Low-temperature IR Research Articles

Influence of triphosphine ligand coordination geometry in Mn(I) hydride complexes [(P∩P∩P)(CO)2MnH] on their kinetic hydricity.

Octahedral Mn(I) complexes bearing tridentate donor ligands [(L∩L'∩L'')(CO)2MnX] have recently emerged as major players in catalytic (de)hydrogenation processes. While most of these systems are still based on structurally rigid pincer scaffolds imposing a meridional coordination mode, for some more flexible tridentate ligands a facial arrangement of donor moieties becomes possible. Accordingly, the geometry of the corresponding Mn(I) hydrides [(L∩L'∩L'')(CO)2MnH] directly involved in the catalytic processes, namely the nature of the donor extremity located in the trans-position of the hydride (CO and L for mer- and fac-configurations, respectively) may influence their hydride transfer ability. Herein, low-temperature IR and NMR spectroscopy studies of two model Mn(I) complexes, mer-[(L1)(CO)2MnH] and fac-[(L2)(CO)2MnH], bearing similar triphosphine ligands (L1 = PhP(CH2CH2PPh2)2; L2 = MeC(CH2PPh2)3) in the presence of B(C6F5)3 as the H- abstractor revealed for the first time a higher kinetic hydricity of the tripodal system. Even for the pincer complex, hydride transfer proceeds from the non-covalent adduct fac-[(L1)(CO)2MnH]⋯B(C6F5)3 with the facial geometry arising from the mer-to-fac isomerization of the initial mer-[(L1)(CO)2MnH]. The higher reactivity of the fac-hydride derivatives was found to be consistent with the catalytic performance of the corresponding Mn(I) bromide complexes in the benchmark ester hydrosilylation.

Synthesis, сharacterization, and сatalytic properties of GdCoO3 for dry reforming of methane

Perovskite oxides (ABO3) due to their high thermal stability and the ability to control the physico-chemical properties are considered as an alternative to traditional catalysts containing noble and transition metals. Herein, the recent research breakthroughs of GdCoO3 catalysts in experimental studies are summarized in detail. First, the perovskite-type GdCoO3 complex oxides were obtained by co-precipitation method with the various precipitators and were characterized by X-ray diffraction (XRD), low temperature nitrogen adsorption and IR spectroscopy. Physical and chemical analysis showed that the choice of precipitant doesn?t significantly affect the phase composition of the perovskites. The catalytic performance of gadolinium cobaltites was discussed. It was found that the use of cobaltites obtained by co-precipitation leads to the inhibition of the side reaction of the reverse steam reforming of carbon monoxide. Finally, the investigation of the used catalysts demonstrated the formation of Gd2O2CO3 and metallic cobalt, which indicates the nature of active centres: gadolinium is the centre of CO adsorption, while hydrogen chemisorption occurs on cobalt-sites.

SYNTHESIS, STRUCTURE AND PROPERTIES OF BENTONITE - ACTIVATED CARBON COMPOSITE

The development of adsorption purification technology in industry is divorced from research in the development of new methods for the synthesis and modification of composites, while the efficiency of the adsorbent directly depends on its structural and surface properties. Currently, there is an increasing demand for hydrogel adsorbents used in pharmaceuticals, medicine, and the chemical industry for wastewater treatment. Dash-Salakhlinsky bentonite, a natural clay mineral and activated carbon of the BAU-A brand, was used as an object of study. By intercalation of bentonite with activated carbon, a bentonite-coal composite was obtained. Grinding equipment of medium energy density was used: roller-ring vibrating mill VM-4. Activation time was 15 min. The average particle size of the initial bentonite and bentonite-carbon composite was determined by laser diffraction on a particle size analyzer. During mechanical activation of bentonite, the nature of the distribution of particles indicates the predominant process of aggregation over grinding. The morphology, structure, and textural properties of the composite were studied by the methods of low-temperature nitrogen adsorption-desorption and IR spectroscopy. An increase in the specific surface area and porosity after mechanochemical activation as compared to the calculated values indicates the interaction between the components. IR spectroscopy established the formation of additional functional groups that can act as active centers, which positively affects the adsorption properties. It is shown that the bentonite-charcoal composite has a high sorption activity with respect to the brilliant green dye. The data obtained can be useful in the development of effective sorbents for wastewater treatment from pollution by organic compounds, as well as in medicine and pharmaceuticals.

Benchmarking the anisotropy of nitroxyl radical solvation with IR spectroscopy.

Two simple nitroxyl radicals, di-tert-butyl nitroxyl (DTBN) and 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) are solvated by one or two water, methanol, tert-butyl alcohol or phenol molecules. The resulting low temperature IR spectra of the vacuum-isolated microsolvates in the OH stretching range are assigned based on harmonic DFT predictions for closed shell solvent dimers and trimers and their offset from experiment, to minimise theory-guided assignment bias. Systematic conformational preferences for the first and second solvent molecule are observed, depending on the conformational rigidity of the radical. These assignments are collected into an experimental benchmark data set and used to assess the spectral predicting power of different DFT approaches. The goal is to find inexpensive computational methods which provide reliable spectral predictions for this poorly explored class of microsolvates.

Vibrational entropy of disordering in omphacite

The cations of an ordered omphacite from the Tauern window were gradually disordered in piston cylinder experiments at temperatures between 850 and 1150 °C. The samples were examined by X-ray powder diffraction and then investigated using low-temperature calorimetry and IR spectroscopy. The low-temperature heat capacity data were used to obtain the vibrational entropies, and the line broadening of the IR spectra served as a tool to investigate the disordering enthalpy. These data were then used to calculate the configurational entropy as a function of temperature. The vibrational entropy does not change during the cation ordering phase transition from space group C2/c to P2/n at 865 °C but increases with a further temperature increase due to the reduction of short-range order.

Hydrogen Bonding with Hydridic Hydrogen–Experimental Low-Temperature IR and Computational Study: Is a Revised Definition of Hydrogen Bonding Appropriate?

Spectroscopic characteristics of Me3Si-H···Y complexes (Y = ICF3, BrCN, and HCN) containing a hydridic hydrogen were determined experimentally by low-temperature IR experiments based on the direct spectral measurement of supersonically expanded intermediates on a cold substrate or by the technique of argon-matrix isolation as well as computationally at harmonic and one-dimensional anharmonic levels. The computations were based on DFT-D, MP2, MP2-F12, and CCSD(T)-F12 levels using various extended AO basis sets. The formation of all complexes related to the redshift of the Si-H stretching frequency upon complex formation was accompanied by an increase in its intensity. Similar results were obtained for another 10 electron acceptors of different types, positive σ-, π-, and p-holes and cations. The formation of HBe-H···Y complexes, studied only computationally and again containing a hydridic hydrogen, was characterized by the blueshift of the Be-H stretching frequency upon complexation accompanied by an increase in its intensity. The spectral shifts and stabilization energies obtained for all presently studied hydridic H-bonded complexes were comparable to those in protonic H-bonded complexes, which has prompted us to propose a modification of the existing IUPAC definition of H-bonding that covers, besides the classical protonic form, the non-classical hydridic and dihydrogen forms.

Energetic coordination compounds of late 3d metals with 1N-(Nitromethyl)-5H-tetrazole, a ligand with astonishing properties from the rare class of N-nitromethyl azoles

The hardly accessible N-nitromethyl moiety is introduced onto the 1,5H-tetrazole scaffold, representing the fourth and fifth ever synthesized N-nitromethyl azoles. Both compounds 1-(Nitromethyl)-5H-tetrazole (2a) and 2-(Nitromethyl)-5H-tetrazole (2b) are thoroughly analyzed by low-temperature single-crystal X-ray diffraction experiments, complemented by elemental analysis, multinuclear (1H, 13C, 14N, 15N) NMR as well as IR spectroscopy. Sensitivity measurements towards external stimuli revealed two highly insensitive compounds. Additionally, 2a was applied as ligand for energetic coordination compounds of 3d transition metals (e.g. Cu, Fe, Ni, and Zn) in combination with oxidizing nitrate, chlorate and perchlorate anions. The synthesized ECCs were analyzed by low-temperature single-crystal X-ray diffraction experiments, elemental analysis and IR spectroscopy. The thermal behavior of all compounds was investigated by differential thermal analysis and the sensitivities towards impact and friction was measured. Due to the lack of information about the properties of N-nitromethyl azoles, the complete insensitivity towards external stimuli of 2a and 2b supports first hints of a sensitivity decreasing influence of the N-nitromethyl moiety, which is highly desired when designing new high explosives. Additionally, the functional group is able to introduce meltability into molecular scaffolds.

Bleachability of pIRIR signal from single-grain K-feldspar

The high-temperature (e.g., >200 °C) post-infrared infrared (pIRIR) stimulated luminescence of feldspar has been widely used for dating, because it usually suffers negligibly from anomalous fading. However, compared to the quartz optically stimulated luminescence (OSL) or low-temperature (e.g., 50 °C) IR stimulated luminescence (IRSL) from feldspars, the high-temperature pIRIR signals have been shown to bleach more slowly and usually have high residual does of a few Gys or more, which limits its application to date relatively young samples and may cause problems for correcting residual dose/signal. In this study, we investigated the bleachability of K-feldspar pIRIR signals for individual grains. A significant variation in the bleachability of the pIRIR signal was observed among different K-feldspar grains from the same sample. Experiments show that such a variability may result in different residual doses for different grains and, consequently, additional scatter in De values. We present a method for quickly testing the bleachability of individual grains. Our result suggests that selecting the easy-to-bleach grains for De determination provides an efficient way to date young samples using high-temperature pIRIR signals, which avoids the compromise of using low-temperature pIRIR signals that may suffer from anomalous fading.

The influence of the local environment on the electron accepting ability of the cation: induced Lewis acidity

The effect of adsorption of acid gases CO2, SO2 and SO3 on the electron accepting ability of cations on the surface of calcium oxide was studied by low-temperature IR spectroscopy of adsorbed CO. It is shown that the formation of carbonate, sulfite and sulfate anions in the surface layer leads to the appearance of Lewis acid sites, the strength of which, estimated by the shift in the vibration frequency of CO molecules associated with them, increases in the appropriate order, approaching that for calcium-containing zeolites. The results of the calculation of cluster models by the DFT method confirm the observed trends in the variation of the vibrational frequency and the absorption coefficient of CO molecules bound through both the carbon and oxygen atoms, and also demonstrate the dependence of the ability of a cation for CO linkage isomerism on its local environment. Keywords: surface, adsorption, acidity, IR spectroscopy, quantum chemical calculation, electron accepting ability.

Влияние локального окружения на электроноакцепторную способность катиона: индуцированная льюисовская кислотность

The effect of adsorption of acid gases CO2, SO2 and SO3 on the electron-accepting ability of cations at the surface of calcium oxide was studied by low-temperature IR spectroscopy of adsorbed CO. It is shown that the formation of carbonate, sulfite and sulfate anions in the surface layer leads to the appearance of Lewis acid sites, the strength of which, estimated by the shift in the vibrational frequency of CO molecules bound to them, increases in the corresponding order, approaching that for calcium- containing zeolites. The results of calculation of cluster models by the DFT method confirm the observed trends in the variation of vibrational frequency and the absorption coefficient of CO molecules bound both via carbon atom or via oxygen, and also demonstrate the dependence of the ability of a cation to linkage isomerism on its local environment.

Ring Opening and Closure Reactions of β‐Propiolactone in the Superacids HF/MF5 (M=Sb, As)

AbstractTransformations of β‐propiolactone in the binary superacidic systems HF/MF5 (M=Sb, As) were investigated at different temperatures. Salts of monoprotonated 3‐hydroxypropanoyl fluoride [H2O(CH2)2C(O)F][SbF6] (1) and [H2O(CH2)2C(O)F][AsF6] (2) were obtained by performing the reactions at temperatures lower than −40 °C. In contrast, temperatures higher than −40 °C yield the respective salts [C9H15O6][SbF6]3 (3) and [C9H15O6][AsF6]3 (4) containing a cyclic trication. The protonated species of β‐propiolactone was not observed. All salts were characterized by low temperature Raman and IR spectroscopy. In addition, single‐crystal X‐ray analyses were performed for (1) and (3). For both cations the existence of a cationic acyl species as a reactive intermediate in the form of [HO(CH2)2CO][MF6] is assumed. The formation of the acyl cation, based on protonated β‐propiolactone, was investigated by quantum chemical calculations on the MP2/aug‐cc‐pVTZ level of theory.

Functionalization strategy influences the porosity of amino-containing porous aromatic frameworks and the hydrogenation activity of palladium catalysts synthesized on their basis

We investigated the influence of functionalization strategy (post- or pre-) on the structure of amino-modified porous aromatic frameworks (PAF-NH2). Different techniques, such as low-temperature N2 adsorption-desorption, IR spectroscopy and solid-state CP-MAS 13C NMR spectroscopy, were used to characterize the synthesized materials. Palladium catalysts were prepared based on obtained PAF-NH2 supports, characterized by TEM, XPS and elemental analysis and studied in the semihydrogenation of C8-acetylenes and dienes. The results show that the modification approach influences structure of PAF and, therefore, the morphology and distribution of palladium nanoparticles. Post-modified catalysts proved to be more active, while pre-modified ones exhibited enhanced selectivity towards desired olefins and improved stability.

Photoreaction of 2-chlorothiophenol studied by low-temperature matrix-isolation IR spectroscopy with DFT calculation

• Photoreaction of 2-chlorothiophenol isolated in cryogenic matrix. • Benzothiirene with HCl complex changes into five-membered-ring thioketene compound. • Isomerization of chloro-cyclohexadienethione compound. The photoreactions of 2-chlorothiophenol are investigated using matrix-isolation IR spectroscopy combined with density functional theory calculation at the B3LYP/6-311++G(3df,3pd) level. In a Ne matrix, 2,4-cyclopentadien-1-ylidenemethanethione, a five-membered-ring thioketene compound, is identified as the major photoproduct of a reaction similar to Wolff rearrangement. The reaction intermediate is spectroscopically detected as benzothiirene, which is produced from the reactant under UV ( λ ≥ 275 nm) irradiation and is then converted to the thioketene under λ ≥ 325 nm irradiation. Moreover, the complex of benzothiirene and HCl changes into 4-chloro-2,5-cyclohexadiene-1-thione under irradiation at λ ≥ 325 nm. By contrast, UV irradiation of 2-chlorothiophenol in an Ar matrix yields primarily 4-chloro-2,5-cyclohexadiene-1-thione via its isomer 2-chloro-2,4-cyclohexadiene-1-thione.

Cooperative effect of PdOx and SiO2 in CO detection by SnO2-based gas sensors: Thorough operando DRIFTS analysis

This work is devoted to the study of the joint effect of SiO2 and PdOx on the SnO2 sensor properties when detecting CO in dry and humid air. Nanocrystalline SnO2 was obtained by wet chemical synthesis, modification with SiO2 was effectuated during hydrothermal treatment, modification with PdOx was carried out by impregnation method. All sensitive materials characterized by XRD, low-temperature nitrogen adsorption, HRTEM, TPR-H2, XPS and IR spectroscopy. The sensor characteristics were studied when detecting CO in dry (RH = 1%) and humid (RH = 20%) air. Tin dioxide, jointly modified by SiO2 and PdOx, demonstrates the greatest sensitivity to CO in both dry and humid atmosphere. To explain the cooperative effect of PdOx and SiO2 in CO detection, the interaction of palladium modified samples with CO in dry and humid air was investigated by DRIFT spectroscopy in operando conditions.

Influence of zeolite content in NiW/Y-ASA-Al2O3 catalyst for second stage hydrocracking

NiW catalysts for second stage hydrocracking were prepared by impregnation of granulated supports with ultrastable zeolite Y content of 5−30 wt%; amorphous silica-alumina content of 45−20 wt%; and γ-Al2O3 content of 50 wt%. The catalysts were studied by low-temperature N2 adsorption, HRTEM, XPS and IR spectroscopy of adsorbed pyridine. Sulfide NiWS particles have been revealed to be similar in all the catalysts according to HRTEM and XPS. It has been shown that concentration of Brønsted acid sites increases with increasing of zeolite content in the catalysts. Hydrocracking tests were carried out at a pressure of 16.0 MPa, a LHSV of 1.4 h−1 and H2 to oil ratio of 750 (v/v). Two samples of unconverted oil with nitrogen content of 11 and 65 ppm obtained from the products of VGO hydrocracking under different severities were used as the feedstocks. The higher zeolite content the higher activity of the catalyst was achieved. However, the increase of zeolite content leaded to the decrease of selectivity to diesel. It has been found that increasing of nitrogen content in the feedstock was followed by a sharp decline in catalyst activity while the change in selectivity to diesel was insignificant. Optimal zeolite content in the support for studied conditions and feedstock with nitrogen content of 65 ppm was 20 wt%.

New Lines in High Resolution IR Luminescence Spectra of SiC Single Crystals of the 4H and 6H Polytypes

We have studied high-resolution low-temperature IR luminescence and absorption spectra of undoped high-quality SiC single crystals of the 4H and 6H hexagonal modifications. Narrow lines with a width of smaller than 0.2 cm–1 have been revealed, with some of which being observed for the first time. We have found that some of the lines in the 4H and 6H modifications have similar structures; however, the lines in SiC-4H are shifted to the high-energy part of the spectrum by ~180 cm–1. For the most intense quartet in the range of 1.3 µm, we have succeeded in constructing the energy structure of levels for both the 4H modification and the 6H modification based on their luminescence and absorption spectra.

The Hydrolysis-Reduction of Arabinogalactan in the Presence of Ru/Cs3HSiW12O40 Catalyst

The hydrolysis-reduction of hemicellulose arabinogalactan to arabitol and galactitol polyalcohols, which are widely used in the food and pharmaceutical industries, was studied. It was shown that the process could be performed with the bifunctional catalyst containing highly dispersed ruthenium deposited on the cesium salt of silicon-tungsten heteropolyacid, Ru/Cs3HSiW12O40. The catalysts with different content of ruthenium (0.3, 0.6 and 1 wt.%) were synthesized for the study. The catalysts and their Cs3HSiW12O40 support were examined by various physicochemical methods (low-temperature nitrogen adsorption, IR spectroscopy, XRD, and TEM). The effect of temperature, substrate/catalyst ratio and ruthenium content in the catalyst on the yields of target products was elucidated. The highest yields of target products were achieved at the metal content of 0.6 wt.% and equal weights of the loaded catalyst and substrate (the 1:1 ratio). At a temperature of 200 °C, arabitol and galactitol can be produced with the yields up to 12 and 48 % for 2 h of the reaction in the presence of catalyst with the composition 0.6%Ru/Cs3HSiW12O40.

Hydrogenation of levulinic acid to γ-valerolactone over Fe-Re/TiO2 catalysts

Hydrogenation of levulinic acid to γ-valerolactone is a key reaction in the valorization of carbohydrates to renewable fuels and chemicals. State-of-the-art catalysts are based on supported noble metal nanoparticle catalysts. We report the utility of a bimetallic Fe-Re supported on TiO2 for this reaction. A strong synergy was observed between Fe and Re for the hydrogenation of levulinic acid in water under mild conditions. Fe-Re/TiO2 shows superior catalytic performance compared to monometallic Fe and Re catalysts at similar metal content. The hydrogenation activity of the bimetallic catalysts increased with Re content. H2-TPR, XPS, XANES, EXAFS, Mössbauer spectroscopy, TEM, and low-temperature CO IR spectroscopy show that the bimetallic catalysts contain metallic Re nanoparticles covered by FeOx species and small amounts of a Fe-Re alloy. Under reaction conditions, the partially reduced surface FeOx species adsorb water and form Brønsted acidic OH groups, which are involved in dehydration of reaction intermediates. Under optimized conditions, nearly full conversion of levulinic acid with a 95 % yield of γ-valerolactone could be achieved at a temperature as low as 180 °C in water at a H2 pressure of 40 bar.

The effect of matrices on the low-temperature IR spectra of a formic acid molecule isolated in inert gas crystals

Using the DFT/M06-2X method, we simulated the structure and vibrational spectra of inert gas (Ne, Ar, Kr, Xe) clusters with an isolated formic acid molecule. The impact of the matrix environment on the vibrational spectra of formic acid is established. The values of the matrix shifts of the vibrational frequencies predicted by calculations matched those obtained experimentally. We found that the best agreement between the calculated and experimental shifts occurred for clusters with the smallest deformation energy of the inert gas crystal. At the same time, the ratio of the volume of the molecule embedded in the matrix, and the volume of substituted matrix gas atoms, allowed one to determine only the minimum possible size of the matrix site. The calculated and experimental values of the matrix shifts are in good agreement, indicating the computation method matches the actual experimental conditions.

Review of the Post-IR IRSL Dating Protocols of K-Feldspar.

Compared to quartz, the infrared stimulated luminescence (IRSL) of K-feldspar saturates at higher dose, which has great potential for extending the dating limit. However, dating applications with K-feldspar has been hampered due to anomalous fading of the IRSL signal. The post-IR IRSL (pIRIR) signal of K-feldspar stimulated at a higher temperature after a prior low-temperature IR stimulation has significantly lower fading rate. Different dating protocols have been proposed with the pIRIR signals and successful dating applications have been made. In this study, we review the development of various pIRIR dating protocols, and compare their performance in estimating the equivalent dose (De). Standard growth curves (SGCs) of the pIRIR signals of K-feldspar are introduced. Single-grain K-feldspar pIRIR dating is presented and the existing problems are discussed.