species present in the silylated samples. These data highlighted the importance of choosing a proper concentration of the silylating reagent and finding the washing and extraction conditions that result in efficient sequestration of the structure directing agent (surfactant) without detachment of grafted species or production of unwanted surface alkoxy groups.

Bifunctional mesoporous silica nanoparticle (MSN) catalysts for esterification reaction, containing a Brønsted acid site of diarylammonium triflate (DAT) and a pentafluorophenyl propyl (PFP) group, were synthesized and thoroughly characterized. Their high reactivity is attributed to the formation of a surface-bound hydrophobic layer of PFP molecules, which facilitates the extrusion of one of the reaction products (water) from the mesopores by suppressing water adsorption onto the surface, thereby shifting the reaction equilibrium to completion.Keywords: esterification; fluorinated surface; heterogeneous catalysis; mesoporous materials; rational catalyst design; solid-state NMR

The use of mixed surfactants in the synthesis of mesoporous silica nanoparticles (MSNs) is of importance in the context of adjusting pore structures, sizes and morphologies. In the present study, the arrangement of molecules in micelles produced from a mixture of two surfactants, cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) was detailed by solid-state NMR spectroscopy. Proximities of methyl protons in the trimethylammonium headgroup of CTAB and protons in the pyridinium headgroup of CPB were observed under fast magic angle spinning (MAS) by 1H−1H double quantum (DQ) MAS NMR and NOESY. This result suggested that CTAB and CPB co-exist in the pores without forming significant monocomponent domain structures. 1H−29Si heteronuclear correlation (HETCOR) NMR showed that protons in the headgroups of CTAB are in closer proximity to the silica surface than those in the CPB headgroups. The structural information obtained in this investigation leads to better understanding of the mechanisms of self-assembly and their role in determining the structure and morphology of mesoporous materials.Graphical AbstractResearch highlights► Molecular ordering of mixed surfactants (CTAB+CPB) in mesoporous silica nanoparticles (MSNs) is described. ► CTAB and CPB co-exist in the pores without forming significant monocomponent domains. ► The study helps ‘tailoring’ the morphologies of MSNs.

The conformations of (pentafluorophenyl)propyl groups (−CH2−CH2−CH2−C6F5, abbreviated as PFP), covalently bound to the surface of mesoporous silica nanoparticles (MSNs), were determined by solid-state NMR spectroscopy and further refined by theoretical modeling. Two types of PFP groups were described, including molecules in the prone position with the perfluorinated aromatic rings located above the siloxane bridges (PFP-p) and the PFP groups denoted as upright (PFP-u), whose aromatic rings do not interact with the silica surface. Two-dimensional (2D) 13C−1H, 13C−19F and 19F−29Si heteronuclear correlation (HETCOR) spectra were obtained with high sensitivity on natural abundance samples using fast magic angle spinning (MAS), indirect detection of low-γ nuclei and signal enhancement by Carr−Purcell−Meiboom−Gill (CPMG) spin−echo sequence. 2D double-quantum (DQ) 19F MAS NMR spectra and spin−echo measurements provided additional information about the structure and mobility of the pentafluorophenyl rings. Optimization of the PFP geometry, as well as calculations of the interaction energies and 19F chemical shifts, proved very useful in refining the structural features of PFP-p and PFP-u functional groups on the silica surface. The prospects of using the PFP-functionalized surface to modify its properties (e.g., the interaction with solvents, especially water) and design new types of the heterogeneous catalytic system are discussed.

A simple method is shown for optimization of 1H homonuclear dipolar decoupling at MAS rates exceeding 10 kHz. By monitoring the intensity of a spin-echo under the decoupling conditions, it is possible to optimize the amplitude of the RF magnetic field, the cycle time of the decoupling sequence and the resonance offset within minutes. As a result, the decoupling efficiency can be quickly and reliably fine-tuned without using a reference sample. The utility of this method has been confirmed by studying the resolution patterns for the supercycled PMLG scheme, which were found to be in excellent agreement with earlier theoretical predictions and verified in high-resolution 2D 1H–1H experiments.

A comparative study of catalytic activity under homogeneous and heterogeneous conditions was carried out using the (salen)CrIII-catalyzed oxidation of tetramethylbenzidine (TMB) with iodosobenzene as a model reaction. Amine-functionalized mesoporous silica nanoparticles (MSN) were synthesized in a co-condensation reaction and functionalized with salenvia a covalent Si–C bond. A Cr(III) complex of this supported ligand, MSN-(salen)CrIII, was prepared and characterized. Data from powder XRD, BET isotherms and BJH pore size distribution all showed that MSN-(salen)CrIII still had the typical MSN high surface area, narrow pore size distribution, and ordered hexagonal pore structure, which were further confirmed by transmission electron microscopy (TEM) images. 13C and 29Si solid-state NMR data provided structural information about the catalyst and verified successful functionalization of the salen ligand and coordination to Cr(III). No unreacted salen or Cr(III) were observed. The loadings of salen and salen-CrIII complex were determined viaTGA and EDX, respectively. Both measurements indicated that approximately 0.5 mmol/g of catalyst was loaded on the surface of MSN. The oxidation of TMB with iodosobenzene using MSN-(salen)CrIII as a heterogeneous catalyst exhibited both similarities and differences with the analogous homogeneous reaction using (salen)CrIII(H2O)+ as a catalyst in aqueous acetonitrile. In the presence of 0.10 M HClO4, the two catalytic reactions proceeded at similar rates and generated the doubly oxidized product TMB2+. In the absence of acid, the radical cation TMB˙+ was produced. The kinetics of the heterogeneous reaction in the absence of added acid responded to concentrations of all three reagents, i.e.(salen)CrIII, TMB, and PhIO.

Two-dimensional through-bond 1H{13C} solid-state NMR experiments utilizing fast magic angle spinning (MAS) and homonuclear multipulse 1H 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 1H 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.

Indirectly detected, through-bond NMR correlation spectra between 13C and 1H 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 13C 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.

A detailed study of the chemical structure of mesoporous silica catalysts containing rhodium ligands and nanoparticles (RhP-MSN) was carried out by multi-dimensional solid-state NMR techniques. The degree of functionalization of the rhodium–phosphinosilyl complex to the surface of the RhP-MSN channels was determined by 29Si NMR experiments. The structural assignments of the rhodium–phosphinosilyl complex were unambiguously determined by employing the novel, indirectly detected heteronuclear correlation (13C–1H and 31P–1H idHETCOR) techniques, which indicated that oxidation of the attached phosphinosilyl groups and detachment of Rh was enhanced upon syngas conversion.

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 π 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.

The recently introduced concept of the combined use of rotor assisted population transfer (RAPT) and Carr–Purcell–Meiboom–Gill (CPMG) techniques to boost the sensitivity of cross polarization (CP) based NMR experiments is applied to a synthetic zeolite (ZSM-4). The sensitivity was increased by a factor of ∼4, which enabled acquisition of a high quality two-dimensional 27Al–29Si HETCOR (heteronuclear correlation) spectrum. By separating the resonances in two dimensions, through-space connectivities between spins were revealed and the effective resolution was improved in both dimensions, which allowed determination of the existing ambiguities in spectral assignments in this material. The spectra provided clear indication of random distribution of aluminum and silicon within the ZSM-4 network. Additionally, unexpected correlations were observed between different components of inhomogeneously broadened 29Si and 27Al lines, which are most likely due to differences in the second coordination sphere environments.

The recently introduced concept of soft pulse added mixing (SPAM) is used in two-dimensional heteronuclear correlation (HETCOR) NMR experiments between half-integer quadrupolar and spin-1/2 nuclei. The experiments employ multiple quantum magic angle spinning (MQMAS) to remove the second order quadrupolar broadening and cross polarization (CP) or refocused INEPT for magnetization transfer. By using previously unexploited coherence pathways, the efficiency of SPAM-MQ-HETCOR NMR is increased by a factor of almost two without additional optimization. The sensitivity gain is demonstrated on a test sample, AlPO4-14, using CP and INEPT to correlate 27Al and 31P nuclei. SPAM-3Q-HETCOR is then applied to generate 27Al–31P spectra of the devitrified 41Na2O–20.5Al2O3–38.5P2O5 glass and the silicoaluminophosphate ECR-40. Finally, the method allowed the acquisition of the first high resolution solid-state correlation spectra between 27Al and 29Si.

A suite of one- and two-dimensional solid-state NMR methods was used to follow the complex structural changes in different types of phosphorus species in P-ZSM-5 zeolites that were generated upon treatment of H-ZSM5 zeolites with 0–15% P2O5, followed by calcination or calcination and steaming. Through space and through bond 27Al–31P correlations were used for the first time to study the interaction of phosphorus with the aluminum species. Homogeneous impregnation of phosphorus inside the zeolite channels was observed even at low concentrations. In the calcined samples, the 31P resonances were assigned mainly to orthophosphates, short chain polyphosphates and condensed phosphates. These species were converted into various types of aluminum phosphates with steaming and with the increase in P content. The mechanisms of formation of phosphates and aluminum phosphates are proposed.

Aluminum species in P-ZSM-5 zeolites were identified and quantified using 27Al MAS and MQMAS NMR methods. Samples containing between 0% and 15% P2O5 were studied after calcination or calcination followed by steaming. In addition to the tetrahedral framework aluminum, Altet-f, and the octahedral aluminum, Aloct, we also observed significant quantities of highly distorted aluminum with tetrahedral coordination, Altet-dis, and octahedrally coordinated aluminum in aluminum phosphate, Aloct-O-P. Furthermore, small quantities of the pentahedrally coordinated species Alpent-O-P were seen in samples with high phosphorus content. There was a good correlation between the amount of framework aluminum and acidity measured by TPD of n-propylamine. In contrast, the aluminum phosphate was shown to be catalytically inactive both by acidity measurements and catalytic tests.

We report a REDOR-based scheme for the measurement of heteronuclear J  -couplings in solid samples with well defined structure, containing spin-12 and quadrupolar nuclei, which can be used with selective RF irradiation to target a specific spin pair, and which provides direct information about the number of coupled spins.

Solid-state nuclear magnetic resonance is used to study the thermal decomposition of lithium tetrahydroaluminate into metallic aluminum, hydrogen and trilithium hexahydroaluminate. Aluminum sites in LiAlH4 and Li3AlH6 were characterized using static, magic angle spinning (MAS) and multiple-quantum MAS NMR. By applying the in situ NMR method, it has been demonstrated that melting is not a prerequisite for the decomposition of LiAlH4. Based on the observed data, a decomposition path has been established that is consistent with the concentrations of observed Al species at various stages of the thermally induced reaction.

A series of 10 binary zinc phosphate glasses with composition xZnO+(1−x)P2O5 (0.35≤x≤0.80) is studied by means of two-dimensional (2D) solid and liquid state 31P nuclear magnetic resonance (NMR). Double quantum (DQ) filtering method is used to probe 31P–31P connectivities through dipolar interactions in solid samples. Similar information is obtained using the correlation spectroscopy (COSY) experiment in solution. It is shown that these 2D NMR methods are capable of scrutinizing the evolution of glass structure versus x with improved precision. Varied relative concentrations of dimers, short or long branched chains and ring structures are found in glasses with different composition. Average chain length in the glasses is determined from the NMR results and compared with HPLC data and theoretical predictions. Finally, the evolution of glass structure is studied as a function of the synthesis temperature.

Nuclear magnetic resonance (NMR) spectroscopy of solids and of liquids was used to study the structure of xZnO+(1−x)P2O5 glasses prepared from a glass melt in the composition range 0.35⩽x⩽0.70. Relative concentrations of Qn units in the solid glasses have been compared with the liquid NMR spectra and with the predictions based on the reorganization theory. Disproportionation reactions in glass liquids of meta- and pyrophosphates are evident. Anisotropies of chemical shift tensor for all phosphorus units are given. Structural changes associated with CSA parameters, including the presence of rings and chains are discussed. Both solid and liquid state NMR spectra show that ultraphosphate glasses consist mainly of ring structures, while only chains are present in metaphosphates.

•New pulse sequences combining D-HMQC and MAT are described.•2D infinite-speed MAS spectra can be acquired with indirect 1H detection.•The application of MAT improves both the sensitivity and resolution.Heavy spin-1/2 nuclides are known to possess very large chemical shift anisotropies that can challenge even the most advanced magic-angle-spinning (MAS) techniques. Wide manifolds of overlapping spinning sidebands and insufficient excitation bandwidths often obfuscate meaningful spectral information and force the use of static, low-resolution solid-state (SS)NMR methods for the characterization of materials. To address these issues, we have merged fast-magic-angle-turning (MAT) and dipolar heteronuclear multiple-quantum coherence (D-HMQC) experiments to obtain D-HMQC-MAT pulse sequences which enable the rapid acquisition of 2D SSNMR spectra that correlate isotropic 1H chemical shifts to the indirectly detected isotropic “infinite-MAS” spectra of heavy spin-1/2 nuclides. For these nuclides, the combination of fast MAS and 1H detection provides a high sensitivity, which rivals the DNP-enhanced ultra-wideline SSNMR. The new pulse sequences were used to determine the Pt coordination environments in a complex mixture of decomposition products of transplatin and in a metal-organic framework with Pt ions coordinated to the linker ligands.

We examine the opportunities offered by advancements in solid-state NMR (SSNMR) methods, which increasingly rely on the use of high magnetic fields and fast magic angle spinning (MAS), in the studies of coals and other carbonaceous materials. The sensitivity of one- and two-dimensional experiments tested on several Argonne Premium coal samples is only slightly lower than that of traditional experiments performed at low magnetic fields in large MAS rotors, since higher receptivity per spin and the use of 1H detection of low-gamma nuclei can make up for most of the signal loss due to the small rotor size. The advantages of modern SSNMR methodology in these studies include improved resolution, simplicity of pulse sequences, and the possibility of using J-coupling during mixing.

Solid-state NMR spectroscopy, both conventional and dynamic nuclear polarization (DNP)-enhanced, was employed to study the spatial distribution of organic functional groups attached to the surface of mesoporous silica nanoparticles via co-condensation and grafting. The most revealing information was provided by DNP-enhanced two-dimensional 29Si–29Si correlation measurements, which unambiguously showed that post-synthesis grafting leads to a more homogeneous dispersion of propyl and mercaptopropyl functionalities than co-condensation. During the anhydrous grafting process, the organosilane precursors do not self-condense and are unlikely to bond to the silica surface in close proximity (less than 4 Å) due to the limited availability of suitably arranged hydroxyl groups.

Dynamic nuclear polarization (DNP) is used to enhance the (ultra)wideline 207Pb solid-state NMR spectra of lead compounds of relevance in the preservation of cultural heritage objects. The DNP SSNMR experiments enabled, for the first time, the detection of the basic lead carbonate phase of the lead white pigment by 207Pb SSNMR spectroscopy. Variable-temperature experiments revealed that the short T′2 relaxation time of the basic lead carbonate phase hinders the acquisition of the NMR signal at room temperature. We additionally observe that the DNP enhancement is twice as large for lead palmitate (a lead soap, which is a degradation product implicated in the visible deterioration of lead-based oil paintings), than it is for the basic lead carbonate. This enhancement has allowed us to detect the formation of a lead soap in an aged paint film by 207Pb SSNMR spectroscopy; which may aid in the detection of deterioration products in smaller samples removed from works of art.

DNP-NMR spectroscopy has been applied to enhance the signal for organic molecules adsorbed on γ-Al2O3-supported Pd nanoparticle catalysts. By offering >2500-fold time savings, the technique enabled the observation of 13C–13C cross-peaks for low coverage species, which were assigned to products from oxidative degradation of methionine adsorbed on the nanoparticle surface.

We report direct hydrogenation of MgB2 in a planetary ball mill. Magnesium borohydride, Mg(BH4)2, and various polyhedral borane anion salts have been synthesized at pressures between 50 and 350 bar H2 without the need for subsequent isothermal hydrogenation at elevated temperature and pressure. The obtained products release ∼4 wt% H2 below 390 °C, and a major portion of Mg(BH4)2 transforms back to MgB2 at around 300 °C, demonstrating the possibility of reversible hydrogen storage in an Mg(BH4)2–MgB2 system.

A comparative study of catalytic activity under homogeneous and heterogeneous conditions was carried out using the (salen)CrIII-catalyzed oxidation of tetramethylbenzidine (TMB) with iodosobenzene as a model reaction. Amine-functionalized mesoporous silica nanoparticles (MSN) were synthesized in a co-condensation reaction and functionalized with salenvia a covalent Si–C bond. A Cr(III) complex of this supported ligand, MSN-(salen)CrIII, was prepared and characterized. Data from powder XRD, BET isotherms and BJH pore size distribution all showed that MSN-(salen)CrIII still had the typical MSN high surface area, narrow pore size distribution, and ordered hexagonal pore structure, which were further confirmed by transmission electron microscopy (TEM) images. 13C and 29Si solid-state NMR data provided structural information about the catalyst and verified successful functionalization of the salen ligand and coordination to Cr(III). No unreacted salen or Cr(III) were observed. The loadings of salen and salen-CrIII complex were determined viaTGA and EDX, respectively. Both measurements indicated that approximately 0.5 mmol/g of catalyst was loaded on the surface of MSN. The oxidation of TMB with iodosobenzene using MSN-(salen)CrIII as a heterogeneous catalyst exhibited both similarities and differences with the analogous homogeneous reaction using (salen)CrIII(H2O)+ as a catalyst in aqueous acetonitrile. In the presence of 0.10 M HClO4, the two catalytic reactions proceeded at similar rates and generated the doubly oxidized product TMB2+. In the absence of acid, the radical cation TMB˙+ was produced. The kinetics of the heterogeneous reaction in the absence of added acid responded to concentrations of all three reagents, i.e.(salen)CrIII, TMB, and PhIO.

14N ultra-wideline (UW), 1H{15N} indirectly-detected HETCOR (idHETCOR) and 15N dynamic nuclear polarization (DNP) solid-state NMR (SSNMR) experiments, in combination with plane-wave density functional theory (DFT) calculations of 14N EFG tensors, were utilized to characterize a series of nitrogen-containing active pharmaceutical ingredients (APIs), including HCl salts of scopolamine, alprenolol, isoprenaline, acebutolol, dibucaine, nicardipine, and ranitidine. A case study applying these methods for the differentiation of polymorphs of bupivacaine HCl is also presented. All experiments were conducted upon samples with naturally-abundant nitrogen isotopes. For most of the APIs, it was possible to acquire frequency-stepped UW 14N SSNMR spectra of stationary samples, which display powder patterns corresponding to pseudo-tetrahedral (i.e., RR′R′′NH+ and RR′NH2+) or other (i.e., RNH2 and RNO2) nitrogen environments. Directly-excited 14N NMR spectra were acquired using the WURST-CPMG pulse sequence, which incorporates WURST (wideband, uniform rate, and smooth truncation) pulses and a CPMG (Carr-Purcell Meiboom-Gill) refocusing protocol. In certain cases, spectra were acquired using 1H → 14N broadband cross-polarization, via the BRAIN-CP (broadband adiabatic inversion – cross polarization) pulse sequence. These spectra provide 14N electric field gradient (EFG) tensor parameters and orientations that are particularly sensitive to variations in local structure and intermolecular hydrogen-bonding interactions. The 1H{15N} idHETCOR spectra, acquired under conditions of fast magic-angle spinning (MAS), used CP transfers to provide 1H–15N chemical shift correlations for all nitrogen environments, except for two sites in acebutolol and nicardipine. One of these two sites (RR′NH2+ in acebutolol) was successfully detected using the DNP-enhanced 15N{1H} CP/MAS measurement, and one (RNO2 in nicardipine) remained elusive due to the absence of nearby protons. This exploratory study suggests that this combination of techniques has great potential for the characterization of solid APIs and numerous other organic, biological, and inorganic systems.

We show both experimentally and numerically on a series of model systems that in experiments involving transfer of magnetization from 1H to the quadrupolar nuclei under magic-angle-spinning (MAS), the PRESTO technique consistently outperforms traditionally used cross polarization (CP), affording more quantitative intensities, improved lineshapes, better overall sensitivity, and straightforward optimization. This advantage derives from the fact that PRESTO circumvents the convoluted and uncooperative spin dynamics during the CP transfer under MAS, by replacing the spin-locking of quadrupolar nuclei with a single central transition selective 90° pulse and using a symmetry-based recoupling sequence in the 1H channel. This is of particular importance in the context of dynamic nuclear polarization (DNP) NMR of quadrupolar nuclei, where the efficient transfer of enhanced 1H polarization is desired to obtain the highest sensitivity.

We systematically studied the enhancement factor (per scan) and the sensitivity enhancement (per unit time) in 13C and 29Si 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 13C and 29Si, 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 1H{13C} and 29Si{1H} 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.