Unstable Intermediate Species Research Articles

Catalytic ammonia decomposition to produce hydrogen: A mini-review

Hydrogen (H2) is considered one of the most promising alternative energy resources instead of fossil fuels, serving as a clean energy carrier. Ammonia (NH3) is regarded as a high percentage hydrogen carrier and utilized as an initial material for H2 production. As for the H2 production through NH3 decomposition, there are various catalytic processes particularly using Ru- and Pd-based catalysts. The Ru- and Ni-based NH3 decomposition catalysts have been studied due to synergetic effect of active metal phase in stabilizing unstable nitrogen intermediate species during NH3 decomposition reaction. The Pd catalyst-based membrane reactors were developed to enhance H2 separation from the product stream of NH3 decomposition. Integrated systems combining Ru catalyst and Pd-based membrane were also explored to achieve a high yield of H2 from NH3. This review summarizes recent literatures on catalytic NH3 decomposition to produce H2. It probably provides useful insights into designing large-scale H2 production processes.

Structural and functional biomimetics of [Fe]-hydrogenase featuring a mono-, di- or tetrasubstituted pyridine ligand with a fac-C, N, and S ligation.

The combined structural and functional modeling study of [Fe]-H2ase has remained a great challenge, to date. Now, we report a series of new structural and functional [Fe]-H2ase models (1-6) that contain a mono-, di- or tetrasubstituted pyridine ligand with a biomimetic fac-C, N, and S ligation. While models 1-3, 5 and 6 are conveniently prepared by a novel flexible pyridine ligand (FPL)-based method, model 4 is prepared simply by an intramolecular CO replacement reaction of model 3. More interestingly, the structural study by spectroscopy and X-ray crystallography proves that these new models include a biomimetic fac-acyl (or methylene) C, pyridyl N, and thioether S ligation to an Fe(II) center and the C-Fe(II) σ bond is trans to an iodo ligand. In addition, the chemical reactivity study proves that they all have the enzyme-like H2 activation and hydride transfer functions in the presence of imidazolium Im+, AgBF4 and Et3N. Particularly interesting is that a possible pathway for such H2 activation and hydride transfer reactions catalyzed by a representative model 4 is proposed and the existence of the highly unstable 5-coordinate intermediate M4 and Fe-H species M5 is supported by the isolation and characterization of their MeCN-coordinated derivative 7 and chloro-substituted derivative 8, respectively.

Glucosamine and Its Analogues as Modulators of Amyloid-β Toxicity.

In Alzheimer's disease (AD), amyloid-β (Aβ) oligomers are considered key mediators of synaptic dysfunction and cognitive impairment. These unstable intermediate Aβ species can interfere with different cellular organelles, leading to neuronal cell death, through the formation of Ca2+-permeable membrane pores, impairment in the levels of acetylcholine neurotransmitters, increased insulin resistance, promotion of pro-inflammatory cascades, among others. Based on a series of evidences that indicate the key role of glycosaminoglycans (GAGs) in amyloid plaque formation, we evaluated the capacity of four monosaccharides, i.e., glucosamine (GlcN), N-acetyl glucosamine (GlcNAc), glucosamine-6-sulfate (GlcN6S), and glucosamine-6-phosphate (GlcN6P), to reduce the Aβ-mediated pathological hallmarks. The tested monosaccharides, in particular, GlcN6S and GlcN6P, were able to interact with Aβ aggregates, reducing neuronal cell death, Aβ-mediated damage to the cellular membrane, acetylcholinesterase activity, insulin resistance, and pro-inflammation levels.

Redox-Sensing Iron-Sulfur Cluster Regulators.

Iron-sulfur cluster proteins carry out multiple functions, including as regulators of gene transcription/translation in response to environmental stimuli. In all known cases, the cluster acts as the sensory module, where the inherent reactivity/fragility of iron-sulfur clusters with small/redox-active molecules is exploited to effect conformational changes that modulate binding to DNA regulatory sequences. This promotes an often substantial reprogramming of the cellular proteome that enables the organism or cell to adapt to, or counteract, its changing circumstances. Recent Advances: Significant progress has been made recently in the structural and mechanistic characterization of iron-sulfur cluster regulators and, in particular, the O2 and NO sensor FNR, the NO sensor NsrR, and WhiB-like proteins of Actinobacteria. These are the main focus of this review. Striking examples of how the local environment controls the cluster sensitivity and reactivity are now emerging, but the basis for this is not yet fully understood for any regulatory family. Characterization of iron-sulfur cluster regulators has long been hampered by a lack of high-resolution structural data. Although this still presents a major future challenge, recent advances now provide a firm foundation for detailed understanding of how a signal is transduced to effect gene regulation. This requires the identification of often unstable intermediate species, which are difficult to detect and may be hard to distinguish using traditional techniques. Novel approaches will be required to solve these problems.

Exploring the acid catalyzed isomerization of phenanthrene under confinement in mordenite

The isomerization reaction of 1,2-dihydrophenanthrene catalyzed by a mordenite zeolite is explored theoretically in the present study. It was found that the electrostatic adducts which resemble ion-pair and π complexes, detected for zeolite catalysts, could enable the reaction to proceed through an energy favorable channel and explain its selectivity. The reaction pathway found for the interaction of 1,2-dihydrophenanthrene with the acid site of a mordenite zeolite proceeds firstly through a hydrogen abstraction, followed by a concerted cyclization pathway with C-C bond β-scission assisted by the reagent interaction with the zeolite catalyst. Then, two sequential migrations take place over the five-ring cycled product leading to unstable carbocation intermediate species which could be hydrogenated by an H2 molecule. The Al site within the MOR zeolite framework, does not activate the H-H bond. The C-C bond scission-cyclization process is detected as the energetic limiting step that could be fostered due to the confinement effect induced by the zeolite porous structure.

Redox-sensing iron-sulfur cluster regulators.

Iron-sulfur cluster proteins carry out a wide range of functions, including as regulators of gene transcription/translation in response to environmental stimuli. In all known cases, the cluster acts as the sensory module, where the inherent reactivity/fragility of iron-sulfur clusters towards small/redox active molecules is exploited to effect conformational changes that modulate binding to DNA regulatory sequences. This promotes an often substantial re-programming of the cellular proteome that enables the organism or cell to adapt to, or counteract, its changing circumstances. Recent Advances. Significant progress has been made recently in the structural and mechanistic characterization of iron-sulfur cluster regulators and, in particular, the O2 and NO sensor FNR, the NO sensor NsrR, and WhiB-like proteins of Actinobacteria. These are the main focus of this review. Striking examples of how the local environment controls the cluster sensitivity and reactivity are now emerging, but the basis for this is not yet fully understood for any regulatory family. Characterization of iron-sulfur cluster regulators has long been hampered by a lack of high resolution structural data. Though this still presents a major future challenge, recent advances now provide a firm foundation for detailed understanding of how a signal is transduced to effect gene regulation. This requires the identification of often unstable intermediate species, which are difficult to detect and may be hard to distinguish using traditional techniques. Novel approaches will be required to solve these problems.

Unexpected benzimidazole ring formation from a quinoneimide species in the presence of ammonium acetate as supporting electrolyte used in the coupling of electrochemistry with mass spectrometry

Electrochemistry (EC) coupled to mass spectrometry (MS) has been used to study different phase-I reactions. Despite of the versatility of EC/MS, the effect of the nature of the supporting electrolyte on the formation of oxidation products has seldom been discussed during EC/MS experiments. Here, we present a comparison of two different supporting electrolytes and their effect on the identification of unstable intermediate oxidation species is discussed. The oxidation of acebutolol was performed with a coulometric cell in the presence of two supporting electrolytes namely ammonium acetate and lithium acetate. Ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UPLC/QTOFMS) using a binary gradient (water/acetonitrile) with positive electrospray ionization was used to identify the oxidation products in the presence and absence of glutathione. Chemical structure elucidations of the oxidation products were performed by high-resolution mass spectrometry (HRMS) and were also supported by nuclear magnetic resonance (NMR) measurements. From the electrochemical study and HRMS measurements, we demonstrate that the quinoneimide species resulting from the oxidative hydrolyses of acebutolol gives a benzimidazole ring product in the presence of ammonium acetate. Through the example of the oxidation of acebutolol, a correlation between the supporting electrolyte nature and oxidation product formation was established. The obtained results were supported by quantum mechanical calculations. We present here evidence of the side reactions induced by the presence of ammonia as supporting electrolyte during EC/MS measurements. Acebutolol was used as a model to postulate an uncommon and unexpected side reaction leading to benzimidazole ring formation. The findings may help to understand the identification of the intermediate species in the oxidative degradation process.

Traversing Hot-Jet Ignition in a Constant-Volume Combustor

Hot-jet ignition of a combustible mixture has application in internal combustion engines, detonation initiation, and wave rotor combustion. Numerical predictions are made for ignition of combustible mixtures using a traversing jet of chemically active gas at one end of a long constant-volume combustor (CVC) with an aspect ratio similar to a wave rotor channel. The CVC initially contains a stoichiometric mixture of ethylene or methane at atmospheric conditions. The traversing jet issues from a rotating prechamber that generates gaseous combustion products, assumed at chemical equilibrium for estimating major species. Turbulent combustion uses a hybrid eddy-breakup model with detailed finite-rate kinetics and a two-equation k-ω model. The confined jet is observed to behave initially as a wall jet and later as a wall-impinging jet. The jet evolution, vortex structure, and mixing behavior are significantly different for traversing jets, stationary centered jets, and near-wall jets. Pressure waves in the CVC chamber affect ignition through flame vorticity generation and compression. The jet and ignition behavior are compared with high-speed video images from a prior experiment. Production of unstable intermediate species like C2H4 and CH3 appears to depend significantly on the initial jet location while relatively stable species like OH are less sensitive.

In situ NMR spectroelectrochemistry for the structure elucidation of unstable intermediate metabolites

In situ NMR spectroelectrochemistry is presented in this study as a useful hybrid technique for the chemical structure elucidation of unstable intermediate species. An experimental setting was designed to follow the reaction in real time during the experimental electrochemical process. The analysis of (1)H NMR spectra recorded in situ permitted us (1) to elucidate the reaction pathway of the electrochemical oxidation of phenacetin and (2) to reveal the quinone imine as a reactive intermediate species without using any trapping reaction. Phenacetin has been considered as hepatotoxic at high therapeutic amounts, which is why it was chosen as a model to prove the applicability of the analytical method. The use of 1D and 2D NMR experiments led to the elucidation of the major species produced from the oxidation process. We demonstrated that in situ NMR spectroelectrochemistry constitutes a fast way for monitoring unstable quinone imines and elucidating their chemical structures.

ATR-FTIR Study of the Decomposition of Acetic Anhydride on Fosfotungstic Wells–Dawson Heteropoly Acid Using Concentration-Modulation Excitation Spectroscopy

The decomposition of acetic anhydride in liquid phase on a fosfotungstic Wells–Dawson heteropoly acid (HPA) was investigated by in situ ATR-FTIR spectroscopy. Transient and concentration-modulation excitation spectroscopy (MES) experiments in combination with phase-sensitive detection (PSD) were used to monitor the solid–liquid interface. The MES method is based on the periodic variation of a parameter of the reaction media such as, the reactant concentration. That periodic alteration causes varying infrared signals of the surface adsorbed species that are subsequently demodulated with the PSD methodology. Thus, the separation of the static signals from the changing ones is achieved, and species with different response can be observed in the spectra. Using MES-PSD coupled with ATR-FTIR, acetic anhydride was observed to decompose to acetic acid, acetate and acyl [CH3C(O)+] species, involving Bronsted acid sites of the HPA catalyst. The CH3C(O)+ is a very unstable intermediate species and it is the key intermediate in the Friedel–Crafts acylation reactions. Moreover, the acetate groups are spectator species since remain strongly adsorbed on the catalyst surface and do not further react.

Enhancement Effects of Gas Components on NO Removal by Calcium Hydroxide

The NO absorption characteristic of calcium hydroxide was investigated in a fixed bed reactor system in the presence of SO2. The effect of O2 and H2O on NO removal was discussed in or without the presence of SO2. When SO2 was excluded from the flue gas, NO removal was slight. The presence of SO2 in the flue gas could enhance NO removal only when both of O2 and H2O were also present. The essence of enhancement of SO2 on NO removal was accelerating the conversion of NO to NO2. The desulfurization reaction product could not oxidize NO to NO2. The bed height experiment revealed that the multimolecular intermediate species, which could decomposed in to NO2, was not formed from the adsorption of NO, H2O, and O2 on the surface of absorbent. The possible reaction path proposed in paper is that there may be some unstable intermediate species formed from the desulfurization reaction, which can enhance the conversion of NO to NO2. The presence of H2O can enhance the presence of unstable intermediate. According to analysis, the possible unstable intermediate is the OH radical.

Direct Observation of Unstable Intermediate Species in the Reaction of trans-2-Butene on Ferrierite Zeolite by Picosecond Infrared Laser Spectroscopy

The reaction dynamics of trans-2-butene adsorbed to acidic hydroxyl groups on the surface of ferrierite zeolite is examined by time-resolved spectroscopy using a tunable infrared picosecond pulse laser system. The transient absorption spectra measured by a two-color pump-probe technique at 188-243 K reveal bleaching and hot bands of the OD stretching mode 2 ps after excitation. This vibrationally excited state relaxes within 20 ps at 188 K, while the bleaching band includes a long-lifetime component that lasts for more than 100 ps at 243 K. Thus, the OD (isotope-exchanged hydroxy groups) stretching band does not entirely recover in this period and is mirrored by an analogous weakening of the CH bending band of the adsorbed trans-2-butene. Simultaneously, three new bands in CH stretching region were observed at 3045, 3095, and 3130 cm(-1). This result suggests the presence of a short-lived intermediate formed by reaction between the acidic hydroxyl groups and adsorbed trans-2-butene.

In Situ Time-Resolved Energy-Dispersive X-ray Absorption Fine Structure Study on the Decarbonylation Processes of Mo(CO)6Entrapped in NaY and HY Zeolites

Temperature-programmed decarbonylation processes of Mo(CO)6 entrapped in NaY and HY zeolites were investigated in situ by the time-resolved energy-dispersive X-ray absorption fine structure (DXAFS) technique. The DXAFS study revealed that the decarbonylation of Mo(CO)6 in both NaY and HY proceeded through molybdenum subcarbonyl species and that the subsequent formation of decarbonylated species is affected by the stability of the subcarbonyl intermediate species. In the case of Mo(CO)6/NaY, stable Mo(CO)3(OL)3 (OL, oxygen atoms of the zeolite framework) species were formed at 400−500 K and Mo(II) oxocarbide dimeric species Mo2(C)Ox were formed above 500 K. In the case of Mo(CO)6/HY, on the other hand, unstable Mo(CO)3(OL)x (x = 1−2) species were formed around 450 K and they reacted easily with protons in zeolite supercages to be converted to Mo(II) monomer species. An unstable intermediate species was observed, and its structure was determined by DXAFS for the first time.

Theoretical sstudy on the Co2+OH2+/Co3+OH2+ electron transfer reactivity

This paper presents a contact distance dependence analysis scheme and an abinitio calculation application for the electron transfer (ET) reactivity of Co2+OH2/Co3+OH2 reacting pair. The applicability of these schemes and the corresponding models has been discussed. The contact distance (R CoCo) dependence of the relevant quantities has been analyzed. The results indicate that the activation energy from the accurate PES method agrees well with that from the anharmonic potential method, and they are obviously better than that from the harmonic potential method. The pair distribution function varies from 10−2 to 10−5 along withR CoCo changing from 1.20 to 0.35 nm. The coupling matrix element exponentially decays along with the increase ofR CoCo, and the effective electronic coupling requiresR CoCo smaller than 0.75 nm. In the range from 0.50 to 0.75 nm forR CoCo, the corresponding electronic transmission coefficient falls within 1.0—10−6. The local ET rate also exponentially decays along with the increase ofR CoCo due to the electronic factor. Since the contribution from the pair distribution function to the total ET rate is an inverse measure of that from the electronic factor, the variation of the spherically averaged local ET rate along withR CoCo exhibits a parabola with a maximum at 0.50 nm ofR CoCo. This maximum is close to the overall observed ET rate value. For this mono-hydrated transition metal ionic system, the ET rate generally is about 106 L·mol−1·s−1 in gaseous process. Further, since it is impossible to experimentally determine the structures and their PESs of these hydrated systems, especially for the unstable intermediate species,ab initio calculations can play an effective auxiliary role in discussing the ET reactivities of these kinds of reacting systems.

Theoretical study on the Co 2+ OH 2 /Co 3+ OH 2 electron transfer reactivity

This paper presents a contact distance dependence analysis scheme and an ab initio calculation application for the electron transfer (ET) reactivity of Co 2+ OH 2 /Co 3+ OH 2 reacting pair. The applicability of these schemes and the corresponding models has been discussed. The contact distance ( R CoCo ) dependence of the relevant quantities has been analyzed. The results indicate that the activation energy from the accurate PES method agrees well with that from the anharmonic potential method, and they are obviously better than that from the harmonic potential method. The pair distribution function varies from 10 ?2 to 10 ?5 along with R CoCo changing from 1.20 to 0.35 nm. The coupling matrix element exponentially decays along with the increase of R CoCo , and the effective electronic coupling requires R CoCo smaller than 0.75 nm. In the range from 0.50 to 0.75 nm for R CoCo , the corresponding electronic transmission coefficient falls within 1.0?/FONT>10 ?6 . The local ET rate also exponentially decays along with the increase of R CoCo due to the electronic factor. Since the contribution from the pair distribution function to the total ET rate is an inverse measure of that from the electronic factor, the variation of the spherically averaged local ET rate along with R CoCo exhibits a parabola with a maximum at 0.50 nm of R CoCo . This maximum is close to the overall observed ET rate value. For this mono-hydrated transition metal ionic system, the ET rate generally is about 10 6 L·mol ?1 ·s ?1 in gaseous process. Further, since it is impossible to experimentally determine the structures and their PESs of these hydrated systems, especially for the unstable intermediate species, ab initio calculations can play an effective auxiliary role in discussing the ET reactivities of these kinds of reacting systems.

Aqueous chemistry of N-halo-compounds

Halogens in aqueous solution are still used world-wide as disinfectants. During the process of halogenation, the substances present in water undergo several chemical processes, yielding relatively unstable intermediate species; their life-times in the medium depend on their structure and on the physico-chemical conditions. Several low molecular weight hydrocarbons are formed during water halogenation, some of them potent mutagens and/or carcinogens. Halogenation also takes place in vivo involving the system myeloperoxidase/H2O2/halide, which increases the relevance of such reactions and opens new research fields.

Phosphido-bridged dimolybdenum complexes with sulfide and thiolate ligands as precursors to mixed-metal clusters

Treatment of [Mo 2(μ-H)(μ-PPh 2)(CO) 4(η-C 5H 5) 2] with an excess of a thiol, RSH, in refluxing toluene produced the novel quadruply-bridged compounds [Mo 2(μ-S) 2(μ-SR)(μ-PPh 2)(η-C 5H 5) 2] (7a, R = Et; 7b, R = Pr 1; 7c, R = Bu 1; 7d, R = p-C 6H 4Me; 7c R = C 12H 25) in moderate to good yield. The sulfur ligands in the product are formed by dealkylation of the thiol at the dimolybdenum centre. The reaction is proposed to proceed via the unstable intermediate species [Mo 2(μ-SR)(μ-PPh 2)(CO) 2(η-C 5H 5) 2], which could not be isolated in a pure form. Reaction of 7a-d with [Ru 3(CO) 12] in refluxing tetrahydrofuran (thf) produced good yields of the mixed-metal clusters [Mo 2Ru 2( μ 3S) 2( μ-SR)( μ-PPh 2)(CO) 4( η-C 5H 5) 2] 9; the X-ray crystal structure of the Et-substituted complex 9a has been determined. The cluster consists of a tetrahedral metal core with both Mo 2Ru faces capped by triply bridging sulfides. The phosphido group bridges the MoMo bond, whereas the thiolate ligand rated to the RuRu edge.

A spectroelectrochemical study of the interaction of riboflavin with β-cyclodextrin

The interaction between β-cyclodextrin (β-CD) and riboflavin together with its electrogenerated intermediate anion radicals has been specifically investigated by in situ electrochemical UV-visible spectroscopy. It appears that the anion radicals are included more tightly than the parents and stabilized in the special microenvironment of the β-CD cavity. The utility of β-CD for probing the existence of an unstable intermediate species at the electrode is also demonstrated.

Structural alterations of highly dispersed Rh/TiO 2 catalyst upon CO adsorption and desorption investigated by infrared spectroscopy

Structural alterations of highly dispersed Rh/TiO 2 catalyst upon CO adsorption and desorption was investigated by Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy. It was found that, during CO adsorption and desorption, the inter-conversions between the three adsorbed species, namely linear Rh 0CO at ca. 2070 cm −1, bridged RH 0 2CO at 1865–1900 cm −1 and gem dicarbonyl Rh I(CO) 2 at 2096 and 2032 cm −1, involve some unstable intermediate species. The CO adsorption at 300 K, resulted in the disruption of rhodium crystallites to isolated rhodium sites, and was found to proceed via a bridged Rh I 2CO species (at ca. 2005 cm −1) as intermediate. The decarbonylation of supported Rh I(CO) 2 at elevated temperatures, which leads to the agglomeration of rhodium sites, takes place involving Rh I 2CO and Rh ICO (at ca. 2087 cm −1) as intermediates. In addition to alteration of the dispersive state of rhodium, the structure of the TiO 2 carrier was also observed to be changed during the adsorption of CO at 300 K, as manifested by the enhancement or depletion of surface hydroxyl groups on the carrier. The presence of hydrogen significantly promoted the decarbonylation of Rh I(CO) 2, via a process which did not involve Rh I 2CO and Rh ICO as intermediates. While no chemical reaction between the gem dicarbonyl species and hydrogen was observed in the temperature range of 300–450 K, dissociation of the CO bond and formation of hydrocarbon fragments, mainly originating from the bridged adsorbed CO, already occurred at temperatures as low as 325 K.

CO migration on Pt(100) and Pt(11 1 1) surfaces studied by time resolved infrared reflection-absorption spectroscopy

Carbon monoxide (CO) migration on Pt(100) and Pt(11 1 1) single crystal electrode surfaces in an acid solution was studied by electrode potential modulated infrared reflection-absorption spectroscopy and time resolved infrared reflection-absorption spectroscopy. The interconversion of CO adsorbed on both electrode surfaces between on-top, asymmetric and symmetric bridge-bonded sites was caused by the applied electrode potentials. Unstable CO intermediate species were created during the interconversion of their sites.