Ring Transition State Research Articles

Unveiling the low‐temperature oxidation chemistry of dipropyl carbonate

AbstractDialkyl carbonates (DACs) own an environmentally friendly synthesis route, making them potential candidates as alternative fuels. However, for DACs to be widely accepted as an alternative fuel, a comprehensive understanding of their combustion behavior is essential. Dipropyl carbonate (DPrC) represents a transition from short‐chain to mid‐chain carbonates, understanding its combustion behaviors holds significance in unraveling the combustion chemistry of carbonates. In this study, the oxidation of DPrC was investigated with the initial fuel mole fraction of 0.5% at three equivalence ratios of 0.5, 1.0, and 2.0 within a temperature range of 550–1100 K in a jet‐stirred reactor for the first time. Gas chromatography was utilized for the quantitative detection of reactants, intermediates, and products. A detailed DPrC mechanism was first developed, and good agreements between measurements and simulations were obtained. A notable negative temperature coefficient (NTC) behavior was first observed in the oxidation of DACs. Such NTC phenomenon occurred at fuel‐lean conditions in the temperature range of 620–660 K, while only a weak low‐temperature consumption was observed at the stoichiometric condition. Kinetic modeling studies showed that this unique low‐temperature chemistry of DPrC can be attributed to the differences in the RO2 isomerization reactions between DPrC and short‐chain DACs. The RO2 isomerization via a six‐member ring transition state could happen in DPrC oxidation but not in dimethyl carbonate and diethyl carbonate oxidation, due to the different fuel molecular structure. Therefore, the subsequent reaction pathways via QOOH → O2QOOH → HO2Q = O + OH → OQ = O + OH were promoted and two OH radicals were released in this process. Moreover, it is conceivable that mid or long‐chain DACs could also exhibit an NTC phenomenon due to the increased potential for RO2 isomerization via a six‐ or seven‐member ring transition state, thereby increasing the likelihood of RO2 isomerization occurrence.

Catalytic Fe2+ Cation Pair Site for Base-free N-Alkylation of Aromatic Amines with Alcohols.

The development of heterogeneous Fe catalysts is very attractive due to the ubiquitous, abundant, and inexpensive nature of Fe as a resource. However, Fe oxides are commonly inert as catalysts and hence, the design and fabrication of active Fe sites are essential. Herein, the fabrication of an active Fe cation pair site by simple reduction treatment of SiO2-supported FeOx (FeOx/SiO2) is presented. The active Fe cation pair site was formed by the removal of the oxygen atom between Fe cations of the FeOx on SiO2, namely oxygen vacancy formation, which is induced by temperature-controlled reduction treatment. 773 K reduction maximized the Fe cation pair sites without the decomposition of FeOx species, which was an effective catalytic one for the N-alkylation of amines mainly proceeded through Meerwein-Ponndorf-Verley (MPV) type reduction, which is achieved by the stabilization of six-membered ring transition state derived from imines and alcohols over the open active site of the Fe cation pair site.

A DFT Mechanistic Study on the Aza-Aldol Reaction of Boron Aza-Enolates: Relative Stability of Six-Membered Transition State and Its Relevance to the Coordination Mode of the Leaving Group.

The mechanism of the aza-aldol reaction between boron aza-enolate and benzaldehyde is investigated by using density functional theory calculations. The result shows that the syn-E isomer is preferentially formed, consistent with experimental observations. The six-membered ring transition state (TS) with the boat form leads to the E isomer, while the more unstable chair TS does to the Z isomer. The preference of the syn isomer is determined by the interactions between the substituents of aza-enolate and benzaldehyde. Structural distortion and intrinsic reaction coordinate analyses of simplified model systems provide insights into the origin of the relative stability of the rate-determining TS with boat and chair forms. The boat TS is an early TS; thus, minimal structural distortions of the reactant are required to reach this TS. The Lewis pair interactions between the boron and imine groups during B-N elimination also influenced the relative stability of the TSs. This interaction involves the nitrogen lone pair in the boat TS, while the π(N═C) orbital is involved in the chair TS. The Lewis pair with the lone pair stabilizes the TS more than that with the π orbital. The boron aza-enolate with 9-BBN generates an ate complex and forms C-C bonds sequentially, whereas that with Bpin does not generate an ate complex and exhibits the concerted formation of B-O and C-C bonds. Thus, the higher electrophilicity of boron such as 9-BBN enhances the reactivity by facilitating the formation of the ate complex. A reaction design is proposed to reverse the syn/anti selectivity. Proof-of-concept DFT calculations suggested that the modification of the imine group would change the relative stability of the boat/chair TSs and give the anti-product.

High-Pressure Rate Rules for Ether Alkylperoxy Radical Isomerization.

The first isomerization reaction of an alkylperoxy (RO2) radical holds significant importance in low-temperature oxidation, as it governs the branching ratios of the hydroperoxyalkyl (QOOH) radicals, which influence the competition between the chain-propagation and chain-branching reactions. In this study, we systematically calculated high-pressure rate rules for the RO2 isomerization reaction of monoethers, exploring 5-, 6-, 7-, and 8-membered ring transition states. Primary, secondary, and tertiary carbon sites, where both the abstracting peroxy group and the abstracted hydrogen are located, were considered, with particular emphasis on distinguishing between secondary carbons adjacent (alpha) and nonadjacent to the ether functional group. Using the G4//B3LYP/6-311++G(2df,2pd) level of theory and the transition state theory, we estimated the rate constants and the Arrhenius coefficient for over 120 possible isomerization reactions. We examined the effect of ring size and ring atoms, revealing that 6- and 7-membered ring isomerizations were generally the fastest. The impact of the ether functional group on transition states was investigated by comparing reactions with identical ring size, peroxy, and radical positions, but with the ether functional group positioned either outside (i.e., out) or inside (i.e., in) the transition state ring, leading to differences in the rate constants. When comparing to analogous alkane rate constants, differences of up to an order of magnitude were observed, underscoring the need for caution when assigning rate rules by analogy. We applied our rate constants in the di-iso-butyl ether kinetic model and evaluated their influence on low-temperature chemistry finding that they altered the branching ratios by up to a factor of 9, highlighting the significance of site-specific rate constants for more accurate low-temperature modeling.

Density functional theory study on frustrated Lewis pairs catalyzed C‐H activation of heteroarenes: Mechanism variation tuning by electronic effect

AbstractUnreactive C‐H bond activation is a new horizon for frustrated Lewis pairs (FLPs) chemistry. Although concerted mechanism (Science 2015, 349, 513) and stepwise carbene mechanism (Org. Lett. 2018, 20, 1102) have been proposed for the FLPs catalyzed C‐H bond activation of 1‐methylpyrrole, the influence of electronic properties of FLPs on the reaction mechanism is far away from well‐understood. In this study, an assortment of P‐B type FLPs with different electronic characteristic was employed to study the catalyzed C‐H bond activation of 1‐methylpyrrole by using density functional theory calculations. Detailed calculations demonstrated that the reactivity variation and the reaction mechanism binary of FLPs catalyzed C‐H activation can be varied by tuning electronic effect of Lewis base center. On the one hand, the concerted C‐H activation reactivity is mainly controlled by the electron donation of the lone pair of Lewis base center; thus, the FLPs with electron‐donating substituents (FLP1, FLP2, and FLP3) catalyzed the C‐H bond activation through concerted mechanism. On the other hand, the reactivity of stepwise carbene mechanism is mostly attributed to the vacant orbital of Lewis acid center; as a result, the FLP5 bearing ‐P(C6F5)2 preferred to catalyzed the bond activation through concerted mechanism. In contrast, a metathesis mechanism through strained four‐membered ring transition state is less feasible. These results should provide deeper insight and broader perspective to understand the structure and function of FLPs for rational design of FLPs catalyzed C‐H bond activation.

Visible light-driven selective cleavage of Cα-Cβ or Cβ-O bond in lignin β-O-4 model into high-value aromatic chemicals over surface modified 2D g-C3N4

Selective cleavage of C–C/C–O linkages in lignin is highly pursued to obtain aromatic monomers but remains challenging due to distinct dissociation energies and steric hindrance. Herein, two photocatalysts were reported to respectively scissor the Cα-Cβ and Cβ-O in lignin β-O-4 model (2-phenoxy-1-phenylethanol) based on the reactive oxygen species control. On one hand, 2D g-C3N4 promotes protons extraction from Cβ-H and ·O2– formation, which further scissored Cα-Cβ via six-membered ring transition state produced via the coupling of ·Cβ and ·HO2, providing aromatic aldehydes and esters. When single-atomic Cu+ species were coordinated with N atoms in 2D g-C3N4 during surface modification, however, the ·O2– formation was prohibited because of the improper redox potentials. Under the catalysis of Cu/2D g-C3N4, CαO-H would be preferentially activated, causing the selective Cβ-O cleavage via self-hydrogen transfer hydrogenolysis to provide phenolics and aromatic ketones. The access and inhibition mechanism for ·O2– formation as well as the different catalytic mechanisms for two reaction pathways were discussed. Current work shows a promising way to selectively supply aromatic chemicals using the renewable biomass and photoenergy.

Pyrolysis of 1,2-bis(2,4,6-tribromophenoxy)ethane: Theoretical and experimental research

1,2-Bis(2,4,6-tribromophenoxy)ethane (BTBPE), as a novel brominated flame retardant, is widely applied to various plastic products to improve flame resistance. In this paper, we have carried out theoretical and experimental studies on the pyrolysis of BTBPE. The density functional theory (DFT) method at M06/cc-pVDZ theoretical level was used to calculate thermodynamic and kinetic parameters, and the pyrolysis mechanisms of BTBPE were in detail analyzed based on theoretical calculation data and relevant experimental findings. In unimolecular pyrolysis reaction, concerted reaction mechanism with a four-membered ring transition state at low temperatures leads to high yields of 2,4,6-tribromophenol and vinyl 2,4,6-tribromophenol ether, and free radical reaction at high temperatures generates 2,4,6-tribromophenoxy and 2,4,6-tribromophenoxy-ethyl by direct breakage of the CO bond. The 2,4,6-tribromophenoxy radical can further catalyze BTBPE decomposition to form 2,4,6-tribromophenol and vinyl 2,4,6-tribromophenol ether. Reactions of the BTBPE molecule with H radicals mainly proceed via debromination, which results in the production of HBr and low-brominated congeners. The 2,4,6-tribromophenol and 2,4,6-tribromophenoxy undergo a CO coupling reaction to eliminate OH, which is the principal pathway for the generation of polybrominated diphenyl ethers (PBDEs). The formation of polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs) involves the self-condensation of 2,4,6-tribromophenoxys, debromination, rearrangement, and cyclization. The energy barrier of the reaction path forming PBDD is lower than that of the reaction path forming PBDFs. Therefore, the yield of brominated dibenzo-p-dioxin is higher than that of brominated dibenzofuran. The experimental results of pyrolysis are in good agreement with our theoretical research.

Atomic-Level Asymmetric Tuning of the Co1-N3P1 Catalyst for Highly Efficient N-Alkylation of Amines with Alcohols.

Despite the extensive development of non-noble metals for the N-alkylation of amines with alcohols, the exploitation of catalysts with high selectivity, activity, and stability still faces challenges. The controllable modification of single-atom sites through asymmetric coordination with a second heteroatom offers new opportunities for enhancing the intrinsic activity of transition metal single-atom catalysts. Here, we prepared the asymmetric N/P hybrid coordination of single-atom Co1-N3P1 by absorbing the Co-P complex on ZIF-8 using a concise impregnation-pyrolysis process. The catalyst exhibits ultrahigh activity and selectivity in the N-alkylation of aniline and benzyl alcohol, achieving a turnover number (TON) value of 3480 and a turnover frequency (TOF) value of 174-h. The TON value is 1 order of magnitude higher than the reported catalysts and even 37-fold higher than that of the homogeneous catalyst CoCl2(PPh3)2. Furthermore, the catalyst maintains its high activity and selectivity even after 6 cycles of usage. Controlling experiments and isotope labeling experiments confirm that in the asymmetric Co1-N3P1 system, the N-alkylation of aniline with benzyl alcohol proceeds via a transfer hydrogenation mechanism involving the monohydride route. Theoretical calculations prove that the superior activity of asymmetric Co1-N3P1 is attributed to the higher d-band energy level of Co sites, which leads to a more stable four-membered ring transition state and a lower reaction energy barrier compared to symmetrical Co1-N4.

Acid-base synergistic effect towards catalytic transfer hydrogenation reactions

Catalytic transfer hydrogenation (CTH) of aldehydes/ketones by using alcohols as hydrogen donors is an attractive and environmentally friendly hydrogenation technology, which has evoked widespread attention in synthesis of fine chemicals from biomass resources. In this work, a Co-Al mixed metal oxide catalyst (Co1Al2-MMO-200) was obtained through structural topological transformation process of layered double hydroxides, which was used in the CTH reaction of furfural with isopropanol. Notably, the reaction rate achieves 0.023 mol g−1 h−1, which is preponderate to acid-base catalysts earlier reported. The joint research based on CO2-TPD, NH3-TPD and poisoning experiment substantiates that the synergy effect of acid-base sites plays an essential role in this catalytic system. Isotopic labelling MS, in-situ FT-IR and DFT studies further verify that the CTH reaction of furfural occurs via the Meerwein-Ponndorf-Verley (MPV) route, in which the basic site promotes the deprotonation of isopropanol whilst the acid site facilitates the formation of a six-membered ring transition state. This work demonstrates an acid-base synergetic catalysis for the CTH reaction by revealing the structure-property correlation and exploring reaction route, which is instructive for designing high-performance heterogenous catalysts.

Pyrolysis mechanistic study on sulfated polysaccharide from marine algal biomass with density functional theory method

Sulfated polysaccharide (SPS) derived from marine algae represents a promising renewable carbon biomass source. Through pyrolysis, SPS can be converted into C5 platform chemicals, including furfural (FF) and 5-methylfurura (5-MF). Understanding the fundamental reaction mechanisms during pyrolysis is essential for the advancing pyrolysis techniques. This work presents a detailed reaction mechanism that reveal new pathways leading to the most important products and intermediates. Simulations focusing on the initial steps in SPS pyrolysis showed that the unimolecular degradation of SPS is dominated by SO3 elimination, with a barrier height of 157.2 kJ/mol; ring-opening and glycosidic bond fission were identified as important competitive reactions, both of which have a barrier height of 207.6 kJ/mol and 260.1 kJ/mol, respectively. The 4,5-unsaturated acyclic rhamnose derived from the glycosidic bond fission of acyclic reducing end group can transform into 5-MF with high selectivity. In addition, the acyclic reducing end group is readily dehydrated to form a 2,3-unsaturated acyclic reducing end group, a reactive unsaturated intermediate. Its decomposition involves an 8-membered ring transition state that requires a low barrier height and results in the formation of a C5 fragment, which can be cyclized directly and dehydrated to form 5-MF. The majority of CO2 is mostly formed by the synergistic breakage of C-Ccarboxyl bonds and glycosidic bonds, with a barrier height of 197.9 kJ/mol. Moreover, a mechanism is uncovered for the formation of formic acid (FA) from a 4,5-unsaturated no-reducing end group. The major pyrolysis channel is initiated by a retro Diels-Alder reaction and followed by a retro-ene reaction to produce a 3,4-unsaturated non-reducing end group, which further decomposes into FA and a new 4,5-unsaturated non-reducing end group.

High Selectivity in Csp2-Csp2 versus Csp3-O Reductive Elimination from Cycloplatinated(IV) Complexes.

The cycloplatinated(IV) complexes trans-[Pt(p-MeC6H4)(C∧N)(OAc)2(H2O)] (C∧N = benzo[h]quinolate, bhq, 2a, and 2-phenylpyridinate, ppy, 2b) were prepared by reacting the corresponding [Pt(p-MeC6H4)(C∧N)(SMe2)] precursors with PhI(OAc)2 through an oxidative addition (OA) reaction. Thermolysis of 2a at 65 °C generates cis-[Pt(κ1N-10-(p-MeC6H4)-bhq)(OAc)2(H2O)], 3a, which is the product of a Csp2Ar-Csp2bhq reductive elimination (RE). The observed coupling reaction is significantly different from the previously reported analogous thermolysis of trans-[PtMe(C∧N)(OAc)2(H2O)] (C∧N = bhq, 2c, and ppy, 2d) that selectively releases Me-OAc (C-O RE). The density functional theory (DFT) calculations and experimental observations reveal that the Csp2Ar-Csp2bhq coupling reaction occurs through the dissociation of a coordinated water ligand. This in turn is followed by the concomitant bond forming and bond breaking process via a three-center ring transition state, in contrast to the Csp3Me-OAc coupling, which had taken place by an outer sphere SN2 type RE reaction in methyl complexes.

Structural diversity of decalin forming Diels-Alderase.

The Diels-Alder (DA) reaction, specifically referring to the [4 + 2] cycloaddition reaction in pericyclic reactions, is a process that forms two carbon-carbon covalent bonds in a single step via an electron ring transition state. Among the secondary metabolites produced by microorganisms, numerous compounds are biosynthesized through DA reactions, most of which are enzymatic. Our research group has discovered an enzyme named Diels-Alderase (DAase) that catalyzes the DA reaction in filamentous fungi, and we have been investigating its catalytic mechanism. This review describes the reported microbial DAase enzymes, with a particular focus on those involved in the construction of the decalin ring.

Fluorinated Propionic Acids Unmasked: Puzzling Fragmentation Phenomena of the Deprotonated Species.

Environmental contamination by per- and polyfluorinated substances (PFAS) is an emerging concern for the public. In this study, short-chain PFAS such as deprotonated per- and polyfluorinated propionic acids are investigated using a combination of infrared multiple-photon dissociation (IRMPD) spectroscopy, collision-induced dissociation (CID), and density functional theory calculations. IRMPD and CID proceed via multiple competing pathways: (1) production of fluoroformate (FCO2-) and the associated ethylene derivative, (2) production of HF and the associated carbanion, or (3) loss of CO2 and the associated carbanion. Fluorinated propionic acids with at least one fluorine atom bound to the terminal carbon yield FCO2-, whereas loss of HF is observed in polyfluorinated species with at least one fluorine atom bound to the α-carbon. To explore the reaction pathways of the various fluorinated propionic acids, the nudged elastic band method is employed. The relative energy of the four-membered ring transition state leading to FCO2- dictates which product channel is observed in dissociation.

Unravelling the underlying mechanism of the reduction of aldehydes/ketones with metal borohydride in an aprotic solvent.

The reduction mechanism of aldehyde/ketones with M(BH4)n is not fully understood, even though the hydroboration mechanism of weak Lewis base borane complexes is known to involve a four-membered ring transition state. Herein, the reduction mechanism of M(BH4)n in aprotic solvents has been elucidated for a six-membered ring, in which hydride transfer to the C atom from the B atom, formation of an L·BH3 adduct, and disproportionation of (BH3(OR)-) borane are involved. The metal cations and solvents participate in and significantly influence the reaction procedure. We believe that this mechanistic study would provide a further reference for the application of M(BH4)n in organic reactions.

Phosphoric acid catalytic mechanism in lignin pyrolysis: Phosphoric-acid-assisted hydrogen transfer for the decomposition of β-O-4 linkage

Phosphoric acid is an effective and widely used catalyst for biomass pyrolysis, with significant catalytic impact on the pyrolytic reactions and product distribution. However, its fundamental catalytic mechanism in lignin pyrolysis remains unclear. In this work, we proposed and investigated three possible H3PO4-assisted pyrolysis mechanisms, i.e., hydrogen-bond-induced decomposition, phosphoric-acid-assisted hydrogen transfer (H3PO4-AHT), and phosphorolysis, and carefully discussed them through theoretical calculations, electronic structure analyses, catalytic pyrolysis experiments using the β-O-4 linked lignin model. The results indicated that the H3PO4-AHT mechanism played a dominant role in lignin catalytic pyrolysis, superior to the other two mechanisms. The phosphoric acid molecule could take advantage of its unsaturated P=O and saturated P−OH groups for hydrogen donation and acceptance in concerted H3PO4-AHT reactions. Alternatively, it could also participate in concerted intermolecular hydrogen transfer with the active hydroxyl group as a mediator. The Cβ−O bond cleavage could be significantly enhanced by effectively reducing the tension of the transition state ring, leading to the generation of more phenol and styrene in pyrolysis experiments. Due to the inhibition of hydrogen abstraction, Cα−Cβ breakage of β-O-4 linkage, side chain removal of primary products, etc., the formation of other products in phosphoric acid catalytic pyrolysis of β-O-4 linked lignin was restrained. This H3PO4-AHT catalytic mechanism was also confirmed to be conductive to tautomerism, elimination, dehydration, α-O-4 linkage fracture, β-1 linkage fracture, and other important decomposition reactions in lignin pyrolysis. The understanding of these mechanisms provides valuable insights into the role of phosphoric acid in lignin pyrolysis and may guide the development of effective biomass catalytic conversion methods.

Mechanistic study on levoglucosan condensation and decomposition in the condensed phase during cellulose pyrolysis

In this study, the condensation, as the fundamental step of polymerization, and decomposition mechanisms of levoglucosan in the condensed phase during cellulose pyrolysis were investigated using density functional theory (DFT) calculations to investigate non-specific solvent effects. The condensed phase was simulated using an implicit solvent model, while the dielectric constants of levoglucosan at different temperatures were determined by theoretical calculations. This study considered levoglucosan condensation reactions to form anhydro-disaccharides with various α and β linkages (including 1,2-, 1,3- and 1,4-glycosidic bonds) and levoglucosan decomposition reactions including 1,2-dehydration, 1,3-dehydration, and ring-opening reactions. Our results show that the formation of β-anhydro-disaccharides prefers the direct condensation mechanism of two levoglucosan molecules through a four-member ring transition state, while kinetically more disadvantageous α-anhydro-disaccharides are generally formed by the condensation of levoglucosan molecule and α-d-glucose from levoglucosan hydrolysis. However, levoglucosan condensation without catalysis is unlikely to take place at typical temperatures of cellulose pyrolysis, due to the low reaction rate at low temperatures. At high temperatures above 625 K, levoglucosan condensation is also inhibited due to the higher ΔGθ and lower reaction rates, compared to those of levoglucosan decomposition. A high temperature above 800 K is needed for the obvious decomposition of levoglucosan, mainly through the ring-opening reaction to form 5-keto-6-deoxy-glucose, which is further promoted by the dielectric environment of the condensed phase during cellulose pyrolysis. Further calculations on acetic acid-catalyzed levoglucosan condensation and decomposition suggest that the consumption of levoglucosan at low temperatures is mainly due to catalysis of acetic acid on polymerization and further decomposition of polymers.

Trivinylphosphine Oxide: Synthesis, Characterization, and Polymerization Reactivity Investigated Using Single-Crystal Analysis and Density Functional Theory.

Organophosphorus chemicals are versatile and important in industry. Trivinylphosphine oxide (TVPO), for example, exhibited a promising precursor as a flame-retardant additive for industrial applications. Density functional theory (DFT) simulations were used to explore the kinetic and thermodynamic chemical processes underlying the nucleophilic addition reactions of TVPO in order to better understand their polymerization mechanisms. An experimental X-ray single-crystal study of TVPO supported this work's theory based on its computed findings. TVPO was prepared using POCl3 and VMB in a temperature-dependent reaction. TVPO, the thermodynamically favourable product, is preferentially produced at low temperatures. The endothermic anionic addition polymerization reaction between TVPO and VMB begins when the reaction temperature rises. An implicit solvation model simulated TVPO and piperazine reactions in water, whereas a hybrid model modelled VMB interactions in tetrahydrofuran. The simulations showed a pseudo-Michael addition reaction mechanism with a four-membered ring transition state. The Michael addition reaction is analogous to this process.

Diastereoselective Petasis-Borono-Mannich Crotylation Reactions of Chiral α-Heteroatom (F, OBz, OH) Aldehydes: Rapid Access to Valuable Mono and Bicyclic Heterocyclic Scaffolds.

The crotylation reactions of chiral α-F, α-OBz and α-OH aldehydes under Petasis-borono-Mannich conditions using (E)- or (Z)-crotylboronates and primary amines resulted in γ-addition products in high dr and high er. α-F and α-OBz aldehydes gave 1,2-anti-2,3-syn and 1,2-anti-2,3-anti, products, respectively while an α-OH aldehyde gave 1,2-syn-2,3-syn products. The stereochemical outcomes of reactions of the former aldehydes can be explained using a six-membered ring transition state (TS) model in which a Cornforth-like conformation around the imine intermediate is favoured resulting in 1,2-anti products. The 2,3-stereochemical outcome is dependent upon the geometry of the crotylboronate. These TS models were also supported by DFT calculations. The stereochemical outcomes of reactions employing an α-OH aldehyde can be rationalised as occurring via an open-TS involving H-bonding in the imine intermediate between the α-OH group and the imine N atom. Representative products were converted to highly functionalized 1,2,3,6-tetrahydropyridines and 3H-oxazolo[3,4-a]pyridine-3-ones which will be valuable scaffolds in synthesis.

Unveiling the low-temperature oxidation mechanism of a carbon-neutral fuel – Monoglyme via theoretical study

Monoglyme is a promising carbon-neutral for engines due to its renewability, compatibility with existing infrastructure, high energy content and cetane number, etc. However, its low-temperature oxidation mechanism is not well understood, which hinders its application in advantage combustion technologies. Therefore, this study carried out an in-depth theoretical analysis to bridge this gap in knowledge. All potential reaction pathways related to oxygen addition, hydrogen transfer, concerted elimination, and QOOH decomposition were identified using the density functional theory method. The corresponding rate constants were calculated by solving the time-dependent master equation based on the Rice-Ramsperger-Kassel-Marcus theory in conjunction with the transition state theory, and were subsequently fitted to Arrhenius expressions. It is discovered that the oxygen addition to the monoglyme radical formed by dehydrogention from the intermediate site is more competitive than that from the first site. The QOOH formed by the initial adduct RO2 via the 6-member ring transition state exerts a stronger impact on the second oxygen addition of monoglyme than that via the 5, 7, and 9-member ring transition states. After comparing the simulated and measured concentration distribution of the key substances during the oxidation of monoglyme in a jet-stirred reactor, the reliability and accuracy of the established kinetic model was confirmed.

A quantum chemical computation and model investigation for autoignition kinetic of a long chain oxygenate: Tri-propylene glycol methyl ether

Tri-propylene glycol methyl ether (TPGME) is a promising bio-based oxygenated fuel for advanced engines. The internal H-migration of peroxy (ROO) radicals is a critical factor in the process of TPGME autoignition. In this work, the potential energy surfaces (PESs) of the internal H-migration of ROO radicals as well as alcohol elimination of TPGME were calculated with a high-level DLPNO-CCSD(T)/cc-pVTZ//B3LYP-D3/6–311++G(d,p) method. Rate constants for target reactions were then obtained over a temperature range of 300–1000 K using conventional transition state (CTST) theory. The rate constants for above reactions in the Burke’s model [Combust. Flame. 2015, 16 2(7), 2916–2927] were updated with the calculated results, followed by the modification for important reaction using an analogy strategy, to obtain a revised model. It was found that the feasibility of the H-migration of ROO radicals depended on the transition state ring size and the C-H bond energy of the transferred hydrogen atom, with 1, 5H-migration and 1, 6H-migration played a leading role in the process of TPGME autoignition. Validation against the shock tube (ST), jet stirred reactor (JSR) and collaborative fuel research (CFR) engine measurements from the literature showed that the revised model improved the prediction accuracy. In comparison to n-heptane, the CFR simulation proved that more available hydrogens of 1,5H-migration and 1,6H-migration, as well as higher concentrations and rate of production (ROP) for key low-temperature intermediate species, were contributing factors that stronger low-temperature oxidation activity of TPGME.