Mechanism Of Isomerization Research Articles

Probing the Hidden Photoisomerization of a Symmetric Phosphaalkene Switch

In this study, we present the synthesis and analysis of a novel, air‐stable, and solvent‐resistant phosphaalkene switch. Using this symmetric switch, we have demonstrated degenerate photoisomerization experimentally for the first time. With a combination of photochemical‐exchange NMR spectroscopy, ultrafast transient absorption spectroscopy, and quantum chemical calculations, we elucidate the isomerization mechanism of this symmetric phosphaalkene. Our findings highlight the critical role of the isolobal analogy between C=P and C=C bonds in governing nanoscale molecular motion and break new ground for our understanding of light‐induced molecular processes in symmetric heteroalkene systems.

Probing the Hidden Photoisomerization of a Symmetric Phosphaalkene Switch.

In this study, we present the synthesis and analysis of a novel, air-stable, and solvent-resistant phosphaalkene switch. Using this symmetric switch, we have demonstrated degenerate photoisomerization experimentally for the first time. With a combination of photochemical-exchange NMR spectroscopy, ultrafast transient absorption spectroscopy, and quantum chemical calculations, we elucidate the isomerization mechanism of this symmetric phosphaalkene. Our findings highlight the critical role of the isolobal analogy between C=P and C=C bonds in governing nanoscale molecular motion and break new ground for our understanding of light-induced molecular processes in symmetric heteroalkene systems.

Enantioselective regiospecific addition of propargyltrichlorosilane to aldehydes catalyzed by biisoquinoline N,N'-dioxide.

Distilled propargyltrichlorosilane with >99% isomeric purity was prepared for the first time, and its asymmetric catalytic regiospecific addition reaction to aldehydes was developed through a systematic catalyst structure-reactivity and selectivity relationship study. The observed catalyst structure-enantioselectivity relationship of the present allenylation reaction was found exactly opposite to that of the analogous allylation reaction. The method provided eleven α-allenic alcohols in 22-99% yield with 61:39-92:8 enantiomeric ratios. Furthermore, possible mechanisms of propargyl-allenyl isomerization of propargyltrichlorosilane were computationally investigated.

Quality aspects during pine resin storage: Appearance deterioration, turpentine chemical components change, kinetic model and insights into isomerization mechanisms

Pine resin is a promising natural resource with various industrial applications and economic value. However, the quality of pine resin deteriorates during storage, resulting in a decline in the quality of rosin and turpentine. This study compares the effects of deionized and surface water, as well as deionized and surface water containing 5 % impurities, on the quality of pine resin over a six-month storage period. The color of pine resin was evaluated using the CIE L*a*b* color space, and the softening point and acid value of rosin and the refractive index of turpentine obtained from the distillation of pine resin were examined. Furthermore, the alterations in the chemical constitution of turpentine were examined through GC-MS. The results demonstrated that the appearance of pine resin gradually deteriorated with increasing storage time. The acid value and softening point are within normal limits (159 ∼ 170 mg/g, approximately 75 °C), and the color of rosin begins to redden, yellow, and darken with time, with a maximum deterioration to grade 5 at 6 months. The refractive index of turpentine gradually increased with time, from 1.4661 to 1.4687, after 6 months of storage. The pinene content declined, with a maximum decrease of 51.88 % at 6 months. In addition, the network diagram was constructed, which delineates the reaction pathways of pinene during storage. The isomerization of α-pinene and β-pinene produces monocyclic terpenoids, primarily including p-cymene and isoterpinolene. A first-order kinetic model was constructed to describe the mechanism of the transformation of α-pinene and β-pinene. The results provided a scientific rationale for further endeavors to enhance the quality of pine resin and associated commodities during the storage.

Aromatics Alkylated with Olefins Utilizing Zeolites as Heterogeneous Catalysts: A Review

The alkylation reaction of aromatic compounds gains considerable attention because of its wide application in bulk and fine chemical production. Aromatics alkylated with olefins is a well-known process, particularly for linear alkylbenzene, phenyloctanes, and heptyltoluene production. As octane boosters and precursors for various petrochemical and bulk chemical products, a wide range of alkylated compounds are in high demand. Numerous unique structures have been proposed in addition to the usual zeolites (Y and beta) utilized in alkylation procedures. The inevitable deactivation of industrial catalysts over time on stream, which is followed by a decrease in catalytic activity and product selectivity, is one of their disadvantages. Therefore, careful consideration of catalyst deactivation regarding the setup and functioning of the process of catalysis is necessary. Although a lot of work has been carried out to date to prevent coke and increase catalyst lifespan, deactivation of the catalyst is still unavoidable. Coke deposition can lead to catalyst deactivation in industrial catalytic processes by obstructing pores and/or covering acid sites. It is very desirable to regenerate inactive catalysts in order to remove the coke and restore catalytic activity at the same time. Depending on the kind of catalyst, the deactivation processes, and the regeneration settings, each regeneration approach has pros and cons. In this comprehensive study, the focus was on discussing the reaction mechanism of 1-octene isomerization and toluene alkylation as an example of isomerization and alkylation reactions that occur simultaneously, shedding light in detail on the catalysts used for this type of complex reaction, taking into account the challenges facing the catalyst deactivation and reactivation procedures.

Research on the dynamics and mechanism of thermal cracking of coal-based endothermic hydrocarbon fuels

Thermal cracking of endothermic hydrocarbon fuels (EHF) plays an important role in the endothermic process when the fuel temperature exceeds the supercritical temperature. To gain a deeper understanding of the thermal cracking behavior of EHF, this work discusses the thermal cracking law and reaction mechanism of coal-based EHF at different temperatures through static thermal cracking experiments, and establishes a total molecular reaction dynamics model containing 34 species and 24-step reactions based on product distribution. The results show that in the temperature range of 450℃ to 500℃, the gas phase yield of the fuel increases linearly from 8 % to 56 %, and the conversion rate reaches up to 96.1 %. The kinetic reaction of thermal cracking conforms to the primary kinetic equation, the cracking rate constant k is between 1.08×10−4∼7.92×10−4 s−1, the activation energy Ea=184.47±11.5 kJ∙mol−1, and the precursor factor lnA=21.61±3.0. Gas phase products include hydrogen, methane, ethane, ethylene, propane and butane. Liquid phase products include paraffins, alkyldecahydronaphthalenes and aromatic hydrocarbons. As the cracking depth increases, the degree of fuel branching first increases and then decreases, up to 0.33. Paraffins and alkyldecahydronaphthalenes are gradually converted into olefins, cyclic olefins, benzene, naphthalene, indene, fluorene and pyrene and other compounds. Based on the product distribution, the reaction mechanism of thermal cracking of coal-based EHF is speculated to include single-molecule β-cracking, bimolecular F-S-S, intramolecular H-transfer, cyclization and isomerization mechanisms.

Theoretical study on the isomerization mechanism of azobenzene derivatives on graphene substrate

Theoretical study on the isomerization mechanism of azobenzene derivatives on graphene substrate

Photochromic reaction pathways of 1,1′-azobis-1,2,3-triazole: A CASSCF and spin-flip DFT study

This study employed multiconfigurational CASSCF and MS-CASPT2 methods to investigate the photoinduced isomerization mechanism of 1,1′-azobis-1,2,3-triazole. The MS-CASPT2//CASSCF computational results indicate that the photoisomerization reaction of 1,1′-azobis-1,2,3-triazole involves a non-adiabatic transition pathway through the S2 state. After photoexcitation to the S2 state, the molecule undergoes internal conversion to reach S1-min, followed by a non-radiative transition back to the ground state. The S1/S0-CI and S1-min have similar structures and close energies, which facilitates unidirectional rotation. The weak coupling between the ground state and excited states may be due to the strong electron-withdrawing nature of the N8 chain. Additionally, using the MS-CASPT2//CASSCF computational results as a reference, we compared the differences in describing the photoisomerization process of this compound using the SF-DFT method, both qualitatively and quantitatively. The results demonstrate that the SF-DFT method significantly underestimates the energy of the S1 state. These findings are expected to deepen the understanding of non-adiabatic transitions in the photoinduced rotation of high-nitrogen compounds and provide theoretical insights for other high-nitrogen compounds that may exhibit photochromism.

Proline substitutions in the ASIC1 β11-12 linker slow desensitization

Desensitization is a prominent feature of nearly all ligand-gated ion channels. Acid-sensing ion channels (ASICs) undergo desensitization within hundreds of milliseconds to seconds upon continual extracellular acidification. The ASIC mechanism of desensitization is primarily due to the isomerization or “flipping” of a short linker joining the 11th and 12th β sheets in the extracellular domain. In the resting and active states this β11-12 linker adopts an “upward” conformation while in the desensitized conformation the linker assumes a “downward” state. It is unclear if a single linker adopting the downward state is sufficient to desensitize the entire channel, or if all three are needed or some more complex scheme. To accommodate this downward state, specific peptide bonds within the linker adopt either trans-like or cis-like conformations. Since proline-containing peptide bonds undergo cis-trans isomerization very slowly, we hypothesized that introducing proline residues in the linker may slow or even abolish ASIC desensitization, potentially providing a valuable research tool. Proline substitutions in the chicken ASIC1 β11-12 linker (L414P and Y416P) slowed desensitization decays approximately 100- to 1000-fold as measured in excised patches. Both L414P and Y416P shifted the steady-state desensitization curves to more acidic pH values while activation curves and ion selectivity were largely unaffected (except for a left-shifted activation pH50 of L414P). To investigate the functional stoichiometry of desensitization in the trimeric ASIC, we created families of L414P and Y416P concatemers with zero, one, two, or three proline substitutions in all possible configurations. Introducing one or two L414P or Y416P substitutions only slightly attenuated desensitization, suggesting that conformational changes in the single remaining faster wild-type subunits were sufficient to desensitize the channel. These data highlight the unusual cis-trans isomerization mechanism of ASIC desensitization and support a model where ASIC desensitization requires only a single subunit.

Aluminiumalkylinduzierte Isomerisierung von Gruppe IV meso Metallocen Komplexen

AbstractBei der Synthese von Gruppe IV Metallocen‐Präkatalysatoren für die Polymerisation von Propylen werden im Allgemeinen zwei verschiedene Isomere gebildet: Das racemische Isomer, das isotaktisches Polypropylen (iPP) produziert und das meso‐Isomer, das ataktisches Polypropylen (aPP) produziert. Aufgrund seiner vergleichbar schlechten physikalischen Eigenschaften ist die Anwendung von aPP sehr begrenzt. Um Mischungen beider Polymere zu vermeiden, was die mechanischen und thermischen Eigenschaften von iPP zu verschlechtern würde, müssen die meso Metallocenkomplexe mühsam von den racemischen Komplexen getrennt werden, was die Metallocen‐basierte Polymerisation von Propylen industriell weit weniger attraktiv macht als das Ziegler/Natta‐Verfahren. Um dieses Problem zu umgehen, haben wir ein Isomerisierungsprotokoll zur Umwandlung von meso Metallocenkomplexen in ihre racemischen Analoga entwickelt. Dieses Protokoll steigerte die Ausbeute an iPP um 400 %, wobei die hervorragenden physikalischen Eigenschaften des Polymers beibehalten wurden; es war sowohl auf Hafnocen‐, als auch auf Zirkonocenkomplexe, sowie auf verschiedene Methoden zur Aktivierung des Präkatalysators anwendbar. Durch gezielte Variation der Liganden wurden Methoxygruppen an den Indenylsubstituenten als die für eine Isomerisierung verantwortlichen Strukturmotive gefunden ‐ dieser experimentelle Nachweis wurde durch Dichtefunktionaltheorieberechnungen bestätigt. Flüssig‐Injektion‐Feld‐Desorption‐Ionisierung‐Massenspektrometrie, sowie 1H‐ und 29Si‐Kernresonanzstudien ermöglichten den Vorschlag eines Isomerisierungsmechanismus.

Aluminum Alkyl Induced Isomerization of Group IV meso Metallocene Complexes.

The synthesis of group IV metallocene precatalysts for the polymerization of propylene generally yields two different isomers: The racemic isomer that produces isotactic polypropylene (iPP) and the meso isomer that produces atactic polypropylene (aPP). Due to its poor physical properties, aPP has very limited applications. To avoid obtaining blends of both polymers and thus diminish the mechanical and thermal properties of iPP, the meso metallocene complexes need to be separated from the racemic ones tediously-rendering the metallocene-based polymerization of propylene industrially far less attractive than the Ziegler/Natta process. To overcome this issue, we established an isomerization protocol to convert meso metallocene complexes into their racemic counterparts. This protocol increased the yield of iPP by 400 % while maintaining the polymer's excellent physical properties and was applicable to both hafnocene and zirconocene complexes, as well as different precatalyst activation methods. Through targeted variation of the ligand frameworks, methoxy groups at the indenyl moieties were found to be the structural motifs responsible for an isomerization to take place-this experimental evidence was confirmed by density functional theory calculations. Liquid injection field desorption ionization mass spectrometry, as well as 1H and 29Si nuclear magnetic resonance studies, allowed the proposal of an isomerization mechanism.

"On-the-Fly" Nonadiabatic Dynamics Simulation on the Ultrafast Photoisomerization of a Molecular Photoswitch Iminothioindoxyl: An RMS-CASPT2 Investigation.

Iminothioindoxyl (ITI) is a new class of photoswitch that exhibits many excellent properties including well-separated absorption bands in the visible region for both conformers, ultrafast Z to E photoisomerization as well as the millisecond reisomerization at room temperature for the E isomer, and switchable ability in both solids and various solvents. However, the underlying ultrafast photoisomerization mechanism at the atomic level remains unclear. In this work, we have employed a combination of high-level RMS-CASPT2-based static electronic structure calculations and nonadiabatic dynamics simulations to investigate the ultrafast photoisomerization dynamics of ITI. Based on the minimum-energy structures, minimum-energy conical intersections, linear interpolation internal coordinate paths, and nonadiabatic dynamics simulations, the overall photoisomerization scenario of ITI upon excitation is established. Upon excitation around 416 nm, the molecule will be excited to the S2 state considering its close energy to the experimentally measured absorption maximum and larger oscillator strength, from which ultrafast decay of S2 to S1 state can take place efficiently with a time constant of 62 fs. However, the photoisomerization is not likely to complete in the S2 state since the dihedral associated with the Z to E isomerization changes little during the relaxation. Upon relaxing to the S1 state, the molecule will decay to the S0 state ultrafast with a time constant of 232 fs. In contrast, the decay of the S1 state is important for the isomerization considering that the dihedral related to the isomerization of the hopping structures is close to 90°. Therefore, the S1/S0 intersection region should be important for the isomerization of ITI. Arriving at the S0 state, the molecule can either go back to the original Z reactant or isomerize to the E products. At the end of the 500 fs simulation time, the E configuration accounts for nearly 37% of the final structures. Moreover, the photoisomerization mechanism is different from the isomerization mechanism in the ground state; i.e., instead of the inversion mechanism in the ground state, the photoisomerization prefers the rotation mechanism. Our results not only agree well with previous experimental studies but also provide some novel insights that could be helpful for future improvements in the performance of the ITI photoswitches.

Influence of different metal-modified Beta zeolites on the liquid-phase isomerisation of xylene and reaction mechanisms

PX is a very important chemical raw material. There are few studies on the liquid-phase isomerisation of xylene. Here, a series of Beta catalysts with different metal modifications were prepared and tested for many characterisations. The reaction performance of the catalysts was evaluated in the liquid-phase MX isomerisation reaction. The results show that the loading of La, Ce, Mo,Fe and Mg metals decreases the specific surface area and acid content of Beta zeolite. Among them, Mo has a strong interaction with the skeleton Al atoms in Beta. The side reactions of MX isomerisation were inhibited by metal loading, and the selectivity of PX and OX was improved. The effect of Mo content on the liquid-phase isomerisation of MX was further investigated. The selectivity of PX can be improved by appropriate Mo content. This is related to the reaction mechanism of the liquid-phase isomerisation of MX at mainly low temperatures. The acidic site density of B and L acids on the catalyst surface was successfully modulated by Mo loading. At 260 °C, the conversion of MX over Mo/Beta catalyst was about 33 % and the PX yield was 12.83 %. In addition, based on the results of liquid-phase isomerisation of MX, OX and PX, the transformation relationship between the three isomers was investigated.

Nickel-Catalyzed Alkene Isomerization to Access Bench-Stable Enamines and Their [3 + 2] Annulation.

Enamines are difficult to prepare on the bench due to their instability, which results in side reactions, decompositions, poor yields, etc. Herein, we developed a simple and effective method for making bench-stable enamines using a very low amount of nickel catalyst loading. The deuterium exchange, competitive reaction, and radical clock experiment have all been found to favor the ionic mechanism of this alkene isomerization. Scale-up and [3 + 2] annulation reaction of enamines with activated cyclopropane to deliver cyclopentane derivatives have shown the value of this method in organic synthesis.

Theoretical study on the isomerization mechanism of visible light-driven azobenzene-based materials

This article investigates the geometric structure and isomerization mechanism of 4,4-acetamidazobenzene molecules with halogen atoms (F, Cl, Br) substituted at four ortho positions through density functional theory (DFT) calculations. The calculation method was b3lyp/g, and the base set was def2-tzvp. According to DFT calculations, the ortho substitution of the halogen atoms differently distorted the molecular plane of the trans isomers, resulting in obvious absorption peaks in the visible range, giving them visible-light driven properties. In addition, the transition state (TS) of the cis–trans isomerization reaction in the ground state was also found. To study the molecular isomerization mechanism, the potential energy surfaces (PESs) of the ground state (S0), first excited state (S1), and second excited state (S2) of halogen substituted azobenzene derivatives were scanned separately. Through the potential energy surface curve of the ground state, the mechanism of isomerization reaction in the ground state of the four molecules is realized through a rotation-assisted inversion pathway, which is consistent with the calculation of TS. When the molecules are in the excited states, the isomerization pathways are realized through rotation pathways.

Selective Deep Hydrogenation of Acenaphthene to High‐Density Fuel On Supported Ru Catalysts: The Role of Strong Metal‐Support Interaction

AbstractDeveloping high‐performance aviation fuel is highly desired for the rapid development of supersonic aircraft. Herein, selective deep hydrogenation of polycyclic aromatics to a high‐density fuel is developed over a Ru‐based catalyst. The influences of strong Ru‐support interaction on tailoring the reaction pathways of acenaphthene hydrogenation were investigated using different oxide supports (SiO2, CeO2, TiO2). Ru/CeO2 and Ru/TiO2 with strong SMSI possess abundant electron‐deficient Ruδ+ species which exhibit a good ability to promote H2 dissociation and increase the isomerization rate of saturated aromatic compounds. Due to the enhanced adsorption of aromatic molecules, Ru/CeO2 (1.30 μmol min−1 g−1) exhibits superior catalytic activity in comparison to Ru/TiO2 (1.25 μmol min−1 g−1) during acenaphthene hydrosaturation, while Ru/TiO2 exhibits the highest isomerization reaction rate (7.60×10−2 μmol min−1 g−1) because of its excellent hydrogen spillover ability and high content of Ruδ+ species. The best catalytic activity with 96 % selectivity of ccc‐perhydroacenaphthene was achieved on Ru/TiO2, and an isomerization mechanism of perhydroacenaphthene isomers was proposed.

Application of Ce modified zeolite to improve the performance of cresol isomerization

This study clarifies the mechanism behind the isomerization of cresol, focusing on a shaped cerium-modified Beta zeolite. The catalysts were characterized using a series of characterization techniques, including XRD, Py-IR, XPS, and TGA. The results emphasize the pivotal role of cerium, particularly at a 5 % modification concentration, in improving catalyst acidity. The mechanism of isomerization is revealed through kinetic calculation of reversible monomolecular systems, emphasizing the significance of acid sites. TGA analysis of catalyst deactivation reveals distinctive carbon deposition patterns, with the cerium-modified zeolite displaying enhanced resistance to pore blockage through Type II carbon deposition. XPS analysis post-deactivation indicates an increased proportion of Ce3+, suggesting elevated oxygen vacancies and unsaturated chemical bonds, influencing the catalytic activity negatively. In summary, an appropriate cerium modification, especially at the 5 % concentration, strikes a balance between enhanced acidity and resistance to carbon deposition, providing valuable insights for tailoring zeolite catalysts in cresol isomerization.

Photo/Mechanical/Acidic Multi-Stimuli Responses and Information Encryption Design of Acylhydrazone Derivative.

Stimuli-responsive crystalline materials have received much attention for being potential candidates of smart materials. However, the occurrence of polymorphism-driven stimuli responses in crystalline materials remains interesting but rare. Herein, three polymorphs of an acylhydrazone derivative, N'-[(E)-(1-benzofuran-2-yl) methylidene] pyridine -4-carbohydrazide (BFMP) were prepared. Form-1 undergoes a photomechanical response via E→Z photoisomerization under UV irradiation, accompanied by a decrease in fluorescence intensity and a change from colorless to yellow. Two types of Z→E thermal isomerization mechanisms with significant differences in conversion rate were observed at different temperatures in form-1. The solid-melt-solid transition has a faster conversion rate compared to the solid-solid transition due to freedom from lattice confinement. The transition from form-2 to form-3 can be achieved under grinding, coupled with a significant decrease in fluorescence intensity. The similar molecular stacking pattern of form-2 and form-3 provides a structural basis for the grinding-induced crystalline transition behavior. In addition, the presence of the pyridine moiety imparts an acidochromic property. The combination of photochromism and acidochromism explores the possible applications of acylhydrazone derivatives in information encryption.

Hydrosulfonylation of Alkynes for Stereodivergent Synthesis of Vinyl Sulfones: Synthetic Strategy and Mechanistic Insights.

Direct synthesis of thermodynamically less favorable (Z)-vinyl sulfones presents a notable challenge in organic synthesis. In addition, the development of a stereodivergent synthesis for (E)- and (Z)-vinyl sulfones is crucial but remains elusive. In this study, we present a hydrosulfonylation of aryl-substituted alkynes, achieving a stereodivergent synthesis of (E)- and (Z)-vinyl sulfones by leveraging both thermodynamic and kinetic controls. Notably, the synthesis of challenging (Z)-vinyl sulfones was achieved through a kinetically controlled process without the need for a catalyst. To synthesize (E)-vinyl sulfones, unconventional visible light-mediated isomerization was employed as a means of facilitating the transition to the thermodynamically favored form. The present study encompasses a comprehensive experimental and computational investigation, which provides valuable insights into the reaction mechanism. This investigation reveals two plausible isomerization pathways: a novel double spin-flip mechanism and a hydrogen atom transfer process in the presence of eosin Y. This study not only advances our understanding of isomerization mechanisms beyond conventional energy-transfer routes but also offers a robust and switchable strategy for synthesizing (E)- and (Z)-vinyl sulfones, thereby providing a versatile avenue for the creation of valuable compounds in the fields of organic synthesis and medicinal chemistry.

Ultrafast Primary Dynamics and Isomerization Mechanism of a Far-Red Sensing Cyanobacteriochrome.

Far-red cyanobacteriochromes (CBCRs) are bilin-based photosensory proteins that promise to be novel optical agents in optogenetics and deep tissue imaging. Recent structural studies of a far-red CBCR 2551g3 have revealed a unique all-Z,syn chromophore conformation in the far-red-absorbing Pfr state. Understanding the photoswitching mechanism through bilin photoisomerization is important for developing novel biomedical applications. Here, we employ femtosecond spectroscopy and site-directed mutagenesis to systematically characterize the dynamics of wild-type 2551g3 and four critical mutants in the 15Z Pfr state. We captured local relaxations in several picoseconds and isomerization dynamics in hundreds of picoseconds. Most mutants exhibited faster local relaxation, while their twisting dynamics and photoproducts depend on specific protein-chromophore interactions around the D-ring and C-ring. These results collectively reveal a unique dynamic pattern of excited-state evolution arising from a relatively rigid protein environment, thereby elucidating the molecular mechanism of Pfr-state photoisomerization in far-red CBCRs.