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- ItemStability studies of ionic liquid [EMIm][NTf2] under short-term thermal exposure(London : RSC Publishing, 2016) Neise, Christin; Rautenberg, Christine; Bentrup, Ursula; Beck, Martin; Ahrenberg, Mathias; Schick, Christoph; Keßler, Olaf; Kragl, UdoIonic liquids (ILs) as new media for synthesis and as functional fluids in technical applications are still of high interest. Cooling a steel component from an annealing temperature of nearly 850 °C down to room temperature in a liquid bath is a technically important process. The use of ionic liquids offers advantages avoiding film boiling of the quenching medium. However, such a high immersion temperature exceeds the thermal stability of the IL, for example such as [EMIm][NTf2]. To obtain information about formation of potential toxic decomposition products, potential fragments at varied states of decomposition of [EMIm][NTf2] were studied by various spectroscopic and gravimetric methods. For the first time it was possible to quantify fluorine-containing products via mass spectrometry coupled directly with thermogravimetric (TG) measurements. While chemical and spectroscopic analysis of thermally stressed ILs revealed no hints concerning changes of composition after quenching hot steel for several times, the mass-spectrometer (MS) coupled TG analysis gives information by comparing the decomposition behaviour of fresh and used ILs. A number of fragments were detected in low amounts confirming the proposed decomposition mechanism.
- ItemTheoretical mechanistic investigation of zinc(ii) catalyzed oxidation of alcohols to aldehydes and esters(London : RSC Publishing, 2016) Nisa, Riffat Un; Mahmood, Tariq; Ludwig, Ralf; Ayub, KhurshidThe mechanism of the Zn(II) catalyzed oxidation of benzylic alcohol to benzaldehyde and ester by H2O2 oxidant was investigated through density functional theory methods and compared with the similar oxidation mechanisms of other late transition metals. Both inner sphere and intermediate sphere mechanisms have been analyzed in the presence and absence of pyridine-2-carboxylic acid (ligand). An intermediate sphere mechanism involving the transfer of hydrogen from alcohol to H2O2 was found to be preferred over the competitive inner sphere mechanism involving β-hydride elimination. Kinetic barriers associated with the intermediate sphere mechanism are consistent with the experimental observations, suggesting that the intermediate sphere mechanism is a plausible mechanism under these reaction conditions. The oxidation of alcohols to aldehydes (first step) is kinetically more demanding than the oxidation of hemiacetals to esters (second step). Changing the oxidant to tert-butyl hydrogen peroxide (TBHP) increases the activation barrier for the oxidation of alcohol to aldehyde by 0.4 kcal mol−1, but decreases the activation barrier by 3.24 kcal mol−1 for oxidation of hemiacetal to ester. Replacement of zinc bromide with zinc iodide causes the second step to be more demanding than the first step. Pyridine-2-carboxylic acid ligand remarkably decreases the activation barriers for the intermediate sphere pathway, whereas a less pronounced inverse effect is estimated for the inner sphere mechanism.
- ItemGas hydrates model for the mechanistic investigation of the Wittig reaction “on water”(London : RSC Publishing, 2016) Ayub, Khurshid; Ludwig, RalfTheoretical mechanistic details for “on water” Wittig reaction of a stabilized ylide with benzaldehyde are presented and compared with a similar reaction under neat conditions. A gas hydrate structure consisting of 20 water molecules has been applied as a water surface for the reaction. The model is chosen to capture non-bonding interactions over a larger area in order to better account for the “on water” effect. The calculated acceleration for the cis-selective Wittig reaction is more than that for the trans-selective Wittig reaction. The “on water” acceleration for the Wittig reaction is due to greater number of non-bonding interactions in the transition state, compared to the starting material. The greater acceleration for the cis-selective Wittig over the trans-selective Wittig has been rationalized on the basis of non-bonding interactions in addition to hydrogen bonding. Besides accelerating the reaction, water also affects the pathway for the reaction. Decomposition of cisOP2 to alkene is estimated as a barrierless process. Moreover OP2 is more stable than OP1 for both cis and trans-selective Wittig reactions, opposite to what is observed for the neat reaction.
- ItemCopolymerization of CO2 and epoxides mediated by zinc organyls(London : RSC Publishing, 2018) Wulf, Christoph; Doering, Ulrike; Werner, ThomasHerein we report the copolymerization of CHO with CO2 in the presence of various zinc compounds R2Zn (R = Et, Bu, iPr, Cy and Ph). Several zinc organyls proved to be efficient catalysts for this reaction in the absence of water and co-catalyst. Notably, readily available Bu2Zn reached a TON up to 269 and an initial TOF up to 91 h-1. The effect of various parameters on the reaction outcome has been investigated. Poly(ether)carbonates with molecular weights up to 79.3 kg mol-1 and a CO2 content of up to 97% were obtained. Under standard reaction conditions (100 °C, 2.0 MPa, 16 h) the influence of commonly employed co-catalysts such as PPNCl and TBAB has been investigated in the presence of Et2Zn (0.5 mol%). The reaction of other epoxides (e.g. propylene and styrene oxide) under these conditions led to no significant conversion or to the formation of the respective cyclic carbonate as the main product.
- ItemA chemometric study in the area of feasible solution of an acid-base titration of N-methyl-6-oxyquinolone(London : RSC Publishing, 2018) Sawall, Mathias; Schmode, Stella; Schröder, Henning; Ludwig, Ralf; Neymeyr, KlausMultivariate curve resolution methods aim at recovering the underlying chemical components from spectroscopic data on chemical reaction systems. In most cases the spectra and concentration profiles of the pure components cannot be uniquely determined from the given spectral data. Instead continua of possible factors exist. This fact is known as rotational ambiguity. The sets of all possible pure component factors can be represented in the so-called area of feasible solutions (AFS). This paper presents an AFS study of the pure component reconstruction problem for a series of UV/Vis spectra taken from an acid-base titration of N-methyl-6-oxyquinolone. Additional information on the equilibrium concentration profiles for a varying acid concentration is taken from fluorescence measurements. On this basis chemometric duality arguments lead to the construction of a unique final solution.
- ItemRoom temperature synthesis of an amorphous MoS2 based composite stabilized by N-donor ligands and its light-driven photocatalytic hydrogen production(London : RSC Publishing, 2015) Niefind, Felix; Djamil, John; Bensch, Wolfgang; Srinivasan, Bikshandarkoil R.; Sinev, Ilya; Grünert, Wolfgang; Deng, Mao; Kienle, Lorenz; Lotnyk, Andriy; Mesch, Maria B.; Senker, Jürgen; Dura, Laura; Beweries, TorstenHerein an entirely new and simple room temperature synthesis of an amorphous molybdenum sulfide stabilized by complexing ammonia and hydrazine is reported. The resulting material exhibits an outstanding activity for the photocatalytic hydrogen evolution driven by visible light. It is chemically stable during the reaction conditions of the photocatalysis and shows unusual thermal stability up to 350 °C without crystallization. The new material is obtained by a reaction of solid ammonium tetrathiomolybdate and gaseous hydrazine. In the as-prepared state Mo atoms are surrounded by μ2-briding S2−, NH3 and hydrazine, the latter being coordinated to Mo(IV) in a bridging or side-on mode. Heating at 450 °C or irradiation with an electron beam generates nanosized crystalline MoS2 slabs. The two modes for crystallization are characterized by distinct mechanisms for crystal growth. The stacking of the slabs is low and the material exhibits a pronounced turbostratic disorder. Heat treatment at 900 °C yields more ordered MoS2 but structural disorder is still present. The visible-light driven hydrogen evolution experiments evidence an outstanding performance of the as-prepared sample. The materials were thoroughly characterized by optical spectroscopy, chemical analysis, in situ HRTEM, XRD, 1H and 15N solid-state NMR, XPS, and thermal analysis.
- ItemAcridinedione as selective flouride ion chemosensor: A detailed spectroscopic and quantum mechanical investigation(London : RSC Publishing, 2018) Iqbal, Nafees; Ali, Syed Abid; Munir, Iqra; Khan, Saima; Ayub, Khurshid; al-Rashida, Mariya; Islam, Muhammad; Shafiq, Zahid; Ludwig, Ralf; Hameed, AbdulThe use of small molecules as chemosensors for ion detection is rapidly gaining popularity by virtue of the advantages it offers over traditional ion sensing methods. Herein we have synthesized a series of acridine(1,8)diones (7a-7l) and explored them for their potential to act as chemosensors for the detection of various anions such as fluoride (F-), acetate (OAc-), bromide (Br-), iodide (I-), bisulfate (HSO4-), chlorate (ClO3-), perchlorate (ClO4-), cyanide (CN-), and thiocyanate (SCN-). Acridinediones were found to be highly selective chemosensors for fluoride ions only. To investigate in detail the mechanism of selective fluoride ion sensing, detailed spectroscopic studies were carried out using UV-visible, fluorescence and 1H NMR spectroscopy. Fluoride mediated (NH) proton abstraction of acridinedione was found to be responsible for the observed selective fluoride ion sensing. Quantum mechanical computational studies, using time dependent density functional theory (TDDFT) were also carried out, whereupon comparison of acridinedione interaction with fluoride and acetate ions explained the acridinedione selectivity for the detection of fluoride anions. Our results provide ample evidence and rationale for further modulation and exploration of acridinediones as non-invasive chemosensors for fluoride ion detection in a variety of sample types.
- ItemKinetic investigation of para-nitrophenol reduction with photodeposited platinum nanoparticles onto tunicate cellulose(London : RSC Publishing, 2022) Thiel, T.A.; Zhang, X.; Radhakrishnan, B.; van de Krol, R.; Abdi, F.F.; Schroeter, M.; Schomäcker, R.; Schwarze, M.Photodeposition is a specific method for depositing metallic co-catalysts onto photocatalysts and was applied for immobilizing platinum nanoparticles onto cellulose, a photocatalytically inactive biopolymer. The obtained Pt@cellulose catalysts show narrow and well-dispersed nanoparticles with average sizes between 2 and 5 nm, whereby loading, size and distribution depend on the preparation conditions. The catalysts were investigated for the hydrogenation of para-nitrophenol via transfer hydrogenation using sodium borohydride as the hydrogen source, and the reaction rate constant was determined using the pseudo-first-order reaction rate law. The Pt@cellulose catalysts are catalytically active with rate constant values k from 0.09 × 10−3 to 0.43 × 10−3 min−1, which were higher than the rate constant of a commercial Pt@Al2O3 catalyst (k = 0.09 × 10−3 min−1). Additionally, the Pt@cellulose catalyst can be used for electrochemical hydrogenation of para-nitrophenol where the hydrogen is electrocatalytically formed. The electrochemical hydrogenation is faster compared to the transfer hydrogenation (k = 0.11 min−1).
- ItemMechanistic insight of TiCl4catalyzed formal [3 + 3] cyclization of 1,3-bis(silyl enol ethers) with 1,3-dielectrophiles(London : RSC Publishing, 2015) Nisa, Riffat Un; Maria, Maria; Wasim, Fatima; Mahmood, Tariq; Ludwig, Ralf; Ayub, KhurshidThe mechanism of TiCl4 mediated formal [3 + 3] cyclization of 1,3-bis(silyl enol ethers) with 1,3-dielectrophiles is studied at the B3LYP level of density functional theory (DFT) to rationalize the experimental regioselectivity. Methyl and trifluoromethyl substituted 1,3 dielectrophiles are studied theoretically since they show different regioselectivities. Two different mechanisms involving 1,2 and 1,4 addition of 1,3-bis(silyl enol ethers) on 1,3-dielectrophiles are studied for each dienophile. The intramolecular transition metal catalyzed and non-catalyzed dynamic shift of the silyl moiety is also studied. The structure of the 1,3 dienophile and the associated Mulliken charges are the driving forces for different regioselectivities in methyl and trifluoromethyl dienophiles.
- ItemBrønsted acid-catalyzed hydroarylation of activated olefins(London : RSC Publishing, 2014) Fleischer, Ivana; Pospech, JolaA mild, regiospecific Brønsted acid-catalyzed hydroarylation of activated olefins, capable of the formation of quinone methide-like intermediates, has been investigated. Variously substituted 2- and 4-vinylphenols, 4-vinylaniline or 6-vinyl-naphthalen-2-ol were successfully implemented in a sequential protonation and Friedel–Crafts-type alkylation reaction of electron-rich arenes.