Determining the surface structure of silicated alumina catalysts via isotopic enrichment and dynamic nuclear polarization surface-enhanced NMR spectroscopy

We would like to thank SASOL and EPSRC (EP/L505079/1) for studentship funding for AGMR. SEA would also like to thank the Royal Society and Wolfson Foundation for a merit award. PBW would like to thank the Royal Society for the award of an Industry Fellowship. The University of Nottingham DNP MAS NMR Facility used in this research was funded by EPSRC and the University of Nottingham, and assistance from the Facility Manager (Subhradip Paul, University of Nottingham) is also acknowledged. This work was also supported by ERC Advanced Grant No. 320860. The research data (and/or materials) supporting this publication can be accessed at DOI: http://dx.doi.org/10.17630/00533fb3-e938-498d-bfe4-f07d82c309d6.Isotopic enrichment of 29Si and DNP-enhanced NMR spectroscopy are combined to determine the detailed surface structure of a silicated alumina catalyst. The significant sensitivity enhancement provided by DNP is vital to the acquisition of multinuclear and multidimensional experiments that provide information on the atomic-level structure of the species present at the surface. Isotopic enrichment not only facilitates spectral acquisition, particularly given the low (1.5 wt%) Si loading, but also enables spectra with higher resolution than those acquired using DNP to be obtained. The unexpected similarity of conventional, CP and DNP NMR spectra is attributed to the presence of adventitious surface water that forms a sufficiently dense 1H network at the silica surface so as to mediate efficient polarization transfer to all Si species regardless of their chemical nature. Spectra reveal the presence of Si-O-Si linkages at the surface (identified as Q4(3Al)-Q4(3Al)), and confirm that the anchoring of the surface overlayer with the alumina occurs through AlIV and AlV species only. This suggests the presence of Q3/Q4 Si at the surface affects the neighboring Al species, modifying the surface structure and making it less likely AlVI environments are in close spatial proximity. In contrast, Q1/Q2 species, bonded to the surface by fewer covalent bonds, have less of an effect on the surface and more AlVI species are consequently found nearby. The combination of isotropic enrichment and DNP provides a definitive and fully quantitative description of the Si-modified alumina surface, and we demonstrate that almost one-third of the silicon at the surface is connected to another Si species, even at the low level of coverage used, lowering the propensity for the formation of Brønsted acid sites. This suggests that a variation in the synthetic procedure might be required to obtain a more even coverage for optimum performance. The work here will allow for more rigorous future investigations of structure-function relationships in these complex materials.PostprintPeer reviewe

Crystal structure of tetrabutylammonium bromide–1,2-diiodo-3,4,5,6-tetrafluorobenzene–dichloromethane (2/2/1)

The crystallization of a 1:1 molar solution of 1,2-diiodo-3,4,5,6-tetrafluorobenzene (o-DITFB) and tetrabutylammonium bromide (n-Bu4NBr) from dichloromethane yielded pure white crystals of a halogen-bonded compound, C16H36N+·Br−·C6F4I2·0.5CH2Cl2 or [(n-Bu4NBr)(o-DITFB)]·0.5CH2Cl2. The compound may be described as a quaternary system and may be classified as a salt–cocrystal solvate. The asymmetric unit contains one molecule of solvent, two o-DITFB molecules, two cations (n-Bu4N+) and two crystallographically distinct bromide ions [θI...Br-...I = 144.18 (1) and 135.35 (1)°]. The bromide ion is a bidentate halogen-bond acceptor which interacts with two covalently bonded iodines (i.e. halogen-bond donors), resulting in a one-dimensional polymeric zigzag chain network approximately along the a axis. The observed short contacts and angles are characteristic of the non-covalent interaction [dC—I...Br = 3.1593 (4)–3.2590 (5) Å; θC—I...Br = 174.89 (7) and 178.16 (7)°]. It is noted that iodine acts as both a halogen-bond donor and a weak CH hydrogen-bond acceptor, while the bromide ions act as acceptors for weak CH hydrogen bonds and halogen bonds

Intracluster ligand rearrangement: an NMR-based thermodynamic study

Ligand and metal exchange reactions are powerful methods to tailor the properties of atomically precise metal nanoclusters. Hence, a deep understanding of the mechanisms behind the dynamics that rule the ligand monolayer is crucial for its specific functionalization. Combining variable-temperature NMR experiments and dynamic-NMR simulations, we extract the thermodynamic activation parameters of a new exchange reaction: the intracluster ligand rearrangement between the two symmetry-unique positions in [Ag25(DMBT)18]− and [Ag24Au(DMBT)18]− clusters. We report for the first time that this peculiar intracluster modification does not seem to proceed via metal–sulphur bond breaking and follows a first-order rate law, being therefore a process independent from the well-described collisional ligand exchange.</p

Core-Shell Structure of Organic Crystalline Nanoparticles Determined by Relayed Dynamic Nuclear Polarization NMR

The structure of crystalline nanoparticles (CNPs) is determined using dynamic nuclear polarization (DNP) enhanced NMR spectroscopy experiments. The CNPs are composed of a crystalline core containing an active pharmaceutical ingredient (compound P), coated with a layer of PEG (DSPE-PEG 5000) located at the crystal surface, in a D2O suspension. Relayed DNP experiments are performed to study H-1-H-1 spin diffusion and to determine the size of the crystalline core as well as the thickness of the PEG overlayer. This is achieved through selective doping to create a heterogeneous system in which the D2O contains glycerol and organic radicals, which act as polarization sources, and the CNPs are exempt of radical molecules. We observe features that are characteristic of a core-shell system: high and constant DNP enhancement for components located in the surrounding radical solution, short build-up times for the PEG layer, and longer build-up times and time dependent enhancements for compound P. By comparing numerical simulations and experimental data, we propose a structural model for the CNPs with a core-shell organization and a high affinity between the radical and the PEG molecules

Role of Intercluster and Interligand Dynamics of [Ag₂₅(DMBT)₁₈]⁻ Nanoclusters by Multinuclear Magnetic Resonance Spectroscopy

Even though gold and silver belong to the same group of the periodic table, they provide significantly different nanocluster (NC) structures in terms of both nuclearities and ligand coordination motifs. Until today, only one isostructural gold analogue has been found for silver nanoclusters, [Ag25(DMBT)18]−, and it is only obtained by using 2,4-dimethylbenzenethiol (DMBT) as the ligand. Our study of the dynamics of DMBT ligands in metal NCs using multinuclear magnetic resonance spectroscopy demonstrates that DMBT favors the formation of two types of interligand interactions, i.e., H−π and π–π. These interactions stabilize the entire nanocluster, yet we observe that thermal and chemical stimuli have the capability to weaken the [Ag25(DMBT)18]− structure triggering irreversible decomposition. Moreover, employing 2D-NMR spectroscopy we demonstrate the intercluster exchange of DMBT ligands and their temperature dependence

Chemical exchange at the ferroelectric phase transition of lead germanate revealed by solid state Pb-207 nuclear magnetic resonance

The nature of the dynamics and structural changes that take place at the ferroelectric phase transition in lead oxides is a rich field of study. Solid-state nuclear magnetic resonance (NMR) of Pb-207 is well suited to study the local structure and disorder in lead oxide ferroelectric transitions at the atomic level. However, very large Pb-207 shielding anisotropy results in poor resolution in 1D static and magic angle spinning (MAS) NMR spectra. We address this problem by using short high-power adiabatic pulses (SHAPs) with magic-angle-turning sequences to correlate the isotropic and anisotropic parts of the Pb-207 chemical shift tensor in a 2D NMR experiment, yielding resolved Pb-207 NMR spectra of the nine distinct lead sites in uniaxial ferroelectric lead germanate (Pb5Ge3O11). Using this technique we detect the magnetic environments of displaced Pb2+ ions and unambiguously identify the nature of the phase transition as mixed displacive and order-disorder. We also observe that the atomic-level process responsible for the phase transition in ferroelectric lead germanate is chemical exchange on the kilohertz timescale. We derive an activation energy of 103.4 +/- 1.7 kJ mol(-1) and compare it to dielectric spectroscopy studies on similar materials. These results show that this method can be used to characterize ferroelectric phase transitions of complex materials with high resolution using nuclei that are typically inaccessible due to their large shielding anisotropy

Direct Investigation of Halogen Bonds by Solid-State Multinuclear Magnetic Resonance Spectroscopy and Molecular Orbital Analysis

Noncovalent interactions play a ubiquitous role in the structure, stability, and reactivity of a wide range of molecular and ionic cocrystals, pharmaceuticals, materials, and biomolecules. The halogen bond continues to be the focus of much attention, due in part to its strength and unique directionality. Here, we report a multifaceted experimental and computational study of halogen bonds in the solid state. A series of cocrystals of three different diiodobenzene molecules and various onium halide (Cl^– or Br^–) salts, designed to exhibit moderately strong halogen bonds (C–I···X^–) in the absence of competing hydrogen bonds, has been prepared and characterized by single-crystal X-ray diffraction. Interestingly, a wide range of geometries about the halide anion are observed. ^35/37Cl and ^79/81Br solid-state NMR spectroscopy is applied to characterize the nuclear quadrupolar coupling constants (<i>C</i>_Q) and asymmetry parameters (η_Q) for the halogen-bonded anions at the center of bonding environments ranging from approximately linear to distorted square planar to octahedral. The relationship between the halogen bond environment and the quadrupolar parameters is elucidated through a natural localized molecular orbital (NLMO) analysis in the framework of density functional theory (DFT). These calculations reveal that the lone pair type orbitals on the halogen-bonded anion govern the magnitude and orientation of the quadrupolar tensor as the geometry about the anion is systematically altered. In −C–I<b>···</b>X^–<b>···</b>I–C– environments, the value of η_Q is well-correlated to the I<b>···</b>X^–<b>···</b>I angle. ¹³C NMR and DFT calculations show a correlation between chemical shifts and halogen bond strength (through the C–I distance) in <i>o</i>-diiodotetrafluorobenzene cocrystals. Overall, this work provides a chemically intuitive understanding of the connection between the geometry and electronic structure of halogen bonds and various NMR parameters with the aid of NLMO analysis

Direct Investigation of Halogen Bonds by Solid-State Multinuclear Magnetic Resonance Spectroscopy and Molecular Orbital Analysis

Noncovalent interactions play a ubiquitous role in the structure, stability, and reactivity of a wide range of molecular and ionic cocrystals, pharmaceuticals, materials, and biomolecules. The halogen bond continues to be the focus of much attention, due in part to its strength and unique directionality. Here, we report a multifaceted experimental and computational study of halogen bonds in the solid state. A series of cocrystals of three different diiodobenzene molecules and various onium halide (Cl^– or Br^–) salts, designed to exhibit moderately strong halogen bonds (C–I···X^–) in the absence of competing hydrogen bonds, has been prepared and characterized by single-crystal X-ray diffraction. Interestingly, a wide range of geometries about the halide anion are observed. ^35/37Cl and ^79/81Br solid-state NMR spectroscopy is applied to characterize the nuclear quadrupolar coupling constants (<i>C</i>_Q) and asymmetry parameters (η_Q) for the halogen-bonded anions at the center of bonding environments ranging from approximately linear to distorted square planar to octahedral. The relationship between the halogen bond environment and the quadrupolar parameters is elucidated through a natural localized molecular orbital (NLMO) analysis in the framework of density functional theory (DFT). These calculations reveal that the lone pair type orbitals on the halogen-bonded anion govern the magnitude and orientation of the quadrupolar tensor as the geometry about the anion is systematically altered. In −C–I<b>···</b>X^–<b>···</b>I–C– environments, the value of η_Q is well-correlated to the I<b>···</b>X^–<b>···</b>I angle. ¹³C NMR and DFT calculations show a correlation between chemical shifts and halogen bond strength (through the C–I distance) in <i>o</i>-diiodotetrafluorobenzene cocrystals. Overall, this work provides a chemically intuitive understanding of the connection between the geometry and electronic structure of halogen bonds and various NMR parameters with the aid of NLMO analysis

Time‐Dependent Hydrogen Bond Network Formation in Glycerol‐Based Deep Eutectic Solvents

Over the last few years, Deep Eutectic Solvents have gained popularity as a novel class of green solvents, due to their feasible synthesis and overall low production costs. The properties of glycerol (Gly)-based Deep Eutectic Solvents are frequently associated with the formation of an extended hydrogen bond network. In this study, two-dimensional Nuclear Magnetic Resonance (NMR) spectroscopy is employed to analyse the effect of glycerol oversaturation of the hydrogen bond acceptor, choline chloride (ChCl) on the structural arrangement of glyceline (molar ratio 1 : 2 ChCl:Gly), selected to represent Gly-based Deep Eutectic Solvents. The rearrangement of glycerol molecules, additionally trapping water molecules inside of isolated clusters, is revealed during a time-resolved analysis, performed in the presence of various fractions of water added to solvent. 200 % oversaturated Deep Eutectic Solvent (1 : 4 ChCl:Gly) is found to be a suitable cryoprotectant candidate, based on the revealed glycerol-water interactions