Bond Fission Research Articles

Double-Imaging Photoelectron Photoion Coincidence Spectroscopy Reveals the Unimolecular Thermal Decomposition Mechanism of Dimethyl Carbonate.

We studied the thermal decomposition of dimethyl carbonate (DMC, C3H6O3) in a flash vacuum pyrolysis reactor in the 1100-1700 K range. The reaction products and intermediates were probed by vacuum ultraviolet synchrotron radiation in a photoelectron photoion coincidence (PEPICO) spectrometer to record isomer-specific photoion mass-selected threshold photoelectron (ms-TPE) spectra. Reaction pathways were explored using quantum chemical calculations, which confirmed the experimental observation that the intramolecular migration of a methyl group, yielding dimethyl ether (DME, C2H6O) and carbon dioxide, dominates the initial unimolecular decomposition chemistry. The role of a second potentially important channel, namely, C-O bond fission to yield methyl radicals, could not be determined experimentally due to the short lifetime of the ·C2H3O3 radical and overlapping sequential decomposition products. However, potential energy surface and microcanonical rate constant calculations predict 2 to 3 orders of magnitude lower rates for this channel than for decarboxylation to yield DME. Consequently, DMC pyrolysis shows bewilderingly similar products and product abundances as DME pyrolysis. This coincides with DMC combustion modeling studies, which found that DME is a key intermediate in the mechanism. Furthermore, we have detected traces of methyl formate and formaldehyde, produced after the hydrogen shift to the central carbon atom in DMC. Ethylene and acetylene could be established as bimolecular reaction products because their abundance depended strongly on the DMC concentration. It is intriguing to compare the decomposition of DMC with that of the structurally similar methylal (dimethoxymethane, DMM). While methanol and formaldehyde are produced in similar quantities in DMM, thanks to low-energy hydrogen-transfer reactions, the methanol channel is almost fully suppressed in DMC due to the absence of hydrogens at the central carbon atom and the thermodynamically favored decarboxylation. These new mechanistic insights may help the development of predictive combustion models for fuel additives and biofuels.

HCl elimination in the photolysis of chlorobenzene at 266 nm: An FT-IR spectroscopy and quantum chemical study

The photolysis of chlorobenzene (C6H5Cl) in the gas phase has been studied at 266 nm using repetitive scan FT-IR spectroscopy and density functional theory (DFT) to understand the degradation mechanism relevant to combustion and atmospheric chemistry. Following 266 nm photolysis of C6H5Cl, ro-vibrational lines were observed in the region 3060–2625 cm−1, at 3317.8/ 3262.7 cm−1 and 1346.2/1301.2 cm−1, and at 3341.2 and 1232.7 cm−1. These infrared features are assigned to the hydrochloric acid (HCl), acetylene (C2H2), and 1,3-butadiyne (C4H2), respectively. Identification of C2H2 and C4H2 but not expected HCl co-product ortho-benzyne (o-C6H4) indicates, possibly, o-C6H4 further degraded into C2H2 + C4H2. B3PW91/aug-cc-pVTZ and CBS-QB3 calculated potential energy surfaces for the possible degradation channels of C6H5Cl shows that HCl elimination and C–Cl bond fission are major degradation paths. Their experimental branching ratio was determined to be 1:1. The RRKM rate of HCl elimination of C6H5Cl at 266 nm was found to be 3.8 × 102 s−1 and compared with the HF elimination rate in C6H5F. The possible degradation mechanism is discussed.

Two-Dimensional Master Equation Modeling of Some Multichannel Unimolecular Reactions.

Multichannel thermal decomposition reactions of n-propyl radicals, 1-pentyl radicals, and toluene are investigated by solving a two-dimensional master equation formulated as a function of total energy (E) and angular momentum (J). The primary aim of this study is to elucidate the role of angular momentum in the kinetics of multichannel unimolecular reactions. The collisional transition processes of the reactants colliding with argon are characterized based on the classical trajectory calculations and implemented in the master equation. The rate constants calculated by using the two-dimensional master equation are compared with those of one-dimensional master equations. The consequence of the explicit treatment of angular momentum depends on the J dependence of the microscopic rate constants and is particularly emphasized in the thermal decomposition of toluene, for which the C-H and C-C bond fission channels are considered. The centrifugal effect is insignificant in the energetically favored C-H bond fission but is substantial in the energetically higher C-C bond fission, which causes rotational channel switching of the microscopic rate constants. The proper treatment of the J-dependent channel coupling effect, weak collisional transfer of J, and initial-J-dependent collisional energy transfer are found to be essential for predicting the branching fractions at low pressures.

Nickel‐Catalyzed Acyl Group Transfer of o‐Alkynylphenol Esters Accompanied by C−O Bond Fission for Synthesis of Benzo[b]furan

AbstractHerein, we report the nickel‐catalyzed cascade C−O bond cleavage/cyclization of ortho‐alkynylphenyl ester to construct a 3‐acylbenzo[b]furan skeleton. As a result of reaction condition screening, the Ni(0)/IAd (1,3‐Di(1‐adamantyl)imidazole‐2‐ylidene) system was found to be optimal for catalytic conversion. Interestingly, the reaction exclusively gives 3‐acylbenzo[b]furan instead of a decarbonylated product frequently observed in reactions mediated by acyl‐nickel species. The catalyst loadings could be reduced to 5–10 mol %. We demonstrated the synthesis of a variety of functionalized 3‐acylbenzofuran derivatives. We conducted stoichiometric study of nickel complexes as well as density functional theory (DFT) calculations to support a possible reaction mechanism.

Direct Observation of the C + S2 Channel in CS2 Photodissociation.

Carbon disulfide (CS2) is a typical triatomic molecule. Its photodissociation process has generally been assumed to proceed to CS and S primary products via single bond fission. However, recent theoretical calculations suggested that an exit channel to produce C + S2 should also be energetically accessible. Here, we report the direct experimental evidence for the C + S2 channel in CS2 photodissociation by using the velocity map ion imaging technique with two-photon UV and one-photon vacuum UV (VUV) excitations. The detection of the C (3P) products illustrates that the ground state and the electronically excited states of S2 coproducts are formed within highly excited vibrational states. The very weak anisotropic distributions indicate relatively slow dissociation processes. The possible dissociation mechanism involves molecular isomerization of CS2 to linear-CSS from the excited 1B2 (21Σ+) state via vibronic coupling with the 1Π state followed by an avoided crossing with the ground state surface. Our results imply that the S2 molecules observed in comets might be primarily formed in CS2 photodissociation.

Thermo-kinetic theoretical studies on pyrolysis of dimethoxymethane fuel additive

Many recent studies have been considered dimethoxymethane (DMM) as a potent fuel additive which reduces soot production and nitrogen oxides (NOx) emissions of diesel and biodiesel fuel. This study reports thermo-kinetic investigation of DMM pyrolysis in temperature range 300–2000 K using M06-2X and ωB97XD density functional methods and modest cost ab initio/complete basis set-quadratic Becke3 (CBS-QB3) procedure. The energy profile has been constructed using possible unimolecular H-atom transfer reactions (R1–R4) and simple bond fission reactions (R5–R11). Rate constant calculations were conducted at high-pressure (HP) limit and in the fall-off regimes using classical transition state (TST) and the statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theories, respectively. Based on CBS-QB3 energies, branching ratios analysis reveals unimportance of all H-atom transfer reactions at T ≥ 400 K, while the homolytic bond cleavage reaction R5 which produces CH3 and gauche– OCH2OCH3 fragments is considered the main dominated decomposition channel with high competition from R6 (CH3 and trans– CH3OCH2O) and R7 (CH3O and CH2OCH3) channels especially at elevated temperature.

Distribution Changes during Thermal Degradation of Poly(styrene peroxide) by Pairing Tree-Based Kinetic Monte Carlo and Artificial Intelligence Tools

A tree-based kinetic Monte Carlo (kMC) model is presented that differentiates between 38 end-group pairs for isothermal degradation of poly(styrene peroxide) (PSP). The binary trees allow for fast and accurate calculation of reaction probabilities, with mass-weighted binary trees for the accurate sampling of peroxide bond fissions and hydrogen abstractions along chains. The kinetic parameters are tuned via artificial neural networks (ANNs) to successfully predict literature experimental data, among other lumped product yields. ANNs are also utilized for sensitivity analysis to unravel the effects of individual reactions on the time evolution of experimental responses and other simulation outputs, including the variations of the chain length distributions of the macrospecies. PSP degradation is characterized by three stages of degradation considering both instantaneous and time-averaged concentrations. The first stage features rapid unzipping and results in the fast production of major products benzaldehyde and formaldehyde, the second stage features the most significant level of hydrogen abstractions involving PSP and other macrospecies types, and the third stage exhibits the consumption of the remaining peroxide bonds toward oligomeric species in a wide time frame until the degradation process is finalized by the depletion of peroxide bonds. This proof-of-concept study based on unprecedentedly detailed analyses of the chemistry via kinetic Monte Carlo paves the way to further improve our understanding of chemical recycling of solid plastic waste.

Comparing the pyrolysis kinetics of dimethoxymethane and 1,2-dimethoxyethane: An experimental and kinetic modeling study

This work investigates the reaction kinetics of the thermal decomposition of dimethoxymethane (DMM) and 1,2-dimethoxyethane (1,2-DME), which are both representative molecules of the promising clean polyether fuels. Pyrolysis experiments are carried out with helium diluted mixtures containing individual fuels in a flow tube at two different pressures of 760 Torr and 30 Torr. By varying the temperature distributions along the flow tube, the resulting species composition is sampled downstream the reactor and analyzed by a photoionization mass spectrometer. A kinetic model, including sub-mechanisms for the thermal decomposition of both fuels, is proposed, which can well characterize important measurements, such as the fuel decomposition reactivity and the speciation of crucial products. Under identical conditions, 1,2-DME exhibits a higher decomposition reactivity than DMM, because of the characteristic C–C bond fission as well as the easier hydrogen abstraction reactions by methyl radical. Some fuel-specific intermediates are observed, including methyl formate in DMM pyrolysis, methyl vinyl ether and methoxy acetaldehyde in 1,2-DME pyrolysis. These species mainly come from fuel radical dissociations following the hydrogen abstractions from the central (–CH2O–) and (–CH2CH2O–) moieties in DMM and 1,2-DME, respectively. The formation of some other intermediates are closely related to the consumption of these fuel-specific species. Particularly, the decomposition of methyl formate produces high concentrations of methanol in DMM pyrolysis. In 1,2-DME pyrolysis, the consumption of methyl vinyl ether and methoxy acetaldehyde brings about the formation of C2 oxygenated intermediates such as acetaldehyde, ethenol and ketene. Higher concentrations of unsaturated C2 (ethylene and acetylene) and larger hydrocarbon intermediates present in 1,2-DME pyrolysis, due to the existence of C–C bond in the fuel molecule.

Effects of Ring Fluorination on the Ultraviolet Photodissociation Dynamics of Phenol.

The dynamics of photoinduced O-H bond fission in five fluorinated phenols (2-fluorophenol, 3-fluorophenol, 2,6-difluorophenol, 3,4,5-trifluorophenol, and pentafluorophenol) have been investigated by H Rydberg atom photofragment translational spectroscopy following excitation at many wavelengths in the range 220 ≤ λ ≤ 275 nm. The presence of multiple fluorine substituents reduces the efficiency of O-H bond fission (by tunneling) from the first excited (11ππ*) electronic state, whereas all bar the perfluorinated species undergo O-H bond fission when excited at shorter wavelengths (to the 21ππ* state). As in bare phenol, O-H bond fission is deduced to occur by non-adiabatic coupling at conical intersections between the photoprepared "bright" ππ* states and the 11πσ* potential energy surface. In all cases, the fluorophenoxyl photoproducts are found to be formed in a range of vibrational levels, all of which include an odd number of quanta (typically one) in an out-of-plane (a″) vibrational mode; this product vibration is viewed as a legacy of the parent out-of-plane motions that promote non-adiabatic coupling to the dissociative 11πσ* potential. The radical products also show activity in in-plane vibrations involving coupled (both in- and out-of-phase) C-O and C-F wagging motions, which can be traced to the impulse between the recoiling O and H atoms and, in detail, are sensitive to the presence (or not) of an intramolecular F···H-O hydrogen bond. Upper limit values for the O-H bond dissociation energies are reported for all molecules studied apart from pentafluorophenol.

Ultraviolet photodissociation of gas-phase transition metal complexes: dicarbonylcyclopentadienyliodoiron(II)

The ultraviolet photodissociation of the prototypical organometallic half-sandwich compound dicarbonylcyclopentadienyliodoiron(II) [η 5-CpFe(CO)2I] has been studied in the gas phase across the wavelength range 260 ≤ λ ≤ 310 nm using multi-mass velocity-map ion imaging with photoproducts detected using both resonance enhanced multiphoton and vacuum ultraviolet (λ = 118.2 nm) single photon ionisation methods. Ion images recorded for the atomic iodine and the cyclopentadienyl photoproducts reveal fast, anisotropic components to their recoil velocity distributions. The experimental work is supported by multi-reference (spin–orbit averaged) electronic structure calculations that suffice to illustrate the high electronic state density in such transition metal complexes and provide insights into the rival fragmentation dynamics. The ground state parent molecule has singlet spin multiplicity, but the product energy disposal measured following Fe–Cp bond fission shows the involvement of nominally spin-forbidden transitions. The Fe–I and Fe–Cp bond fissions should both be viewed as homolytic and occurring on excited state potentials that are dissociative in the relevant ligand elimination coordinate.

Hydrogen migration as a potential driving force in the thermal decomposition of dimethoxymethane: New insights from pyrolysis imaging photoelectron photoion coincidence spectroscopy and computations

Pyrolysis and low-temperature oxidation of dimethoxymethane (methylal, MeOCH2OMe) play an important role in the ignition of blended diesel fuels, but the underlying mechanisms are still debated. In these kinetic models, bimolecular hydrogen abstraction or unimolecular C–O bond fission are considered as the primary initial steps, while MeOCH2OMe isomerization is sometimes disregarded. In this work, we investigate the pyrolysis of MeOCH2OMe combining imaging photoelectron photoion coincidence spectroscopy with vacuum ultraviolet (VUV) synchrotron radiation and CBS-QB3 theoretical calculations to unveil reaction paths and energetics. In the mass spectrum of MeOCH2OMe, pyrolysis products and radical intermediates were observed at m/z 15 (CH3), 28 (CO), 29 (HCO), 30 (H2CO), 31 (CH2OH), 32 (CH3OH), 45 (CH3OCH2), and 75 (H-loss from methylal). Only the m/z 45 and 75 ions are found to be dissociative photoionization products of MeOCH2OMe, the other mass spectral peaks are attributed to ionization of the neutral MeOCH2OMe pyrolysis products. The m/z 31 peak was assigned to the methoxy radical in the previous studies. However, our photoion mass-selected threshold photoelectron spectrum (ms-TPES) confirms that it originates from dissociative photoionization of the primary pyrolysis fragment methanol. Based on the experimental and computational results, a thermal decomposition mechanism of MeOCH2OMe is proposed. Here, H-migration precedes the production of methoxymethylene (CH3OCH) and methanol, while dimethyl ether and formaldehyde are probably formed in multi-step processes, too. The sequential dissociation of CH3OCH and of dimethyl ether yields enhanced m/z 15, 28 and 29 signals at high temperature. Rate constants have been calculated to confirm the dominant role of MeOCH2OMe isomerization and to help improve predictive combustion models.

Synthesis and Thermally and Light Driven Cleavage of an N‐Heterocyclic Diphosphine with Inorganic Backbone

A diphosphine with an unsupported PP bond connecting two carbon‐free “inorganic” 1,3,2,4,5‐diazaphosphadisilolidine rings was prepared by reductive coupling of a P‐chloro‐substituted monocyclic precursor molecule. VT‐EPR studies revealed that the diphosphine exists in solution, like other compounds of this kind, in dynamic equilibrium with the corresponding phosphinyl radicals. Determination of the radical concentration from the EPR spectra permitted to calculate thermochemical parameters for the homolytic PP bond fission. The results disclose that both the enthalpy and entropy of dissociation are higher than in topologically related bi(diazaphospholidines). The impact of the entropy term allows explaining that, regardless of the presence of an energetically rather stable PP bond, the onset of dissociation is observable even at ambient temperature. Irradiation experiments showed that radical formation cannot only be induced thermally, but also by photolysis.

Looking at the bigger picture: Identifying the photoproducts of pyruvic acid at 193 nm.

Photodissociation of pyruvic acid (PA) was studied in the gas-phase at 193 nm using two complementary techniques. The time-sliced velocity map imaging arrangement was used to determine kinetic energy release distributions of fragments and estimate dissociation timescales. The multiplexed photoionization mass spectrometer setup was used to identify and quantify photoproducts, including isomers and free radicals, by their mass-to-charge ratios, photoionization spectra, and kinetic time profiles. Using these two techniques, it is possible to observe the major dissociation products of PA photodissociation: CO2, CO, H, OH, HCO, CH2CO, CH3CO, and CH3. Acetaldehyde and vinyl alcohol are minor primary photoproducts at 193 nm, but products that are known to arise from their unimolecular dissociation, such as HCO, H2CO, and CH4, are identified and quantified. A multivariate analysis that takes into account the yields of the observed products and assumes a set of feasible primary dissociation reactions provides a reasonable description of the photoinitiated chemistry of PA despite the necessary simplifications caused by the complexity of the dissociation. These experiments offer the first comprehensive description of the dissociation pathways of PA initiated on the S3 excited state. Most of the observed products and yields are rationalized on the basis of three reaction mechanisms: (i) decarboxylation terminating in CO2 + other primary products (∼50%); (ii) Norrish type I dissociation typical of carbonyls (∼30%); and (iii) O-H and C-H bond fission reactions generating the H atom (∼10%). The analysis shows that most of the dissociation reactions create more than two products. This observation is not surprising considering the high excitation energy (∼51 800 cm-1) and fairly low energy required for dissociation of PA. We find that two-body fragmentation processes yielding CO2 are minor, and the expected, unstable primary co-fragment, methylhydroxycarbene, is not observed because it probably undergoes fast secondary dissociation and/or isomerization. Norrish type I dissociation pathways generate OH and only small yields of CH3CO and HOCO, which have low dissociation energies and further decompose via three-body fragmentation processes. Experiments with d1-PA (CH3COCOOD) support the interpretations. The dissociation on S3 is fast, as indicated by the products' recoil angular anisotropy, but the roles of internal conversion and intersystem crossing to lower states are yet to be determined.

A comparative kinetic study of C8–C10 linear alkylbenzenes pyrolysis in a single-pulse shock tube

This work presents a comparative study on the pyrolysis of C8–C10 linear alkylbenzenes including ethylbenzene, n-propylbenzene and n-butylbenzene. Experiments were performed with highly diluted mixtures in argon containing respectively the three fuels under nearly identical conditions in a single-pulse shock tube, at a nominal pressure of 20 bar and over a temperature range of 950–1700 K. Post-shock gas mixtures were sampled and analyzed with the gas chromatographic technique so that species concentration evolutions as function of temperature were obtained for the pyrolysis of each fuel. A kinetic model was developed to interpret the similarities and differences regarding the fuel decomposition and species formation behaviors observed in the experiments. The fuel conversion of n-propylbenzene and n-butylbenzene proceeds along a similar curve, which is much faster than that of ethylbenzene. All three fuels are consumed mainly through the bond fission producing benzyl radical. The simultaneously formed C1–C3 alkyl radicals in separate cases significantly impact the fuel reactivity and the formation of small C1–C4 and monocyclic aromatic hydrocarbons. Specifically, in n-propylbenzene pyrolysis, the decomposition of ethyl radicals produces a considerable amount of hydrogen atoms, which enhances the reactivity of the reaction system and meanwhile results in relatively high production of benzene during the fuel consumption. The formation of other monocyclic aromatic hydrocarbon intermediates, such as toluene and styrene, is also influenced by fuel-related pathways. Concerning PAH formation, the kinetic schemes in the pyrolysis of linear C8–C10 alkylbenzenes are very similar, which are dominated by the reactions of benzyl and other resonantly-stabilized radicals produced from benzyl decomposition. The major PAH formation reactions are barely influenced by the fuel chemistry. The only notable fuel-specific pathway is the indene formation from 1-phenyl-2-propenyl in n-propylbenzene and n-butylbenzene pyrolysis at relatively low temperatures. Styrene is an abundant product and its reaction with phenyl is found to be an important channel of phenanthrene formation.

Investigating the Photochemical Decomposition of Solid 1,3,5-Trinitro-1,3,5-triazinane (RDX).

Energetic materials such as 1,3,5-trinitro-1,3,5-triazinane (RDX) are known to photodissociate when exposed to UV light. However, the fundamental photochemical process(es) that initiate the decomposition of RDX is (are) still debatable. In this study we investigate the photodissociation of solid-phase RDX at four distinct UV wavelengths (254 nm (4.88 eV), 236 nm (5.25 eV), 222 nm (5.58 eV), 206 nm (6.02 eV)) exploiting a surface science machine at 5 K. We also conducted dose-dependent studies at the highest and lowest photon energy of 206 nm (6.02 eV) and 254 nm (4.88 eV). The products were monitored online and in situ via infrared spectroscopy. During the temperature-programmed desorption phase, the subliming products were detected with a reflectron time-of-flight mass spectrometer coupled with soft-photoionization at 10.49 eV (PI-ReTOF-MS). Infrared spectroscopy revealed the formation of small molecules including nitrogen monoxide (NO), nitrogen monoxide dimer ([NO]2), dinitrogen trioxide (N2O3), carbon dioxide (CO2), carbon monoxide (CO), dinitrogen monoxide (N2O), water (H2O), and nitrite group (-ONO) while ReTOF-MS identified 32 cyclic and acyclic products. Among these, 11 products such as nitryl isocyanate (CN2O3), 5-nitro-1,3,5-triazinan-2-one (C3H6N4O3) and 1,5-dinitro-1,3,5-triazinan-2-one (C3H5N5O5) were detected for the first time in photodecomposition of RDX. Dose-dependent in combination with wavelength-dependent photolysis experiments aid to identify key primary and secondary products as well as distinguished pathways that are more preferred at lower and higher photon energies. Our experiments reveled that N-NO2 bond fission and nitro-nitrite isomerization are the initial steps in the UV photolysis of RDX. Reaction mechanisms are derived by comparing the experimental findings with previous electronic structure calculations to rationalize the origin of the observed products. The present study can assist in understanding the complex chemistry behind the photodissociation of electronically excited RDX molecule, thus bringing us closer to unraveling the decomposition mechanisms of nitramine-based explosives.

Peroxymonosulfate Activation by Fe-Co-O-Codoped Graphite Carbon Nitride for Degradation of Sulfamethoxazole.

Graphite carbon nitride (g-C3N4) has a stable structure but poor catalytic capability for activating peroxymonosulfate (PMS). In this study, the codoping of g-C3N4 with bimetallic oxides (iron and cobalt) and oxygen was investigated to enhance its catalytic capability. The results showed that iron, cobalt, and oxygen codoped g-C3N4 (Fe-Co-O-g-C3N4) was successfully prepared, which was capable of completely degrading sulfamethoxazole (SMX) (0.04 mM) within 30 min, with a reaction rate of 0.085 min-1, indicating the superior catalytic activity of Fe-Co-O-g-C3N4. The mineralization efficiency of SMX was 22.1%. Sulfate radicals and singlet oxygen were detected during the process of PMS activation. However, the role that singlet oxygen played in degrading SMX was not obvious. Surface-bound reactive species and sulfate radicals were responsible for SMX degradation, in which sulfate radicals contributed to 46.6% of SMX degradation. The superior catalytic activity was due to the synergistic effect of metal oxides and O-g-C3N4, in which O-g-C3N4 could act as a carrier and an activator as well as an electron mediator to promote the conversion of Fe(III) to Fe(II) and Co(III) to Co(II). Four main steps of SMX degradation were proposed, including direct oxidation of SMX, bond fission of N-C, bond fission of N-S, and bond fission of S-C. The effect of the pH, temperature, PMS concentration, chloridion, bicarbonate, and humic acids on SMX degradation was investigated. Cycling experiments demonstrated the good stability of Fe-Co-O-g-C3N4. This study first reported the preparation of bimetallic oxide and oxygen codoped g-C3N4, which was an effective PMS activator for degradation of toxic organic pollutants.

UV light-assisted persulfate activation by Cu0-Cu2O for the degradation of sulfamerazine

Sulfate radicals-based advanced oxidation process (SRAOP) has received increasing attention in the wastewater treatment area because sulfate radicals have high redox potential. The key for SRAOP is to develop high efficient catalysts that can effectively and stably activate persulfate (PS) to produce sulfate radicals. Cu2O is a good activator of PS and photo-catalyst. However, its catalytic activity is relatively low because of the poor stability and the recombination of electron and holes. In theory, Cu0 can reduce Cu2+ to form Cu+, which can decrease the recombination of electron and holes from the Cu2O by the formation of heterojunction. In this study, Cu0-Cu2O was synthesized by a simple ball-milling method. The as-prepared Cu0-Cu2O was characterized and used to activate PS for sulfamerazine (SMZ) degradation by means of UV illumination. In the Cu0-Cu2O/PS/UV system, the removals of 100% SMZ and 30% TOC within 30 min were achieved, respectively, and the removal efficiency of SMZ decreased by only 12.2% after 5 cycles. The high efficiency and good catalytic stability of Cu0-Cu2O for the activation of PS were due to that Cu0 and photo-induced electron of Cu2O promoted the conversion of Cu2+ to Cu+. Quenching experiments demonstrated that the hydroxyl radicals and sulfate radicals contributed to SMZ degradation. However, the concentrations and fractions of radicals was affected by the initial pH of solution. Nine intermediate products were identified, and the degradation pathways were proposed, including the fission of sulfur and nitrogen bond, the oxidation of methyl group and the hydroxylation of benzene ring. The effects of initial pH, PS concentration, Cu0-Cu2O dosage and SMZ initial concentration on the degradation of SMZ were investigated. This study provided an effective catalyst that could simultaneously activate PS and utilize UV to form photo-induced electron, which could enhance the degradation and mineralization of organic pollutants.

Decomposition of 3-methylcyclohexene radicals: Beginning of its mechanism development

Cyclohexenes are important oxidation intermediates for cyclohexanes and precursors of aromatic hydrocarbons. 3-methylcyclohexene is one of important intermediate products of methylcyclohexane oxidation. In order to improve the understanding of oxidation for 3-methylcyclohexene, the decomposition pathways of 3-methylcyclohexene radical isomers were investigated using ab initio theoretical method. The molecular geometric structures of stationary points on the potential energy surfaces were optimized at B3LYP/6–311++G(d,p) level and then the single point energies were extrapolated using complete basis set (CBS) based on the results obtained using MP2 and QCISD(T) methods with basis sets of cc-pVDZ and cc-pVTZ. The reactivity of reaction channels are discussed with the energy barriers on potential energy surfaces. It is observed the decompositions of allylic radicals have clearly high barrier heights. The products of decomposition of 3-methylcyclohexene radicals are alkyl and alkenyl chain radicals via ring opening reactions, cyclic diolfins plus H atom via C–H bond fission, and cyclohexadienes plus methyl via side chain dissociation. Then it is shown that energies delivered with various quantum chemistry methods are of great similarity. Rate constants of major channels for all radicals were computed with information of stable and saddle points collected on the PESs. The high pressure limits of ring opening and dehydrogenation reactions are comparable for all alkyl radicals, while the other two allylic radicals possess much distinguished values. Finally, the pressure-dependent rate constants are computed based on RRKM/ME theory. The rate constants of ring opening reactions are compared with literature data for similar molecules.

How to complete the tautomerization and substrate-assisted activation prior to C–C bond fission by meta-cleavage product hydrolase LigY?

Two feasible binding modes could complete the C–C bond fission of the substrate. One is the bidentate mode and five-coordination, and the other is the monodentate mode and five-coordination.

Subpicosecond HI elimination in the 266 nm photodissociation of branched iodoalkanes.

The 266 nm photodissociation dynamics of 1-iodopropane and 2-iodopropane were studied using photofragment translational energy spectroscopy using vacuum ultraviolet (VUV) photoionization and electron impact ionization detection of products. The photochemistry of 1-iodopropane was found to be similar to that of iodomethane and iodoethane, with dominant production of I*(2P1/2), and no evidence (<0.21%) for HI + alkene formation. Significantly different behavior was observed for 2-iodopropane, with dominant production of ground state I(2P3/2), and a HI yield >10.5%. The anisotropy (β) parameters for all channels approached the limiting value of 2.0, indicating that 1,2-HI elimination occurs on subpicosecond timescales, like direct C-I bond fission, following excitation to 3Q0. The HI translational energy and angular distributions were similar to those for I(2P3/2), suggesting that motion of the heavy I atom in HI is largely derived from the repulsive nature of the 1Q1 surface correlating to R + I with the light H atom picked up by ground state I late in the exit channel producing highly vibrationally excited HI.