Theoretical study on the reaction mechanisms and stereoselectivities of DABCO‐catalyzed Rauhut–Currier/cyclization reaction of methyl acrylate with 2‐benzoyl‐3‐phenyl‐acrylonitrile

With the aid of density functional theory (DFT) calculations, we have investigated the mechanisms and stereoselectivities of the tandem cross Rauhut–Currier/cyclization reaction of methyl acrylate R₁ with (E)‐2‐benzoyl‐3‐phenyl‐acrylonitrile R₂ catalyzed by a tertiary amine DABCO. The results of the DFT calculations indicate that the favorable mechanism (mechanism A) includes three steps: the first step is the nucleophilic attack of DABCO on R₁ to form intermediates Int1 and Int1‐1, the second step is the reaction of Int1 and Int1‐1 with R₂ to generate intermediate Int2(SS,RR,SR&RS), and the last step is an intramolecular S_N2 process to give the final product P(SS,RR,SR&RS) and release catalyst DABCO. The S_N2 substitution is computed to be the rate‐determining step, whereas the second step is the stereoselectivity‐determining step. The present study may be helpful for understanding the reaction mechanism of similar tandem reactions.     

Thermodynamics and kinetics of steam splitting over a potassium aluminosilicate electrolyte

A novel system using a potassium aluminosilicate electrolyte under applied potential that is able to split H₂O (or OHˉ) into H₂ and 1/2O₂ (or O₂ ^2-) with higher yields than the value deduced from Faraday"s law is presented. There were three steps by which H₂ and O₂ were generated stoichiometrically, and it was predicted that the high yields were due to the occurrence of chemically endothermic reactions: dehydration of the catalytic cell at a temperature below 100°C (step I), disproportionation of KOH (2KOH→H₂+K₂O₂) at a temperature around 200°C (step II), and disproportionation of K₂O (2K₂O→K₂+K₂O₂) at a temperature above 500°C (step III). So-called Nemca might be caused in the course of step III, since the rate of H₂ was ca 10² times larger than the value deduced from Faraday"s law. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal‐ and Light‐Induced Spin Crossover in a Guest‐Dependent Dinuclear Iron(II) System

We previously reported the dinuclear material [Fe^II₂(ddpp)₂(NCS)₄] ? 4 CH₂Cl₂ ( 1? 4 CH₂Cl₂; ddpp=2,5‐di(2′,2′′‐dipyridylamino)pyridine) and its partially desolvated analogue ( 1? CH₂Cl₂), which undergo two‐ and one‐step spin‐crossover (SCO) transitions, respectively. Here, we manipulate the type and degree of solvation in this system and find that either a one‐ or two‐step spin transition can be specifically targeted. The chloroform clathrate 1? 4 CHCl₃ undergoes a relatively abrupt one‐step SCO, in which the two equivalent Fe^II sites within the dinuclear molecule crossover simultaneously. Partial desolvation of 1? 4 CHCl₃ to form 1? 3 CHCl₃ and 1? CHCl₃ occurs through single‐crystal‐to‐single‐crystal processes (monoclinic C2/c to P2₁/n to P2₁/n) in which the two equivalent Fe^II sites become inequivalent sites within the dinuclear molecule of each phase. Both 1? 3 CHCl₃ and 1? CHCl₃ undergo one‐step spin transitions, with the former having a significantly higher SCO temperature than 1? 4 CHCl₃ and the latter, and each has a broader SCO transition than 1? 4 CHCl₃, attributable to the overlap of two SCO steps in each case. Further magnetic manipulation can be carried out on these materials through reversibly resolvating the partially desolvated material with chloroform to produce the original one‐step SCO, or with dichloromethane to produce a two‐step SCO reminiscent of that seen for 1? 4 CH₂Cl₂. Furthermore, we investigate the light‐induced excited spin state trapping (LIESST) effect on 1? 4 CH₂Cl₂ and 1? CH₂Cl₂ and observe partial LIESST activity for the former and no activity for the latter.     

Influence of sample pan on the thermal behaviour of KSCN measured with TG

In this study, the influence of the sample pan on the thermal behaviour of potassium thiocyanate (KSCN) was investigated. The measurements were performed with thermogravimetry (TG) and the two sample pans used were a platinum pan and a ceramic crucible. The samples were heated to 400-950 °C and the thermal products were identified by powder diffraction.The thermal behaviour of KSCN was found to be dependent on the sample pan used. With the platinum sample pan KSCN reacted in the first step into a mixture of K₂SO₄ and potassium tetracyanoplatinate (K₂Pt(CN)₄). In the second step, the mixture reacted further to pure K₂SO₄. In the ceramic sample crucible, however, the reaction in the first step resulted in a mixture of K₂SO₄ and KOCN. In the second step, the mixture reacted further to pure K₂SO₄.The results were verified with additional measurements of rubidium thiocyanate (RbSCN) and cesium thiocyanate (CsSCN). The reactions of these compounds proved to be similar to those of KSCN, thereby confirming that the thermal behaviour of the alkali metal thiocyanates mentioned in this study, depends on the sample pan used.     

Kinetics and mechanism of degradation of Co(II)–<Emphasis Type="Italic">N</Emphasis>-benzyloxycarbonylglycinato complex

The kinetics of multi-step thermal degradation of Co(II) complex with N-benzyloxycarbonyl glycinato ligand [Co(N-Boc-gly)₂(H₂O)₄]·2H₂O, in non-isothermal conditions was studied using isoconversional and non-isoconversional methods. The degradation of complex occurs in three well-separated steps involving the loss of water molecules in first step followed by two degradation steps of dehydrated complex. The dependence of Arrhenius parameters on conversion degree showed that all observed steps of thermal degradation are very complex, involving more than one elementary step, as can be expected for most solid-state heterogeneous reactions with solid reactants and solid and gaseous products. It was shown that step 1, corresponding to the dehydration, involves a series of competitive dehydration steps of differently bound water molecules complicated by diffusion. Second step involves two parallel reactions related to the loss of two identical C₆H₅CH₂O– ligand fragments complicated by the presence of products in gaseous state. Further degradation in step 3 corresponds to complex process with a change in the limiting stage, in this case from the kinetic to the diffusion regime, connected with the presence of gaseous products diffusing through the solid product.     

A Comparative Study of CO Oxidation on Nitrogen‐ and Phosphorus‐Doped Graphene

The geometry, electronic structure, and catalytic properties of nitrogen‐ and phosphorus‐doped graphene (N‐/P‐graphene) are investigated by density functional theory calculations. The reaction between adsorbed O₂ and CO molecules on N‐ and P‐graphene is comparably studied via Langmuir–Hinshelwood (LH) and Eley–Rideal (ER) mechanisms. The results indicate that a two‐step process can occur, namely, CO+O₂→CO₂+O_ads and CO+O_ads→CO₂. The calculated energy barriers of the first step are 15.8 and 12.4 kcal mol^?1 for N‐ and P‐graphene, respectively. The second step of the oxidation reaction on N‐graphene proceeds with an energy barrier of about 4 kcal mol^?1. It is noteworthy that this reaction step was not observed on P‐graphene because of the strong binding of O_ads species on the P atoms. Thus, it can be concluded that low‐cost N‐graphene can be used as a promising green catalyst for low‐temperature CO oxidation.     

Adsorption study of xanthates on PbSO<Subscript>4</Subscript> by titration microcalorimetry

Isoperibol (pseudo-adiabatic) titration microcalorimetry was used to study the adsorption of various xanthates [CH₃(CH₂)_nOCS₂^?] at the PbSO₄/aqueous solution interface. The effect of the xanthate alkyl chain length (1n–3n) on the adsorption heat was evaluated. Xanthate adsorption isotherms were also determined. Furthermore, the amount of SO₄ into the aqueous solution was quantified to correlate it with the xanthate uptake by PbSO₄. The adsorption isotherms and the adsorption heat of the xanthates showed two steps. The first step occurred within a sub-monolayer xanthate coverage and was attributed to chemisorption of the xanthates exchanging surface hydroxyls to form CH₃(CH₂)_nOCS₂Pb. Lead xanthate (CH₃(CH₂)_nOCS₂)₂Pb multilayers formed in the second step, which was attributed to an ionic exchange chemical reaction between the xanthates and PbSO₄(aq). In the chemisorption step, the heat was found to be independent of the xanthate alkyl chain length and to linearly decrease in magnitude with the xanthate adsorption. In the multilayer formation step, the magnitude of the integral heat increased with the chain length of the xanthate. Heat contributions due to both the alkyl chain length and the interaction between the xanthate polar group and PbSO₄(aq) for the formation of lead xanthates are presented. Raman spectroscopy was used to characterize the lead xanthate multilayers on PbSO₄.     

The chemical changes occurring upon cycling of a SnO2 negative electrode for lithium ion cell: In situ Mössbauer investigation

Electrochemical reduction of a SnO₂ electrode for a lithium ion cell is known to result in formation of Li_4.4Sn alloy+2Li₂O. In order to determine to which extent such an electrode can be considered as reversible, we studied the electrochemical oxidation of a previously reduced SnO₂ electrode, using in situ ¹¹⁹Sn Mössbauer spectroscopy. Contrary to what could be expected, the first step does not consist in extraction of lithium from Li_4.4Sn for β-Sn to be obtained. In fact, simple lithium extraction proceeds only down to the Li_1.4Sn composition. Further oxidation (second step) involves formation of unusual species (Sn(0) and oxygen-surrounded Sn(II), both probably in interaction with Li₂O). Then (third step), red SnO-like Sn(II) species are formed, along with some Sn(IV). Especially during the second and third steps, the working electrode is far from thermodynamic equilibrium despite the low oxidation rate. This non-equilibrium behavior is probably related to the ultrafine particle size resulting from electrochemical grinding.     

Untersuchungen an oxidischen Katalysatoren. XVII. Zum Zusammenhang zwischen elektrischen und katalytischen Eigenschaften dotierter Zinkoxidkatalysatoren

Studies on Oxide Catalysts. XVII. On the Relations between Electric and Catalytic Properties of Doped Zinc Oxide Catalysts The electric conductivity and thermo EMF of zinc oxide catalysts doped with Li₂O, Ga₂O₃ and Fe₂O₃, and, on the other hand, their activity of dehydrogenation in the catalytic decomposition of isopropanol have been studied. The electric measurements were carried out preferably in an atmosphere of isopropanol. In washed single-phase catalysts doped with Li₂O (n-type semiconductors) the relation between the catalytic activity of dehydrogenation and the FERMI level position postulated by VOL'KEN?TEJN was verified quantitatively. The absorption of the isopropanol is considered to be the rate-determining acceptor step. In unwashed zinc oxide catalysts doped with Li₂O, however, the inversion from n- to p-type (between 0.3 and 0.5 mole-% Li₂O) causes a change in the character of the rate-determining step from an acceptor to a donor step. For the two-phase and three-phase solid systems of ZnO? Ga₂O₃ and ZnO? Fe₂O₃, respectively, it was not possible to find quantitative relations according to the electron theory of catalysis.     

Heavy reflux PSA cycles for CO2 recovery from flue gas: Part I. Performance evaluation

This study evaluated nine stripping PSA cycle configurations, all with a heavy reflux (HR) step, some with a light reflux (LR) step, and some with a recovery (REC) or feed plus recycle (F+R) step, for concentrating CO₂ from stack and flue gas at high temperature (575 K) using a K-promoted HTlc. Under the process conditions studied, the addition of the LR step always resulted in a better process performance; and in all cases, the addition of a REC or F+R step surprisingly did not affect the process performance except at low feed throughputs, where either cycle step resulted in a similar diminished performance. The best cycle based on overall performance was a 5-bed 5-step stripping PSA cycle with LR and HR from countercurrent depressurization (CnD) (98.7% CO₂ purity, 98.7% CO₂ recovery and 5.8 L STP/hr/kg feed throughput). The next best cycle was a 5-bed 5-step stripping PSA cycle with LR and HR from LR purge (96.5% CO₂ purity, 71.1% CO₂ recovery and 57.6 L STP/hr/kg feed throughput). These improved performances were caused mainly by the use of a very small purge to feed ratio (γ=0.02) for the former cycle and a larger one (γ=0.50) for the latter cycle. The former cycle was good for producing CO₂ at high purities and recoveries but at lower feed throughputs, and the latter cycle was useful for obtaining CO₂ at high purities and feed throughputs but at lower recoveries. The best performance of a 4-bed 4-step stripping PSA cycle with HR from CnD was disappointing because of low CO₂ recoveries (99.2% CO₂ purity, 15.2% CO₂ recovery and 72.0 L STP/hr/kg feed throughput). This last result revealed that the recoveries of this cycle would always be much lower than the corresponding cycles with a LR step, no matter the process conditions, and that the LR step was very important to the performance of these HR cycles for this application and process conditions studied.     

Ba2Cu3O5+δ as an intermediate phase during the synthesis of YBa2Cu4O8

We propose a reaction model for the synthesis of YBa₂Cu₄O₈ under normal pressure conditions, which contains 4 partial reaction steps. In a first step bariumnitrate and copperoxide react to Ba₂Cu₃O_5+δ. This substance will be formed for each mixtures Ba:Cu=2∶3...3∶2. The following two partial reaction steps are connected to Ba₂Cu₃O_5+δ, which reacts with Y₂O₃ and CuO to YBa₂Cu₄O₈ or decomposes to BaCuO₂ and CuO. In a last step parts of BaCuO₂ reacts with Y₂O₃ and CuO to YBa₂Cu₄O₈.     

Combined use of H2SO4 and SO2 impregnation for steam pretreatment of spruce in ethanol production

Fuel ethanol can be produced from softwood through hydrolysis in an enzymatic process. Prior to enzymatic hydrolysis of the softwood, pretreatment is necessary. In this study, two-step steam pretreatment employing dilute H₂SO₄ impregnation in the first step and SO₂ impregnation in the second step, to improve the overall sugar and ethanol yield, was investigated. The first pretreatment step was performed under conditions of low severity (180°C, 10 min, 0.5% H₂SO₄) to optimize the amount of hydrolyzed hemicellulose. In the second step, the washed solid material from the first pretreatment step was impregnated with SO₂ and pretreated under conditions of higher severity to make the cellulose more accessible to enzymatic attack, as well as to hydrolyze a portion of the cellulose. A wide range of conditions was used in the second step to determine the most favorable combination. The temperatures investigated were between 190 and 230°C, the residence times were 2, 5, and 10 min; and the SO₂ concentration was 3%. The effect of pretreatment was assessed by both enzymatic hydrolysis of the solids and by simultaneous saccharification and fermentation (SSF) of the whole slurry, after the second pretreatment step. For each set of pretreatment conditions, the liquid fraction was also fermented to determine any inhibitory effects. Ethanol yield using the SSF configuration reached 66% of the theoretical value for pretreatment conditions in the second step of 210°C and 5 min. The sugar yield using the separate hydrolysis and fermentation configuration reached 71% for pretreatment conditions of 220°C and 5 min.     

A Mechanism for Nitrogenase Including Loss of a Sulfide

Nitrogenase is the only enzyme in nature that can fix N₂ from the air. The active cofactor of the leading form of this enzyme contains seven irons and one molybdenum connected by sulfide bridges. In several recent experimental studies, it has been suggested that the cofactor is very flexible, and might lose one of its sulfides during catalysis. In this study, the possible loss of a sulfide has been investigated by model calculations. In previous studies, we have shown that there should be four activation steps before catalysis starts, and this study is based on that finding. It was found here that, after the four reductions in the activation steps, a sulfide will become very loosely bound and can be released in a quite exergonic step with a low barrier. The binding of N₂ has no part in that release. In our previous studies, we suggested that the central carbide should be protonated three times after the four activation steps. With the new finding, there will instead be a loss of a sulfide, as the barrier for the loss is much lower than the ones for protonating the carbide. Still, it is suggested here that the carbide will be protonated anyway, but only with one proton, in the E₃ to E₄ step. A very complicated transition state for H₂ formation involving a large structural change was obtained. The combined step, with a loss of H₂ and binding of N₂, is calculated to be endergonic by +2.3 kcal mol⁻¹; this is in excellent agreement with experiments in which an easily reversible step has been found.     

Dinitrogen Activation by Fryzuk’s [Nb(P2N2)] Complex and Comparison with the Laplaza–Cummins [Mo{N(R)Ar}3] and Schrock [Mo(N3N)] Systems

The reaction profile of N₂ with Fryzuk’s [Nb(P₂N₂)] (P₂N₂=PhP(CH₂SiMe₂NSiMe₂CH₂)₂PPh) complex is explored by density functional calculations on the model [Nb(PH₃)₂(NH₂)₂] system. The effects of ligand constraints, coordination number, metal and ligand donor atom on the reaction energetics are examined and compared to the analogous reactions of N₂ with the three‐coordinate Laplaza‐Cummins [Mo{N(R)Ar}₃] and four‐coordinate Schrock [Mo(N₃N)] (N₃N=[(RNCH₂CH₂)₃N]^3?) systems. When the model system is constrained to reflect the geometry of the P₂N₂ macrocycle, the N? N bond cleavage step, via a N₂‐bridged dimer intermediate, is calculated to be endothermic by 345 kJ mol^?1. In comparison, formation of the single‐N‐bridged species is calculated to be exothermic by 119 kJ mol^?1, and consequently is the thermodynamically favoured product, in agreement with experiment. The orientation of the amide and phosphine ligands has a significant effect on the overall reaction enthalpy and also the N? N bond cleavage step. When the ligand constraints are relaxed, the overall reaction enthalpy increases by 240 kJ mol^?1, but the N₂ cleavage step remains endothermic by 35 kJ mol^?1. Changing the phosphine ligands to amine donors has a dramatic effect, increasing the overall reaction exothermicity by 190 kJ mol^?1 and that of the N? N bond cleavage step by 85 kJ mol^?1, making it a favourable process. Replacing Nb^II with Mo^III has the opposite effect, resulting in a reduction in the overall reaction exothermicity by over 160 kJ mol^?1. The reaction profile for the model [Nb(P₂N₂)] system is compared to those calculated for the model Laplaza and Cummins [Mo{N(R)Ar}₃] and Schrock [Mo(N₃N)] systems. For both [Mo(N₃N)] and [Nb(P₂N₂)], the intermediate dimer is calculated to lie lower in energy than the products, although the final N? N cleavage step is much less endothermic for [Mo(N₃N)]. In contrast, every step of the reaction is favourable and the overall exothermicity is greatest for [Mo{N(R)Ar}₃], and therefore this system is predicted to be most suitable for dinitrogen cleavage.     

Three‐Step Synthesis of 3‐Aryl‐2‐sulfanylthieno[2,3‐b]‐, ‐[2,3‐c]‐, or ‐[3,2‐c]pyridines from the Corresponding Aryl(halopyridinyl)methanones

A convenient three‐step procedure for the synthesis of three types of 3‐aryl‐2‐sulfanylthienopyridines 4, 8 , and 12 has been developed. The first step of the synthesis of thieno[2,3‐b]pyridine derivatives 4 is the replacement of the halo with a (sulfanylmethyl)sulfanyl group in aryl(2‐halopyridin‐3‐yl)methanones 1 by successive treatment with Na₂S?9 H₂O and chloromethyl sulfides to give aryl{2‐[(sulfanylmethyl)sulfanyl]pyridin‐3‐yl}methanones 2 . In the second step, these were treated with LDA (LiNⁱPr₂) to give 3‐aryl‐2,3‐dihydro‐2‐sulfanylthieno[2,3‐b]pyridin‐3‐ols 3 , which were dehydrated in the last step with SOCl₂ in the presence of pyridine to give the desired products. Similarly, thieno[2,3‐c]pyridine and thieno[3,2‐c]pyridine derivatives, 8 and 12 , respectively, can be prepared from aryl(3‐chloropyridin‐4‐yl)methanones 5 and aryl(4‐chloropyridin‐3‐yl)methanones 9 , respectively.     

Photoinduced release of active proteins from TiO2 surfaces

The present work reports on enzyme attachment on and photoinduced release from TiO₂ surfaces. TiO₂ layers (amorphous and anatase) were modified with 3-aminopropyltriethoxysilane (APTES), followed by attachment of vitamin C and horseradish peroxidase (HRP). Using step by step XPS characterization and vis-spectroscopy we show that upon UV illumination the linker chain to the protein can be cut, releasing active HRP into the environment. The head silane group remains attached to the TiO₂ surface. The kinetics of this photoinduced release is significantly faster for the anatase form of TiO₂ compared with amorphous material. The results indicate that UV induced chain scission represents a very versatile tool for payload release from TiO₂ surfaces.     

A combined density functional theory and molecular mechanics (QM/MM) study of single site ethylene polymerization catalyzed by [Cp{NC(tBu)2}TiR+] in the presence of the counterion,CH3B(C6F5)3−

A density functional study was conducted on the approach and insertion of ethylene monomer into the Ti-C_α bond of the catalyst system, CpNC(^tBu)₂RTi-μ-Me-B(C₆F₅)₃ (R= methyl, propyl). A validated QM/MM model was used to represent the counterion. Solvent effects were incorporated with single point solvent calculations done with cyclohexane (ϵ = 2.023) as the solvent. For R=Me (the initiation step), approach and insertion of the ethylene was found to be endothermic, with the barrier for insertion being 12.7 kcal/mol for the most favourable case. For R=Pr (the propagation step), the insertion barrier was found to decrease slightly (11.5 kcal/mol for the most favorable case), corroborating experimental evidence of decrease in insertion barrier with increase in chain length. Termination by chain transfer to monomer was also considered, and found to be unfavourable, in comparison to insertion, by 8.6 kcal/mol for the propagation step. Solvent effects were found to be significant for the propagation step, changing the rate determining step from insertion to uptake for the most favorable case of insertion.     

Two‐Step Oxidation Synthesis of Sulfur with a Red Aggregation‐Induced Emission

Sulfur is not normally considered a light‐emitting material, even though there have been reports of a dim luminescence of this compound in the blue‐to‐green spectral region. Now, it is shown how to make red‐emissive sulfur by a two‐step oxidation approach using elemental sulfur and Na₂S as starting materials, with a high photoluminescence quantum yield of 7.2 %. Polysulfide is formed first and is partially transformed into Na₂S₂O₃ in the first step, and then turns back to elemental S in the second step. The elevated temperature and relatively oxygen‐deficient environment during the second step transforms Na₂S₂O₃ into Na₂SO₃ incorporated with oxygen vacancies, thus resulting in the formation of a solid‐state powder consisting of elemental S embedded in Na₂SO₃. It shows aggregation‐induced emission properties, attributed to the influence of oxygen vacancies on the emission dynamics of sulfur by providing additional lower energy states that facilitate the radiative relaxation of excitons.     

8th Heyrovský discussion “Electrochemistry in non-aqueous solvents: Liblice,near Prague, 20th–24th May, 1974

The S₈^2? polysulphide ion arising from the first reduction step of elemental sulphur (S₈) in dimethylsulphoxide is partly disproportionated into S₆^2? ions and elemental sulphur. It is proposed that the first step of this reaction consists in the formation of S₆^2? ions and S₂ molecules, according to: S₈^2??S₆^2?+S₂This dissociation would be followed by the polymerization of S₂ into S₈. The theoretical values of the limiting currents of the two reduction waves of sulphur at a rotating disk electrode are derived for the proposed reaction scheme, under certain assumptions. Experimental results are in agreement with these calculations.     

Origin of Thermal and Hyperthermal CO2 from CO Oxidation on Pt Surfaces: The Role of Post‐Transition‐State Dynamics,Active Sites,and Chemisorbed CO2

The post‐transition‐state dynamics in CO oxidation on Pt surfaces are investigated using DFT‐based ab initio molecular dynamics simulations. While the initial CO₂ formed on a terrace site on Pt(111) desorbs directly, it is temporarily trapped in a chemisorption well on a Pt(332) step site. These two reaction channels thus produce CO₂ with hyperthermal and thermal velocities with drastically different angular distributions, in agreement with recent experiments (Nature, 2018 , 558, 280–283). The chemisorbed CO₂ is formed by electron transfer from the metal to the adsorbate, resulting in a bent geometry. While chemisorbed CO₂ on Pt(111) is unstable, it is stable by 0.2 eV on a Pt(332) step site. This helps explain why newly formed CO₂ produced at step sites desorbs with far lower translational energies than those formed at terraces. This work shows that steps and other defects could be potentially important in finding optimal conditions for the chemical activation and dissociation of CO₂.