Solid-State Nuclear Magnetic Resonance as a Versatile Tool To Identify the Main Chemical Components of Epoxy-Based Thermosets

Cross-linked thermosets are investigated by ¹³C solid-state nuclear magnetic resonance (NMR) spectroscopy to determine their structure and to distinguish important epoxy resins and hardening agents. In addition to the epoxy resin and hardening agent, the identification of phosphorus-containing flame retardants is demonstrated by ³¹P solid-state NMR. These studies provide a spectral database for routine use, which is finally applied to analyze commercial products containing an unknown multicomponent system

Water and small organic molecules as probes for geometric confinement in well- ordered mesoporous carbon material

Mesoporous carbon materials were synthesized employing polymers and silica gels as structure directing templates. The basic physico-chemical properties of the synthetic mesoporous materials were characterized by 1H and 13C MAS solid-state NMR, X-ray diffraction, transmission electron microscopy (TEM) and nitrogen adsorption measurements. The confinement effects on small guest molecules such as water, benzene and pyridine and their interactions with the pore surface were probed by a combination of variable temperature 1H-MAS NMR and quantum chemical calculations of the magnetic shielding effect of the surface on the solvent molecules. The interactions of the guest molecules depend strongly on the carbonization temperature and the pathway of the synthesis. All the guest-molecules, water, benzene and pyridine, exhibited high-field shifts by the interaction with the surface of carbon materials. The geometric confinement imposed by the surface causes a strong depression of the melting point of the surface phase of water and benzene. The theoretical calculation of 1H NICS maps shows that the observed proton chemical shifts towards high-field values can be explained as the result of electronic ring currents localized in aromatic groups on the surface. The dependence on the distance between the proton and the aromatic surface can be exploited to estimate the average diameter of the confinement structures

Free-Standing and Self-Crosslinkable Hybrid Films by Core−Shell Particle Design and Processing

The utilization and preparation of functional hybrid films for optical sensing applications and membranes is of utmost importance. In this work, we report the convenient and scalable preparation of self-crosslinking particle-based films derived by directed self-assembly of alkoxysilane-based cross-linkers as part of a core-shell particle architecture. The synthesis of well-designed monodisperse core-shell particles by emulsion polymerization is the basic prerequisite for subsequent particle processing via the melt-shear organization technique. In more detail, the core particles consist of polystyrene (PS) or poly(methyl methacrylate) (PMMA), while the comparably soft particle shell consists of poly(ethyl acrylate) (PEA) and different alkoxysilane-based poly(methacrylate)s. For hybrid film formation and convenient self-cross-linking, different alkyl groups at the siloxane moieties were investigated in detail by solid-state Magic-Angle Spinning Nuclear Magnetic Resonance (MAS, NMR) spectroscopy revealing different crosslinking capabilities, which strongly influence the properties of the core or shell particle films with respect to transparency and iridescent reflection colors. Furthermore, solid-state NMR spectroscopy and investigation of the thermal properties by differential scanning calorimetry (DSC) measurements allow for insights into the cross-linking capabilities prior to and after synthesis, as well as after the thermally and pressure-induced processing steps. Subsequently, free-standing and self-crosslinked particle-based films featuring excellent particle order are obtained by application of the melt-shear organization technique, as shown by microscopy (TEM, SEM)

Dirhodium complex immobilization on modified cellulose for highly selective heterogeneous cyclopropanation reactions

A novel, efficient approach for the functionalization of microcrystalline cellulose (MCC) is presented. The as-obtained material allows the immobilization of chiral dirhodium catalysts preserving their enantioselectivity in asymmetric cyclopropanation reactions. As model, microcrystalline cellulose is modified with a polyethylene glycol derived linker, and Rh₂(S-DOSP)₄ is grafted on the material to produce a heterogeneous catalyst. SEM images at different stages of the immobilization show an unchanging uniform morphology, providing constantly good separation characteristics. The modification of the cellulose material with the polyethylene derived linker and the immobilization process are monitored using DNP enhanced ¹H → ¹³C CP MAS NMR, quantitative ¹⁹F MAS NMR, TGA and ICP-OES analysis, confirming the success of the immobilization as well as the stability of bonds between the used linker molecule and the cellulose material. Finally, the evaluation of the produced catalyst is demonstrated in the asymmetric cyclopropanation reaction between styrene and methyl(E)-2-diazo-4-phenylbut-3-enoate showing excellent enantioselectivity with an ee of nearly 90% over a wide temperature range as well as good recyclability characteristics in four consecutive catalysis cycles

The mechanochemical Friedel‐Crafts polymerization as a solvent‐free cross‐linking approach toward microporous polymers

Herein we report the mechanochemical Friedel‐Crafts alkylation of 1,3,5‐triphenylbenzene (TPB) with two organochloride cross‐linking agents, dichloromethane (DCM) and chloroform (CHCl₃), respectively. During a thorough milling parameter evaluation, the DCM‐linked polymers were found to be flexible and extremely sensitive toward parameter changes, which even enables the synthesis of a polymer with a SSABET of 1670 m²/g, on par with the solution‐based reference. Contrary, CHCl₃‐linked polymers are exhibiting a rigid structure, with a high porosity that is widely unaffected by parameter changes. As a result, a polymer with a SSABET of 1280 m²/g could be generated in as little as 30 minutes, outperforming the reported literature analogue in terms of synthesis time and SSABET. To underline the environmental benefits of our fast and solvent‐free synthesis approach, the green metrics are discussed, revealing an enhancement of the mass intensity, mass productivity and the E‐factor, as well as of synthesis time and the work‐up in comparison to the classical synthesis. Therefore, the mechanochemical polymerization is presented as a versatile tool, enabling the generation of highly porous polymers within short reaction times, with a minimal use of chlorinated cross‐linker and with the possibility of a post polymerization modification

SiCO Ceramics as Storage Materials for Alkali Metals/Ions: Insights on Structure Moieties from Solid‐State NMR and DFT Calculations

Polymer‐derived silicon oxycarbide ceramics (SiCO) have been considered as potential anode materials for lithium‐ and sodium‐ion batteries. To understand their electrochemical storage behavior, detailed insights into structural sites present in SiCO are required. In this work, the study of local structures in SiCO ceramics containing different amounts of carbon is presented. ¹³C and ²⁹Si solid‐state MAS NMR spectroscopy combined with DFT calculations, atomistic modeling, and EPR investigations, suggest significant changes in the local structures of SiCO ceramics even by small changes in the material composition. The provided findings on SiCO structures will contribute to the research field of polymer‐derived ceramics, especially to understand electrochemical storage processes of alkali metal/ions such as Na/Na⁺ inside such networks in the future

Dirhodium Coordination Polymers for Asymmetric Cyclopropanation of Diazooxindoles with Olefins: Synthesis and Spectroscopic Analysis

A facile approach is reported for the preparation of dirhodium coordination polymers [Rh₂(L1)₂]n (Rh₂-L1) and [Rh₂(L2)₂]n (Rh₂-L2; L1=N,N’-(pyromellitoyl)-bis-L-phenylalanine diacid anion, L2=bis-N,N’-(L-phenylalanyl) naphthalene-1,4,5,8-tetracarboxylate diimide) from chiral dicarboxylic acids by ligand exchange. Multiple techniques including FTIR, XPS, and ¹H→¹³C CP MAS NMR spectroscopy reveal the formation of the coordination polymers. ¹⁹F MAS NMR was utilized to investigate the remaining TFA groups in the obtained coordination polymers, and demonstrated near-quantitative ligand exchange. DR-UV-vis and XPS confirm the oxidation state of the Rh center and that the Rh-single bond in the dirhodium node is maintained in the synthesis of Rh₂-L1 and Rh₂-L2. Both coordination polymers exhibit excellent catalytic performance in the asymmetric cyclopropanation reaction between styrene and diazooxindole. The catalysts can be easily recycled and reused without significant reduction in their catalytic efficiency

A novel strategy for site selective spin-labeling to investigate bioactive entities by DNP and EPR spectroscopy

A novel specific spin-labeling strategy for bioactive molecules is presented for eptifibatide (integrilin) an antiplatelet aggregation inhibitor, which derives from the venom of certain rattlesnakes. By specifically labeling the disulfide bridge this molecule becomes accessible for analytical techniques such as Electron Paramagnetic Resonance (EPR) and solid state Dynamic Nuclear Polarization (DNP). The necessary spin-label was synthesized and inserted into the disulfide bridge of eptifibatide via reductive followed by insertion by a double Michael addition under physiological conditions. This procedure is universally applicable for disulfide containing biomolecules and is expected to preserve their tertiary structure with minimal change due to the small size of the label and restoring of the previous disulfide connection. HPLC and MS analysis show the successful introduction of the spin label and EPR spectroscopy confirms its activity. DNP-enhanced solid state NMR experiments show signal enhancement factors of up to 19 in ¹³C CP MAS experiments which corresponds to time saving factors of up to 361. This clearly shows the high potential of our new spin labeling strategy for the introduction of site selective radical spin labels into biomolecules and biosolids without compromising its conformational integrity for structural investigations employing solid-state DNP or advanced EPR techniques

SiCN Ceramics as Electrode Materials for Sodium/Sodium Ion Cells – Insights from ²³Na In‐Situ Solid‐State NMR

Polymer-derived silicon carbonitride ceramic (SiCN) is used as an electrode material to prepare cylindrical sodium/sodium ion cells for solid-state NMR investigations. During galvanostatic cycling structural changes of the environment of sodium/sodium ions are investigated by applying ²³Na in-situ solid-state NMR. Changes of the signals assigned to sodium metal, intercalated sodium cation and sodium cation originating from the electrolyte are monitored as well as the occurrence of an additional signal in the region of metallic sodium. The intensity of this additional signal changes periodically with the cycling process indicating the reversibility of structures formed and deformed during the galvanostatic cycling. To identify interactions of sodium/sodium ions with the SiCN electrode materials, the cycled SiCN material is studied by ²³Na ex-situ MAS NMR at high spinning rates of 20 and 50 kHz to obtain appropriate spectral resolution

Selective C-H Activation at a Molecular Rhodium Sigma-Alkane Complex by Solid/Gas Single-Crystal to Single-Crystal H/D Exchange

The controlled catalytic functionalization of alkanes via the activation of C-H bonds is a significant challenge. Although C-H activation by transition metal catalysts is often suggested to operate via intermediate σ-alkane complexes, such transient species are difficult to observe due to their instability in solution. This instability may be controlled by use of solid/gas synthetic techniques that enable the isolation of single-crystals of well-defined σ-alkane complexes. Here we show that, using this unique platform, selective alkane C-H activation occurs, as probed by H/D exchange using D2, and that five different isotopomers/isotopologues of the σ-alkane complex result, as characterized by single-crystal neutron diffraction studies for three examples. Low-energy fluxional processes associated with the σ-alkane ligand are identified using variable-temperature X-ray diffraction, solid-state NMR spectroscopy, and periodic DFT calculations. These observations connect σ-alkane complexes with their C-H activated products, and demonstrate that alkane-ligand mobility, and selective C-H activation, are possible when these processes occur in the constrained environment of the solid-state