@article{63992, abstract = {{Solid-state NMR combined with dynamic nuclear polarization (DNP NMR) is used to study hydration processes in tricalcium silicate (Ca3SiO5, abbreviated as C3S) samples. The studied C3S samples have experienced early stage hydration (1–30 h) and slow aging (9 years) processes. The appearance of Q3 and Q4 lines in the 29Si MAS and 1H → 29Si CP MAS NMR spectra obtained for partly hydrated C3S samples indicated the formation of amorphous silica which corresponds to their carbonation, which was corroborated by complementary FTIR data. Significant DNP signal enhancements obtained for the studied samples allowed to further investigate the C3S carbonation process in detail employing the 1H → 29Si CP MAS FSLG HETCOR technique. Finally, DNP enhanced 1H → 13C CP MAS and 1H → 13C CP MAS FSLG HETCOR techniques enabled to directly observe the formation of carbonate moieties in partly hydrated C3S samples.}}, author = {{Klimavicius, Vytautas and Hilbig, Harald and Gutmann, Torsten and Buntkowsky, Gerd}}, issn = {{1932-7447}}, journal = {{Journal of Physical Chemistry C}}, number = {{13}}, pages = {{7321–7328}}, publisher = {{American Chemical Society}}, title = {{{Direct Observation of Carbonate Formation in Partly Hydrated Tricalcium Silicate by Dynamic Nuclear Polarization Enhanced NMR Spectroscopy}}}, doi = {{10.1021/acs.jpcc.0c10382}}, volume = {{125}}, year = {{2021}}, } @article{63982, abstract = {{Polyethylene glycol (PEG) is gaining interest as an alternative green solvent in chemical synthesis and processing. This report presents density and viscosity data from 293.15 K to 358.15 K as well as self-diffusion coefficient data from 298.15 K to 358.15 K for oligomers of PEG from di- to nonaethylene glycol. The results were obtained by extrapolation from measurement series where water, the most common impurity in PEGs, was intentionally added in several increments. The obtained results are carefully compared to literature data, which are widely available only for density and viscosity, and only for the lower oligomers. Densities are found to be linearly dependent on temperatures for all studied oligomers. The temperature dependence of viscosity and self-diffusion coefficients show only slight deviations from the Arrhenius equation over the investigated temperature range. The activation energies obtained from the viscosity data agree well with the activation energies from the self-diffusion coefficient data and appear to be linearly dependent with respect to the number of ethylene oxide repeat units in the PEG oligomer. This linearity combined with the observation that the pre-exponential factor appears to be the same for all studied oligomers may serve as a tool to estimate viscosities and self-diffusion coefficients for higher oligomers within the investigated temperature range. The densities of the oligomers all fall within a rather narrow range without a clear trend in homologous series.}}, author = {{Hoffmann, Markus M. and Horowitz, Rachel H. and Gutmann, Torsten and Buntkowsky, Gerd}}, journal = {{Journal of Chemical and Engineering Data}}, number = {{6}}, pages = {{2480–2500}}, publisher = {{American Chemical Society}}, title = {{{Densities, Viscosities, and Self-Diffusion Coefficients of Ethylene Glycol Oligomers}}}, doi = {{10.1021/acs.jced.1c00101}}, volume = {{66}}, year = {{2021}}, } @article{63986, abstract = {{13C and 15N solid-state nuclear magnetic resonance (NMR) combined with dynamic nuclear polarization (DNP) is used to investigate the structure of dye-doped biopolymer-based materials that can be used in amplified spontaneous emission (ASE) experiments. By comparing calligraphic paper prepared from cellulose and scaffolds prepared from chitosan as substrates, differences in the interactions of the carrier material with the dye molecule Calcofluor White are obtained. These are most probably induced by structural changes of the carrier material due to its interaction with water forming hydrogen bonds. Such structural differences may explain the obtained variation of the emission wavelength of Calcofluor White doped on these substrates in ASE experiments.}}, author = {{Höfler, Mark V. and Hoinka, Nicolai and Schäfer, Timmy and Horn, Marilia and Aussenac, Fabien and Fuhrmann-Lieker, Thomas and Gutmann, Torsten}}, issn = {{1932-7447}}, journal = {{Journal of Physical Chemistry C}}, number = {{39}}, pages = {{21550–21558}}, publisher = {{American Chemical Society}}, title = {{{Light Amplification Materials Based on Biopolymers Doped with Dye Molecules—Structural Insights from 15N and 13C Solid-State Dynamic Nuclear Polarization}}}, doi = {{10.1021/acs.jpcc.1c06737}}, volume = {{125}}, year = {{2021}}, } @article{63973, abstract = {{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.}}, author = {{Herr, Kevin and Fleckenstein, Max and Brodrecht, Martin and Höfler, Mark V. and Heise, Henrike and Aussenac, Fabien and Gutmann, Torsten and Reggelin, Michael and Buntkowsky, Gerd}}, journal = {{Scientific Reports}}, number = {{1}}, pages = {{13714}}, title = {{{A novel strategy for site selective spin-labeling to investigate bioactive entities by DNP and EPR spectroscopy}}}, doi = {{10.1038/s41598-021-92975-6}}, volume = {{11}}, year = {{2021}}, } @article{63947, abstract = {{The interactions of molecules such as surfactants with solid interfaces are not sufficiently understood since their study is challenging with standard spectroscopic methods. In this work, octanol-d17 as a model system confined in the mesopores of SBA-15 is studied by variable temperature deuterium solid-state NMR, and the findings are compared to those of bulk octanol-d17. The magic angle spinning (MAS) as well as the static, nonspinning case, are investigated, showing that the described observations are independent of the applied NMR method. The 2H NMR spectra of both the bulk and the confined octanol-d17 show a large and a small quadrupolar Pake pattern below the melting point, suggesting a rigid conformation of the observed molecules with a 3-fold jump motion of the terminal CD3-group. Apart from the melting of the solid, no other phase transition is observed for either sample. The confined octanol-d17 forms a pore solid, exhibiting a melting point 38 K lower than bulk octanol-d17. The interactions of the molecule with the mesoporous SBA-15 bring about a distribution of activation energies for the melting process, resulting in a gradual melting process.}}, author = {{Döller, Sonja C. and Brodrecht, Martin and Haro Mares, Nadia B. and Breitzke, Hergen and Gutmann, Torsten and Hoffmann, Markus and Buntkowsky, Gerd}}, issn = {{1932-7447}}, journal = {{Journal of Physical Chemistry C}}, number = {{45}}, pages = {{25155–25164}}, publisher = {{American Chemical Society}}, title = {{{Deuterium NMR Studies of the Solid–Liquid Phase Transition of Octanol-d17 Confined in SBA-15}}}, doi = {{10.1021/acs.jpcc.1c05873}}, volume = {{125}}, year = {{2021}}, } @article{64037, abstract = {{Cross-linked thermosets are investigated by C-13 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 P-31 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.}}, author = {{Schäfer, T. and Buntkowsky, G. and Gutmann, Torsten}}, journal = {{ACS Omega}}, number = {{10}}, pages = {{5412–5420}}, title = {{{Solid-State Nuclear Magnetic Resonance as a Versatile Tool To Identify the Main Chemical Components of Epoxy-Based Thermosets}}}, doi = {{10.1021/acsomega.9b04482}}, volume = {{5}}, year = {{2020}}, } @article{64004, abstract = {{Abstract 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.}}, author = {{Li, Zhenzhong and Rösler, Lorenz and Herr, Kevin and Brodrecht, Martin and Breitzke, Hergen and Hofmann, Kathrin and Limbach, Hans-Heinrich and Gutmann, Torsten and Buntkowsky, Gerd}}, issn = {{2192-6506}}, journal = {{ChemPlusChem}}, number = {{8}}, pages = {{1737–1746}}, title = {{{Dirhodium Coordination Polymers for Asymmetric Cyclopropanation of Diazooxindoles with Olefins: Synthesis and Spectroscopic Analysis}}}, doi = {{10.1002/cplu.202000421}}, volume = {{85}}, year = {{2020}}, } @article{63985, abstract = {{An experimental study is presented for the reverse micellar system of 15% by mass polydisperse hexaethylene glycol monodecylether (C10E6) in cyclohexane with varying amounts of added water up to 4% by mass. Measurements of viscosity and self-diffusion coefficients were taken as a function of temperature between 10 and 45 °C at varying sample water loads but fixed C10E6/cyclohexane composition. The results were used to inspect the validity of the Stokes–Einstein equation for this system. Unreasonably small reverse average micelle radii and aggregation numbers were obtained with the Stokes–Einstein equation, but reasonable values for these quantities were obtained using the ratio of surfactant-to-cyclohexane self-diffusion coefficients. While bulk viscosity increased with increasing water load, a concurrent expected decrease of self-diffusion coefficient was only observed for the surfactant and water but not for cyclohexane, which showed independence of water load. Moreover, a spread of self-diffusion coefficients was observed for the protons associated with the ethylene oxide repeat unit in samples with polydisperse C10E6 but not in a sample with monodisperse C10E6. These findings were interpreted by the presence of reverse micelle to reverse micelle hopping motions that with higher water load become increasingly selective toward C10E6 molecules with short ethylene oxide repeat units, while those with long ethylene oxide repeat units remain trapped within the reverse micelle because of the increased hydrogen bonding interactions with the water inside the growing core of the reverse micelle. Despite the observed breakdown of the Stokes–Einstein equation, the temperature dependence of the viscosities and self-diffusion coefficients was found to follow Arrhenius behavior over the investigated range of temperatures.}}, author = {{Hoffmann, Markus M. and Too, Matthew D. and Vogel, Michael and Gutmann, Torsten and Buntkowsky, Gerd}}, journal = {{Journal of Physical Chemistry B}}, number = {{41}}, pages = {{9115–9125}}, publisher = {{American Chemical Society}}, title = {{{Breakdown of the Stokes–Einstein Equation for Solutions of Water in Oil Reverse Micelles}}}, doi = {{10.1021/acs.jpcb.0c06124}}, volume = {{124}}, year = {{2020}}, } @article{63976, abstract = {{Mesoporous silica materials (SBA-15) with surfaces modified with aminopropyltriethoxysilane (APTES) of two different surface coverages were synthesized, and their structural pore characteristics were analyzed. These two mesoporous silica materials were impregnated with various solutions of radicals in a nonionic surfactant solvent. Differential scanning calorimetry (DSC) analysis of the impregnated mesoporous silica materials confirmed that the surfactant solutions were confined into the pores. Dynamic nuclear polarization (DNP)-enhanced solid-state 13C magic-angle spinning (MAS) NMR spectra recorded for these impregnated mesoporous silica materials showed the presence of superimposed spectra from direct and indirect channel polarization transfer processes not only for the confined surfactant solvent but also for the APTES surface modification. The observation of the indirect channel resonances implies that the surfactant solvents as well as the APTES exhibit molecular motions with correlation times on the order of or faster than the inverse Larmor frequency. Such motions are unexpected at the experimental temperature conditions of ∼120 K in particular for the immobilized APTES. Spectral line widths and intensities of the observed 13C MAS NMR spectra were sensitive to the specific combination of the radical, surfactant solvent, and APTES surface coverage. One particular combination showed identical widths and intensities for the direct and the oppositely phased indirect channel resonances, resulting in a blank spectrum. The differences in line widths and intensities are discussed with respect to the structural organization of the polarizing agent and surfactant within the pores and the complex interplay of intermolecular interactions between these constituents.}}, author = {{Hoffmann, Markus M. and Bothe, Sarah and Brodrecht, Martin and Klimavicius, Vytautas and Haro-Mares, Nadia B. and Gutmann, Torsten and Buntkowsky, Gerd}}, issn = {{1932-7447}}, journal = {{Journal of Physical Chemistry C}}, number = {{9}}, pages = {{5145–5156}}, publisher = {{American Chemical Society}}, title = {{{Direct and Indirect Dynamic Nuclear Polarization Transfer Observed in Mesoporous Materials Impregnated with Nonionic Surfactant Solutions of Polar Polarizing Agents}}}, doi = {{10.1021/acs.jpcc.9b10504}}, volume = {{124}}, year = {{2020}}, } @article{63954, abstract = {{We propose a mild and versatile synthesis protocol based on N-hydroxysuccinimide-activated esters for the introduction of new functionalities to cellulose, using as starting point established protocols for the tosylation of cellulose and its subsequent reaction with a diamine linker. As a proof of concept, we describe the functionalization of microcrystalline cellulose with a N-hydroxysuccinimide-activated ester of benzophenone, a photoreactive functional group. Irradiation of the final product with UV light yields a self-standing polymer film and is expected to result in cross-linking among cellulose chains. To monitor structural changes at the molecular level, each functionalization step is characterized by a multinuclear solid-state NMR approach. DNP-enhanced 15N CP MAS NMR experiments reveal the formation of the amide bond to the photoreactive linker and deliver further information about the binding situation of nitrogen-containing groups in these materials. The flexible synthesis protocol described here can be easily extended to a broad range of other functionalities of interest, both for the cellulose and macromolecular research.}}, author = {{Groszewicz, Pedro B. and Mendes, Pedro and Kumari, Bharti and Lins, Jonas and Biesalski, Markus and Gutmann, Torsten and Buntkowsky, Gerd}}, issn = {{0969-0239}}, journal = {{Cellulose}}, pages = {{1239–1254}}, title = {{{N-Hydroxysuccinimide-activated esters as a functionalization agent for amino cellulose: synthesis and solid-state NMR characterization}}}, doi = {{10.1007/s10570-019-02864-5}}, volume = {{27}}, year = {{2020}}, } @article{63953, abstract = {{Most porous polymers are notoriously hard to characterize due to their amorphous and completely insoluble nature. On the other hand, they are an interesting class of materials for sorption, catalytic, and electrode applications, thus they warrant in-depth studies. In this contribution, we elaborate on the possibilities that dynamic nuclear polarization offers towards the investigation of the structure of porous polymers. We discuss the advantages and disadvantages of this technique in the investigation of model polymers.}}, author = {{Grätz, Sven and Olivera Junior, Marcos and Gutmann, Torsten and Borchardt, Lars}}, journal = {{Physical Chemistry Chemical Physics}}, number = {{40}}, pages = {{23307–23314}}, publisher = {{The Royal Society of Chemistry}}, title = {{{A comprehensive approach for the characterization of porous polymers using 13C and 15N dynamic nuclear polarization NMR spectroscopy}}}, doi = {{10.1039/D0CP04010J}}, volume = {{22}}, year = {{2020}}, } @article{64035, abstract = {{Trityl and nitroxide radicals are connected by pi-topologically controlled aryl linkers, generating genuinely g-engineered biradicals. They serve as a typical model for biradicals in which the exchange (J) and hyperfine interactions compete with the g-difference electronic Zeeman interactions. The magnetic properties underlying the biradical spin Hamiltonian for solution, including J’s, have been determined by multifrequency CW-ESR and H-1 ENDOR spectroscopy and compared with those obtained by quantum chemical calculations. The experimental J values were in good agreement with the quantum chemical calculations. The g-engineered biradicals have been tested as a prototype for AWG (Arbitrary Wave Generator)-based spin manipulation techniques, which enable GRAPE (GRAdient Pulse Engineering) microwave control of spins in molecular magnetic resonance spectroscopy for use in molecular spin quantum computers, demonstrating efficient signal enhancement of specific weakened hyperfine signals. Dynamic nuclear polarization (DNP) effects of the biradicals for 400 MHz nuclear magnetic resonance signal enhancement have been examined, giving efficiency factors of 30 for H-1 and 27.8 for C-13 nuclei. The marked DNP results show the feasibility of these biradicals for hyperpolarization.}}, author = {{Sato, K. and Hirao, R. and Timofeev, I. and Krumkacheva, O. and Zaytseva, E. and Rogozhnikova, O. and Tormyshev, V. M. and Trukhin, D. and Bagryanskaya, E. and Gutmann, Torsten and Klimavicius, C. and Buntkowsky, G. and Sugisaki, K. and Nakazawa, S. and Matsuoka, H. and Toyota, K. and Shiomi, D. and Takui, T.}}, journal = {{Journal of Physical Chemistry A}}, number = {{34}}, pages = {{7507–7517}}, title = {{{Trityl-Aryl-Nitroxide-Based Genuinely g-Engineered Biradicals, As Studied by Dynamic Nuclear Polarization, Multifrequency ESR/ENDOR, Arbitrary Wave Generator Pulse Microwave Waveform Spectroscopy, and Quantum Chemical Calculations}}}, doi = {{10.1021/acs.jpca.9b07169}}, volume = {{123}}, year = {{2019}}, } @article{64038, abstract = {{An efficient approach for the characterization of core–shell polymer hybrid nanoparticles is presented. Selective signal amplification by dynamic nuclear polarization (DNP) is employed to shed more light on the molecular structure of surface sites and shell of the particles. DNP-enhanced 29Si solid-state NMR is used to clearly prove the core–shell structure of the nanoparticles as well as the success of their functionalization with low amounts of trimethylsiloxy groups. By combination of DNP-enhanced 1H → 29Si and 1H → 13C cross-polarization magic-angle-spinning experiments, differently substituted alkoxysilane moieties, namely, methacryloxypropyltriethoxysilane, 3-methacryloxypropyltriisopropoxysilane, and 3-methacryloxypropyltris(methoxyethoxy)silane, are investigated, revealing various cross-linking capabilities of the particle shell. This knowledge about efficiency of surface functionalization and cross-linking sites strongly influences the application and properties of the core–shell polymer hybrid particles, for instance, as materials for photonic crystals, particle film formation, and coatings. This is of high importance for the design of tailor-made core–shell particle architectures.}}, author = {{Schäfer, Timmy and Vowinkel, Steffen and Breitzke, Hergen and Gallei, Markus and Gutmann, Torsten}}, issn = {{1932-7447}}, journal = {{Journal of Physical Chemistry C}}, number = {{1}}, pages = {{644–652}}, publisher = {{American Chemical Society}}, title = {{{Selective DNP Signal Amplification To Probe Structures of Core–Shell Polymer Hybrid Nanoparticles}}}, doi = {{10.1021/acs.jpcc.8b07969}}, volume = {{123}}, year = {{2019}}, } @article{64033, abstract = {{Abstract The reactions of three metal nanoparticle (MNP) systems Ru/dppb, RuPt/dppb, Pt/dppb (dppb=1,4-bis(diphenylphosphino)butane) with gaseous D2 at room temperature and different gas pressures have been studied using 1H gas phase NMR, GC-MS and solid state 13C and 31P MAS NMR. The main product is gaseous HD arising from the reaction of D2 with surface hydrogen sites created during the synthesis of the nanoparticles. In a side reaction, some of the dppb ligands are decomposed producing surface phosphorus species and gaseous partially deuterated butane and cyclohexane. These findings are fundamental for detailed studies of the reaction kinetics of these particles towards H2 or D2 gas.}}, author = {{Rothermel, Niels and Röther, Tobias and Ayvalı, Tuğçe and Martínez-Prieto, Luis M. and Philippot, Karine and Limbach, Hans-Heinrich and Chaudret, Bruno and Gutmann, Torsten and Buntkowsky, Gerd}}, journal = {{ChemCatChem}}, number = {{5}}, pages = {{1465–1471}}, title = {{{Reactions of D2 with 1,4-Bis(diphenylphosphino) butane-Stabilized Metal Nanoparticles-A Combined Gas-phase NMR, GC-MS and Solid-state NMR Study}}}, doi = {{10.1002/cctc.201801981}}, volume = {{11}}, year = {{2019}}, } @article{64018, abstract = {{CO oxidation is an extensively studied reaction in heterogeneous catalysis due to its seeming simplicity and its great importance for emission control. However, the role of particle size and more specifically structure sensitivity in this reaction is still controversial. In the present study, colloidal “surfactant-free” Pt nanoparticles (NPs) in a size regime of 1–4 nm with narrow size distribution and control over particle size were synthesized and subsequently supported on Al2O3 to prepare model catalysts. CO oxidation was performed using Pt NPs catalysts with particles sizes of 1, 2, 3, and 4 nm at different reaction temperatures. It is shown that the reaction exhibits a particle size effect that depends strongly on the reaction conditions. At 170 °C, the reaction seems to proceed within the same kinetic regime for all particle sizes, but the surface normalized activity depends strongly on the particle size, with maximum activity for nanoparticles 2 nm in diameter. A temperature increase to 200 °C leads to a change of the kinetic regime that depends on the particle size. For Pt NPs 1 nm in diameter a reaction order of 1 for O2 was observed, indicating that O2 adsorbs molecularly and dissociates in a following step, which represents the generally accepted mechanism on Pt surfaces. The reaction order of −1 for CO demonstrates that the surface is saturated with CO under reaction conditions. With increasing particle size, the reaction orders of O2 and CO change. For particles 2 nm in size, an increase in temperature also results in reaction orders of 1 for O2 and −1 for CO; NPs of 3 and 4 nm, even at higher temperatures, show no clear kinetic behavior that can be explained by a single reaction mechanism. Instead, the Boudouard reaction between two adjacent adsorbed CO molecules was identified as an important additional reaction pathway that occurs preferentially on large particles and causes more complex kinetics.}}, author = {{Neumann, Sarah and Gutmann, Torsten and Buntkowsky, Gerd and Paul, Stephen and Thiele, Greg and Sievers, Heiko and Bäumer, Marcus and Kunz, Sebastian}}, journal = {{Journal of Catalysis}}, keywords = {{Solid state NMR, “Surfactant-free” platinum nanoparticles, CO oxidation, Particle size effect, Structure sensitivity}}, pages = {{662–672}}, title = {{{Insights into the reaction mechanism and particle size effects of CO oxidation over supported Pt nanoparticle catalysts}}}, doi = {{10.1016/j.jcat.2019.07.049}}, volume = {{377}}, year = {{2019}}, } @article{64019, abstract = {{A chiral zirconium-based catalyst, DUT-67-Pro containing 8-connected Zr6-clusters is obtained by post synthetic functionalization of Zr6O6(OH)2(TDC)4(HCOO)2 (DUT-67, TDC = 2,5-thiophenedicarboxylate) with the chiral monocarboxylic acid, L-proline. 13C and 15N solid state MAS and DNP NMR studies of DUT-67-Pro confirm the integration of L-proline into the porous framework. The chiral MOF catalyst exhibits an excellent catalytic activity at low temperature (298 K) with an unprecedented syn-(S,S)-product selectivity in an asymmetric aldol addition reaction of cyclohexanone to 4-nitrobenzaldehyde (yield = 95%, ee = 96%). Comparative catalytic studies using a molecular Zr6-cluster model compound indicate the Zr6-moiety to be responsible for this inverse diastereoselectivity compared to well-established L-proline organocatalysis and a mechanism is proposed to explain the Zr6-cluster-mediated syn-selectivity. Masking residual acidic active sites in the cluster of the framework was found to be a key prerequisite to achieve the high enantioselectivity. The purely heterogeneous catalytic system based on DUT-67-Pro is highly stable and can be recycled several times.}}, author = {{Nguyen, Khoa D. and Kutzscher, Christel and Ehrling, Sebastian and Senkovska, Irena and Bon, Volodymyr and Oliveira, Marcos and Gutmann, Torsten and Buntkowsky, Gerd and Kaskel, Stefan}}, journal = {{Journal of Catalysis}}, keywords = {{-proline, -selective aldol reaction, Chirality, Metal-organic framework, Zirconium}}, pages = {{41–50}}, title = {{{Insights into the role of zirconium in proline functionalized metal-organic frameworks attaining high enantio- and diastereoselectivity}}}, doi = {{10.1016/j.jcat.2019.07.003}}, volume = {{377}}, year = {{2019}}, } @article{64023, abstract = {{The structure of vanadium oxide (VOx) species in vanadium containing MCM-41 catalysts prepared by co-condensation or grafting, respectively, was investigated by a combination of Raman scattering, UV-vis diffuse reflectance, ATR-IR, and magic angle spinning (MAS) 51V as well as 29Si NMR spectroscopy techniques. Simulations of the 51V MAS NMR spectra allowed the determination of chemical shift and quadrupole tensor parameters, which give valuable information about the nature of the VOx units. Structural transformations of the supported vanadium oxide species for the catalyst in the dehydrated state and hydrated state were investigated to examine the effect of water molecules on the VOx structures. The results reveal the presence of different VOx structures for the hydrated samples, including dimeric species, oligomeric chains and isolated trigonal pyramid units. Upon dehydration, the predominance of oligomeric and/or dimeric units for the sample prepared by grafting was observed, while a considerable amount of isolated units was additionally detected for the sample prepared by co-condensation.}}, author = {{Oliveira, Marcos and Seeburg, Dominik and Weiß, Jana and Wohlrab, Sebastian and Buntkowsky, Gerd and Bentrup, Ursula and Gutmann, Torsten}}, issn = {{2044-4753}}, journal = {{Catalysis Science & Technology}}, number = {{21}}, pages = {{6180–6190}}, publisher = {{The Royal Society of Chemistry}}, title = {{{Structural characterization of vanadium environments in MCM-41 molecular sieve catalysts by solid state 51V NMR}}}, doi = {{10.1039/C9CY01410A}}, volume = {{9}}, year = {{2019}}, } @article{64001, abstract = {{FSLG CPMAS HETCOR is a 2D solid-state NMR experiment which provides structural information and conformational correlation between a 1H and an X-nucleus. However, practical application of the experiment suffers from the chemical shift referencing problem on the indirect 1H dimension. In our paper, we present a novel 1H–1H MAS FSLG-based approach and its application to reference the FSLG CPMAS HETCOR which overcomes the 1H referencing in the 2D 1H-X HETCOR experiment. This approach works excellently irrespective of the sample type over a wide range of temperature.}}, author = {{Kumari, Bharti and Brodrecht, Martin and Gutmann, Torsten and Breitzke, Hergen and Buntkowsky, Gerd}}, issn = {{1613-7507}}, journal = {{Applied Magnetic Resonance}}, number = {{12}}, pages = {{1399–1407}}, title = {{{Efficient Referencing of FSLG CPMAS HETCOR Spectra Using 2D 1H–1H MAS FSLG}}}, doi = {{10.1007/s00723-019-01156-2}}, volume = {{50}}, year = {{2019}}, } @article{63991, abstract = {{A series of 1 and 2 nm sized platinum nanoparticles (Pt-NPs) deposited on different support materials, namely, gamma-alumina (gamma-Al2O3), titanium dioxide (TiO2), silicon dioxide (SiO2) and fumed silica are investigated by solid-state NMR and dynamic nuclear polarization enhanced NMR spectroscopy (DNP). DNP signal enhancement factors up to 170 enable gaining deeper insight into the surface chemistry of Pt-NPs. Carbon monoxide is used as a probe molecule to analyze the adsorption process and the surface chemistry on the supported Pt-NPs. The studied systems show significant catalytic activity in carbon monoxide oxidation on their surface at room temperature. The underlying catalytic mechanism is the water-gas shift reaction. In the case of alumina as the support the produced CO2 reacts with the surface to form carbonate, which is revealed by solid-state NMR. A similar carbonate formation is also observed when physical mixtures of neat alumina with silica, fumed silica and titania supported Pt-NPs are studied.}}, author = {{Klimavicius, V. and Neumann, S. and Kunz, S. and Gutmann, Torsten and Buntkowsky, G.}}, issn = {{2044-4753}}, journal = {{Catalysis Science & Technology}}, keywords = {{Chemistry, gamma-alumina, hydrogenation, silica, c-13, interactions, metal-catalysts, particle-size, platinum nanoparticles, sites, surface, water-gas shift}}, number = {{14}}, pages = {{3743–3752}}, title = {{{Room temperature CO oxidation catalysed by supported Pt nanoparticles revealed by solid-state NMR and DNP spectroscopy}}}, doi = {{10.1039/c9cy00684b}}, volume = {{9}}, year = {{2019}}, } @article{63969, abstract = {{A number of Ir-N-heterocyclic carbene (Ir-NHC) complexes with asymmetric N-heterocyclic carbene (NHC) ligands have been prepared and examined for signal amplification by reversible exchange (SABRE). Pyridine was chosen as model compound for hyperpolarization experiments. This substrate was examined in a solvent mixture using several Ir-NHC complexes, which differ in their NHC ligands. The SABRE polarization was created at 6mT and the H-1 nuclear magnetic resonancesignals were detected at 7T. We show that asymmetric NHC ligands, because of their favorable chemistry, can adapt the SABREactive complexes to different chemical scenarios.}}, author = {{Hadjiali, S. and Savka, R. and Plaumann, M. and Bommerich, U. and Bothe, S. and Gutmann, Torsten and Ratajczyk, T. and Bernarding, J. and Limbach, H. H. and Plenio, H. and Buntkowsky, G.}}, issn = {{1613-7507}}, journal = {{Applied Magnetic Resonance}}, keywords = {{dynamic nuclear-polarization, hyperpolarization, enhancement, hydrogen induced polarization, olefin-metathesis catalysts, parahydrogen-induced polarization, peptides, Physics, sabre, spectroscopy}}, number = {{7}}, pages = {{895–902}}, title = {{{Substituent Influences on the NMR Signal Amplification of Ir Complexes with Heterocyclic Carbene Ligands}}}, doi = {{10.1007/s00723-019-01115-x}}, volume = {{50}}, year = {{2019}}, }