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Endres, Patrick; Schütt, Timo; Bode, Stefan; Hager, Martin; Geitner, Robert; Schubert, Ulrich Sigmar
Investigation of the cooperative-effects of Lewis- and Brønstedt acids in homogeneously catalyzed OME fuel synthesis by inline-NMR monitoring. - In: RSC Advances, ISSN 2046-2069, Bd. 14 (2024), 21, S. 14942-14948

Via inline-nuclear magnetic resonance measurements, the homogeneously catalyzed poly(oxymethylene dimethyl ether) fuel synthesis using trioxane and dimethoxy methane is investigated. Besides the Brønsted acid (BA) catalyst triflic acid (TfOH) different metal halides are studied as Lewis-acidic (LA) catalysts. Among the used LAs, MgCl2, the weakest based on electronegativity, reveals the highest catalytical activity. Additionally, the influence of the concentration of BA and LA is investigated. An increase in BA concentration leads to an exponential increase of the reaction rate, while increasing the concentration of the LA leads to a volcano plot with its optimum at a LA : BA ratio of 1 : 3. The influence of the LA on the electron density of the intermediate formaldehyde is concluded as the main factor for this behavior.
Huang, Tianbai; Kupfer, Stephan; Geitner, Robert; Gräfe, Stefanie
Computational modelling and mechanistic insight into light-driven CO dissociation of square-planar rhodium(I) complexes. - In: ChemPhotoChem, ISSN 2367-0932, Bd. 8 (2024), 5, e202300219, S. 1-13

The activation step of Vaska-type Rh(I) complexes, such as the photocleavage of the Rh‑CO bond, plays an important role in the subsequent C-H activation. To elucidate the details of the photochemistry of Vaska-type Rh(I) complexes, such as trans-Rh(PMe3)2(CO)(Cl), we here present a computationally derived picture as obtained at the density functional level of theory (DFT) in combination with multireference wavefunction-based methods. We have identified that the photocleavage of CO proceeds via the metal-centered excited state, which is populated through intersystem crossing (ISC) from the dipole-allowed excited state S1. Moreover, the present study unraveled the reasons for the low C-H activation efficiency when using Rh featuring the bidentate ligand 1,2-bis(dimethylphosphino)ethane (dmpe), namely due to its unfavorable photochemical properties, i.e., the small driving force for light-induced CO loss and the fast deactivation of 3MC state back to the singlet ground state. In this study, we provide theoretical insight into mechanistic details underlying the light-induced CO dissociation process, for Rh complexes featuring PMe3 and dmpe ligands.
Jaster, Jonas; Dreßler, Elias; Geitner, Robert; Groß, Gregor Alexander
Synthesis and spectroscopic characterization of furan-2-carbaldehyde-d. - In: Molbank, ISSN 1422-8599, Bd. 2023 (2023), 2, M1654, S. 1-9

Here, we present a protocol for the one-step synthesis of the title compound in quantitative yield using adapted Vilsmeier conditions. The product was characterized by 1H-,2H-,13C-NMR-, as well as IR and Raman spectroscopy. Spectral data are given in detail.
Hack, Jasmin; Jordan, Moritz; Schmitt, Alina; Raru, Melissa; Zorn, Hannes Sönke; Seyfarth, Alex; Eulenberger, Isabel; Geitner, Robert
Ilm-NMR-P31: an open-access 31P nuclear magnetic resonance database and data-driven prediction of 31P NMR shifts. - In: Journal of cheminformatics, ISSN 1758-2946, Bd. 15 (2023), 122, S. 1-12

This publication introduces a novel open-access 31P Nuclear Magnetic Resonance (NMR) shift database. With 14,250 entries encompassing 13,730 distinct molecules from 3,648 references, this database offers a comprehensive repository of organic and inorganic compounds. Emphasizing single-phosphorus atom compounds, the database facilitates data mining and machine learning endeavors, particularly in signal prediction and Computer-Assisted Structure Elucidation (CASE) systems. Additionally, the article compares different models for 31P NMR shift prediction, showcasing the database’s potential utility. Hierarchically Ordered Spherical Environment (HOSE) code-based models and Graph Neural Networks (GNNs) perform exceptionally well with a mean squared error of 11.9 and 11.4 ppm respectively, achieving accuracy comparable to quantum chemical calculations.
Endres, Patrick; Schütt, Timo; Kimmig, Julian; Bode, Stefan; Hager, Martin; Geitner, Robert; Schubert, Ulrich Sigmar
Oxymethylene ether (OME) fuel catalyst screening using in situ NMR spectroscopy. - In: Chemistry - a European journal, ISSN 1521-3765, Bd. 29 (2023), 33, e202203776, S. 1-9

Online NMR measurements are introduced in the current study as a new analytical setup for investigation of the oxymethylene dimethyl ether (OME) synthesis. For the validation of the setup, the newly established method is compared with state-of-the-art gas chromatographic analysis. Afterwards, the influence of different parameters, such as temperature, catalyst concentration and catalyst type on the OME fuel formation based on trioxane and dimethoxymethane is investigated. As catalysts, AmberlystTM 15 (A15) and trifluoromethanesulfonic acid (TfOH) are utilized. A kinetic model is applied to describe the reaction in more detail. Based on these results, the activation energy (A15: 48.0 kJ mol^-1 and TfOH: 72.3 kJ mol^-1) and the order in catalyst (A15: 1.1 and TfOH: 1.3) are calculated and discussed.
Hadzich, Antonella; Flores, Santiago; Masucci, Ashley E.; Gomez, Enrique D.; Groß, Gregor Alexander
NMR and GPC analysis of alkyd resins: influence of synthesis method, vegetable oil and polyol content. - In: Polymers, ISSN 2073-4360, Bd. 15 (2023), 9, 1993, S. 1-14

Alkyd resins are oil-based polymers that have been widely used for generations in the surface coating industry and beyond. Characterization of these resins is of high importance to understand the influence of its components on its behavior, compatibility with other resins, and final quality to ensure high durability. Here, NMR spectroscopy and GPC were used for characterizing differences in the chemical structure, molecular distribution, and dispersity between oil-based and fatty acid-based alkyd polymers made from sacha inchi and linseed oils. Sancha inchi (Plukentia volubilis L.) is a fruit-bearing plant native to South America and the Caribbean, and has a rich unsaturated fatty acid content. The effect of vegetable oil and polyol selection on the synthesis of alkyd resins for coating applications was analyzed. The influence of two different synthesis methods, monoglyceride and fatty acid processes, was also compared. Important structural differences were observed using NMR: one-dimensional spectra revealed the degree of unsaturated fatty acid chains along the polyester backbone, whereas, 2D NMR experiments facilitated chemical shift assignments of all signals. GPC analysis suggested that alkyd resins with homogeneous and high molecular weights can be obtained with the fatty acid process, and that resins containing pentaerythritol may have uniform chain lengths.
Schuett, Timo; Anufriev, Ilya; Endres, Patrick; Stumpf, Steffi; Nischang, Ivo; Höppener, Stephanie; Bode, Stefan; Schubert, Ulrich Sigmar; Geitner, Robert
A user-guide for polymer purification using dialysis. - In: Polymer chemistry, ISSN 1759-9962, Bd. 14 (2023), 1, S. 92-101

Dialysis diffusion kinetics are investigated via in situ NMR spectroscopy for numerous different raw polymeric solutions to result in a general guideline for polymer purification using dialysis. In several approaches, a polymer was on purpose contaminated with its respective monomer, regenerated conducting conventional dialysis and monitored online utilizing in situ NMR spectroscopy. Consequently, polymer type and molar mass, monomer type, molar mass cut-off of the dialysis tubing and type of solvent were varied resulting in 29 different purification approaches and over 40 000 NMR-spectra. As a result, several major parameters were identified affecting the purification process significantly such as the chosen solvent, viscosity and alpha value. On the contrary, parameters such as dialysis tubing molar mass cut-off and molar mass of the polymer did not affect the purification in a significant manner. Furthermore, physical properties such as density, viscosity, alpha value, and dipole moment of the ingredients were combined in a principal component analysis in order to identify the most important parameters.
Geitner, Robert;
Physikalische Chemie : Trendbericht. - In: Nachrichten aus der Chemie, ISSN 1868-0054, Bd. 70 (2022), 5, S. 64-67

Die Aufklärung von Reaktionsmechanismen ist in der Katalyse wichtig, um die geschwindigkeitsbegrenzende Schritte zu verstehen und zu beschleunigen. Mit maschinellem Lernen lassen dann sich auf Basis der Mechanismen neue Katalysatoren entwickeln. Photochemische Umsetzungen in weichen Membranen folgen einer anderen Kinetik als Reaktionen in Lösung. Mikroschwimmer, Mikromotoren oder Phototaxis zählen zu aktiver Materie. Sie wandeln kontinuierlich Energie aus ihrer Umgebung um und bewegen sich autonom.
Huang, Tianbai; Kupfer, Stephan; Richter, Martin; Gräfe, Stefanie; Geitner, Robert
Bidentate Rh(I)-phosphine complexes for the C-H activation of alkanes: computational modelling and mechanistic insight. - In: ChemCatChem, ISSN 1867-3899, Bd. 14 (2022), 18, e202200854, S. 1-9

The C-H activation and subsequent carbonylation mediated by metal complexes, i. e., Rh(I) complexes, has drawn considerable attention in the past. To extend the mechanistic insight from Rh complexes featuring monodentate ligands like P(Me)3 towards more active bisphosphines (PLP), a computationally derived fully conclusive mechanistic picture of the Rh(I)-catalyzed C-H activation and carbonylation is presented here. Depending on the nature of the bisphosphine ligand, the highest lying transition state (TS) is associated either to the initial C-H activation in [Rh(PLP)(CO)(Cl)] or to the rearrangement of the chloride in [Rh(PLP)(H)(R)(Cl)]. The chloride rearrangement was found to play a key role in the subsequent carbonylation. A set of 20 complexes of different architectures was studied, in order to fine tune the C-H activation in a knowledge-driven approach. The computational analysis suggests that a flexible ligand architecture with aromatic rings can potentially increase the performance of Rh-based catalysts for the C-H activation.
Nolte, Oliver; Geitner, Robert; Volodin, Ivan A.; Rohland, Philip; Hager, Martin; Schubert, Ulrich Sigmar
State of charge and state of health assessment of viologens in aqueous-organic redox-flow electrolytes using in situ IR spectroscopy and multivariate curve resolution. - In: Advanced science, ISSN 2198-3844, Bd. 9 (2022), 17, 2200535, S. 1-10

Aqueous-organic redox flow batteries (RFBs) have gained considerable interest in recent years, given their potential for an economically viable energy storage at large scale. This, however, strongly depends on both the robustness of the underlying electrolyte chemistry against molecular decomposition reactions as well as the device's operation. With regard to this, the presented study focuses on the use of in situ IR spectroscopy in combination with a multivariate curve resolution approach to gain insight into both the molecular structures of the active materials present within the electrolyte as well as crucial electrolyte state parameters, represented by the electrolyte's state of charge (SOC) and state of health (SOH). To demonstrate the general applicability of the approach, methyl viologen (MV) and bis(3-trimethylammonium)propyl viologen (BTMAPV) are chosen, as viologens are frequently used as negolytes in aqueous-organic RFBs. The study's findings highlight the impact of in situ spectroscopy and spectral deconvolution tools on the precision of the obtainable SOC and SOH values. Furthermore, the study indicates the occurrence of multiple viologen dimers, which possibly influence the electrolyte lifetime and charging characteristics.