Thermal reactions of benzoxazole. Single pulse shock tube experiments and quantum chemical calculations

The thermal decomposition of benzoxazole diluted in argon was studied behind reflected shock waves in a 2 in. i.d. single-pulse shock tube over the temperature range 1000-1350 K and at overall densities of approximately 3 x 10(-5) mol/cm(3). Two major products, o-hydroxybenzonitrile at high concentration and cyclopentadiene carbonitrile (accompanied by carbon monoxide) at much lower concentration, and four minor fragmentation products resulting from the decomposition were found in the postshock samples. They were, in order of decreasing abundance, benzonitrile, acetylene, HCN, and CH=C-CN and comprised of only a few percent of the overall product distribution. Quantum chemical calculations were carried out to determine the sequence of the unimolecular reactions that led to the formation of o-hydroxybenzonitrile and cyclopentadiene carbonitrile, the major products of the thermal reactions of benzoxazole. A potential energy surface leading directly from benzoxazole to cyclopentadiene carbonitrile could not be found, and it was shown that the latter is formed from the product o-hydroxybenzonitrile. In order that cyclopentadiene carbonitrile be produced, CO elimination and ring contraction from a six- to a five-membered ring must take place. A surface where CO elimination occurs prior to ring contraction was found to have very high barriers compared to the ones where ring contraction occurs prior to CO elimination and was not considered in our discussion. Rates for all the steps on the various surfaces were evaluated, kinetic schemes containing these steps were constructed, and multiwell calculations were performed to evaluate the mole percent of the two major products as a function of temperature. The agreement between the experimental results and these calculations, as shown graphically, is very good.     

Theoretical study of the competitive decomposition and isomerization of 1-hexyl radical

The ground-state potential energy surface of the 1-hexyl system, including the main decomposition and isomerization processes, has been calculated with the MPW1K, BB1K, MPWB1K, MPW1B95, BMK, M05-2X and CBS-QB3 methods. On the basis of these data, thermal rate coefficients of different reaction channels and branching ratios were then calculated using the master equation formulation at 250–2,500 K. The results clearly point out that the 1,5 H atom transfer reaction of 1-hexyl radical with exothermicity proceeds through the lowest reaction barrier, whereas the decomposition processes are thermodynamically unfavorable with large endothermicity. The temperature effect is important on the relative importance of different reactions in the 1-hexyl system. In the low-temperature range of 250–900 K, isomerization reactions, especially 1,5 H atom transfer reaction of 1-hexyl radical, are dominating and responsible for over 82.17% of all the reactions, due to their smaller reaction barriers than those of the decomposition reactions. Furthermore, an equilibrium process involving the isomeric forms of the hexyl radicals appearing at relative low temperature was validated theoretically. However, isomerization and decomposition processes are kinetically competitive and simultaneously important under normal pyrolysis conditions.     

Understanding the complex dissociation dynamics of energy selected dichloroethylene ions: neutral isomerization energies and heats of formation by imaging photoelectron-photoion coincidence

The dissociative photoionization of 1,1-C(2)H(2)Cl(2), (E)-1,2-C(2)H(2)Cl(2), and (Z)-1,2-C(2)H(2)Cl(2) has been investigated at high energy and mass resolution using the imaging photoelectron photoion coincidence instrument at the Swiss Light Source. The asymmetric Cl-atom loss ion time-of-flight distributions were fitted to obtain the dissociation rates in the 10(3) s(-1) < k < 10(7) s(-1) range as a function of the ion internal energy. The results, supported by ab initio calculations, show that all three ions dissociate to the same C(2v) symmetry ClC═CH(2)(+) product ion. The 0 K onset energies thus establish the relative heats of formation of the neutral isomers, that is, the isomerization energies. The experimental rate constants, k(E), as well as ab initio calculations indicate an early isomerization transition state and no overall reverse barrier to dissociation. The major high energy channels are the parallel HCl loss and the sequential ClC═CH(2)(+) → HCCH(+) + Cl process, the latter in competition with a ClC═CH(2)(+) → ClCCH(+) + H reaction. A parallel C(2)H(2)Cl(2)(+) → C(2)HCl(2)(+) + H channel also weakly asserts itself. The 0 K onset energy for the sequential Cl loss reaction suggests no barrier to the production of the most stable acetylene ion product; thus the sequential Cl-atom loss is preceded by a ClC═CH(2)(+) → HC(Cl)CH(+) reorganization step with a barrier lower than that of the second Cl-atom loss. The breakdown diagram corresponding to this sequential dissociation reveals the internal energy distribution of the first C(2)H(2)Cl(+) daughter ion, which is determined by the kinetic energy release in the first, Cl loss reaction at high excess energies. At low kinetic energy release, this distribution corresponds to the predicted two translational degrees of freedom, whereas at higher energies, the excess energy partitioning is characteristic of only one translational degree of freedom. New Δ(f)H(o)(298K) of 3.7, 2.5, and 0.2 ± 1.75 kJ mol(-1) are proposed for 1,1-C(2)H(2)Cl(2), (E)-1,2-C(2)H(2)Cl(2), and (Z)-1,2-C(2)H(2)Cl(2), respectively, and the proton affinity of ClCCH is found to be 708.6 ± 2.5 kJ mol(-1).     

Thermal decomposition of ethylene oxide: potential energy surface, master equation analysis, and detailed kinetic modeling

The unimolecular decomposition of ethylene oxide (oxirane) and the oxiranyl radial is examined by molecular orbital calculations, Rice-Ramsperger-Kassel-Marcus (RRKM)/Master Equation analysis, and detailed kinetic modeling of ethylene oxide pyrolysis in a single-pulse shock tube. It was found that the largest energy barrier to the decomposition of ethylene oxide lies in its initial isomerization to form acetaldehyde, and in agreement with previous studies, the isomerization was found to proceed through the *CH2CH2O* biradical. Because of the biradical nature of the transition states and intermediate, the energy barriers for the initial C-O rupture in ethylene oxide and the subsequent 1,2-H shift remain highly uncertain. An overall isomerization energy barrier of 59 +/- 2 kcal/mol was found to satisfactorily explain the available single pulse shock tube data. This barrier height is in line with the estimates made from an approximate spin-corrected procedure at the MP4/6-31+G(d) and QCISD(T)/6-31G(d) levels of theory. The dominant channel for the unimolecular decomposition of ethylene oxide was found to form CH3 + HCO at around the ambient pressure. It accounts for >90% of the total rate constant for T > 800 K. The high-pressure limit rate constant for the unimolecular decomposition of ethylene oxide was calculated as k(1,infinity)(s(-1)) = (3.74 x 10(10))T(1.298)e(-29990/T) for 600 < T < 2000 K.     

Fragmentation and isomerization of 1,2-dimethylcyclobutane

The potential surface of 1,2-dimethylcyclobutane is investigated with respect to fragmentation and isomerization by the semiempirical MO method SINDO1. Energy and geometry of eight transition states and four intermediates are determined for the nonconcerted fragmentations under optimization of all internal coordinates. We find that symmetric cleavage ofcis-dimethylcyclobutane is the most favored process leading to fragmentation. Whereas the symmetric elimination involves two different barrier heights, the asymmetric elimination involves only two equal barrier heights. The possibility of isomerization ofcis- totrans- dimethylcyclobutane was also studied and revealed two different barrier heights lower than the corresponding heights for fragmentation.     

Photodissociation spectroscopy and dynamics of the CH2CFO radical

The photodissociation spectroscopy and dynamics resulting from excitation of the B (2)A(")<--X (2)A(") transition of CH(2)CFO have been examined using fast beam photofragment translational spectroscopy. The photofragment yield spectrum reveals vibrationally resolved structure between 29 870 and 38 800 cm(-1), extending approximately 6000 cm(-1) higher in energy than previously reported in a laser-induced fluorescence excitation spectrum. At all photon energies investigated, only the CH(2)F+CO and HCCO+HF fragment channels are observed. Both product channels yield photofragment translational energy distributions that are characteristic of a decay mechanism with a barrier to dissociation. Using the barrier impulsive model, it is shown that fragmentation to CH(2)F+CO products occurs on the ground state potential energy surface with the isomerization barrier between CH(2)CFO and CH(2)FCO governing the observed translational energy distributions.     

The pyrolysis of trimethylene sulfide. A unimolecular decomposition

The kinetics of decomposition of trimethylene sulfide to ethylene and thioformaldehyde was investigated in a single-pulse shock tube using the «relative rate» technique. The extent of reaction was measured in the reflected shock regime from 860° to 1170°K, but experimental difficulties limited the useful data to the temperature range of 980°–1040°K. The first-order rate constant was found to be k = 10^13.0 exp (?48,200/RT) sec^?1. This result sets an upper limit of 50 kcal/mole for the standard enthalpy of formation of CH₂S, with 35 kcal/mole as a more likely value. The isomerization of cyclopropane to propene was used for the reference reaction; in turn, this was checked, in a relative rate experiment, against the pyrolysis of cyclohexene.     

Ab initio kinetics for the unimolecular reaction C6H5OH --> CO + C5H6

The unimolecular decomposition of C(6)H(5)OH on its singlet-state potential energy surface has been studied at the G2M//B3LYP/6-311G(d,p) level of theory. The result shows that the most favorable reaction channel involves the isomerization and decomposition of phenol via 2,4-cyclohexadienone and other low-lying isomers prior to the fragmentation process, producing cyclo-C(5)H(6) + CO as major products, supporting the earlier assumption of the important role of the 2,4-cyclohexadienone intermediate. The rate constant predicted by the microcanonical RRKM theory in the temperature range 800-2000 K at 1 Torr--100 atm of Ar pressure for CO production agrees very well with available experimental data in the temperature range studied. The rate constants for the production of CO and the H atom by O-H dissociation at atmospheric Ar pressure can be represented by k(CO) = 8.62 x 10(15) T(-0.61) exp(-37,300/T) s(-1) and k(H) = 1.01 x 10(71) T(-15.92) exp(-62,800/T) s(-1). The latter process is strongly P-dependent above 1000 K; its high- and low-pressure limits are given.     

Rate constant measurements for the overall reaction of OH + 1-butanol → products from 900 to 1200 K

The rate constant for the overall reaction OH + 1-butanol → products was determined in the temperature range 900 to 1200 K from measurements of OH concentration time histories in reflected shock wave experiments of tert-butyl hydroperoxide (TBHP) as a fast source of OH radicals with 1-butanol in excess. Narrow-linewidth laser absorption was employed for the quantitative OH concentration measurement. A detailed kinetic mechanism was constructed that includes updated rate constants for 1-butanol and TBHP kinetics that influence the near-first-order OH concentration decay under the present experimental conditions, and this mechanism was used to facilitate the rate constant determination. The current work improves upon previous experimental studies of the title rate constant by utilizing a rigorously generated kinetic model to describe secondary reactions. Additionally, the current work extends the temperature range of experimental data in the literature for the title reaction under combustion-relevant conditions, presenting the first measurements from 900 to 1000 K. Over the entire temperature range studied, the overall rate constant can be expressed in Arrhenius form as 3.24 × 10(-10) exp(-2505/T [K]) cm(3) molecule(-1) s(-1). The influence of secondary reactions on the overall OH decay rate is discussed, and a detailed uncertainty analysis is performed yielding an overall uncertainty in the measured rate constant of ±20% at 1197 K and ±23% at 925 K. The results are compared with previous experimental and theoretical studies on the rate constant for the title reaction and reasonable agreement is found when the earlier experimental data were reinterpreted.     

N-甲硝胺异构化和分解反应机理和动力学的理论研究

The complex potential energy surface and reaction mechanisms for the unimolecular isomerization and decomposition of methyl-nitramine (CH3NHNO2) were theoretically probed at the QCISD(T)/6-311+G*//B3LYP/6-311+G* level of theory. The results demonstrated that there are four low-lying energy channels: (i) the NN bond fission pathway; (ii) a sequence of isomerization reactions via CH3NN(OH)O; (IS2a); (iii) the HONO elimination pathway; (iv) the isomerization and the dissociation reactions via CH3NHONO (IS3). The rate constants of each initial step (rate-determining step) for these channels were calculated using the canonical transition state theory. The Arrhenius expressions of the channels over the temperature range 298-2000 K are k6(T)=1014:8e-46:0=RT , k7(T)=1013:7e-42:1=RT , k8(T)=1013:6e-51:8=RT and k9(T)=1015:6e-54:3=RT s-1, respectively. The calculated overall rate constants is 6.9￡10-4 at 543 K, which is in good agreement with the experimental data. Based on the analysis of the rate constants, the dominant pathway is the isomerization reaction to form CH3NN(OH)O at low temperatures, while the NN bond fission and the isomerization reaction to produce CH3NHONO are expected to be competitive with the isomerization reaction to form CH3NN(OH)O at high temperatures.     

Thermal decomposition of 1,2 butadiene

The thermal decomposition of 1,2 butadiene has been studied behind reflected shock waves over the temperature and total pressure ranges of 1300–2000 K and 0.20–0.55 atm using mixtures of 3% and 4.3% 1,2 butadiene in Ne. The major products of the pyrolysis are C₂H₂, C₄H₂, C₂H₄, CH₄ and C₆H₆. Toluene was observed as a minor product in a narrow temperature range of 1500–1700 K. In order to model successfully the product profiles which were obtained by time-of-flight mass spectrometry, it was necessary to include the isomerization reaction of 1,2 to 1,3 butadiene. A reaction mechanism consisting of 74 reaction steps and 28 species was formulated to model the time and temperature dependence of major products obtained during the course of decomposition. The importance of C₃H₃ in the formation of benzene is demonstrated.     

A novel synthesis of 3-Amino-1,2-benzisoxazoles — an entry into the isoxazolo[3,4,5-ef][1,4]benzoxazepine ring system

A novel synthesis of 3-amino-1,2-benzisoxazole (3) from 2-[(isopropylideneamino)oxy]benzonitrile (2) is described. This methodology was used to synthesize 3-amino-4-hydroxy-1,2-benzisoxazole (10), which served as an intermediate for a number of isoxazolo[3,4,5-ef][1,4]benzoxazepines.     

Isomerization reaction dynamics and equilibrium at the liquid-vapor interface of water. A molecular-dynamics study

The gauche-trans isomerization reaction of 1,2-dichloroethane at the liquid-vapor interface of water is studied using molecular-dynamics computer simulations. The solvent bulk and surface effects on the torsional potential of mean force and on barrier recrossing dynamics are computed. The isomerization reaction involves a large change in the electric dipole moment, and as a result the trans/gauche ratio is considerably affected by the transition from the bulk solvent to the surface. Reactive flux correlation function calculations of the reaction rate reveal that deviation from the transition-state theory due to barrier recrossing is greater at the surface than in the bulk water. This suggests that the system exhibits non-Rice-Ramsperger-Kassel-Marcus behavior due to the weak solvent-solute coupling at the water liquid-vapor interface.     

Dynamic behavior of the diphosphine ligand in H4Ru4(CO)10(dppe) revisited: kinetic data supporting a nondissociative isomerization of the dppe ligand

The kinetics for the bridge-to-chelate isomerization of the dppe ligand in H4Ru4(CO)10(dppe) have been investigated by UV-vis and NMR spectroscopies over the temperature range of 308-328 K. The isomerization of the ligand-bridged cluster 1,2-H4Ru4(CO)10(dppe) (1-br) was found to be reversible by 31P NMR spectroscopy, affording a K(eq) = 15.7 at 323 K in favor of the chelating dppe isomer 1-ch. The forward (k1) and reverse (k(-1)) first-order rate constants for the reaction have been measured in different solvents and in the presence of ligand-trapping agents (CO and PPh3). On the basis of the activation parameters and reaction rates that are unaffected by added CO and PPh3, a sequence involving the nondissociative migration of a phosphine moiety and two CO groups between basal ruthenium centers is proposed and discussed.     

甲基硅乙烯异构化反应的理论研究 用从头算结果讨论其热力学、动力学性质

The titled isomerizaton of methylsilene has been studied at ab initio level. Theharmonic vibrational frequencies of reactant, product and transition state, the barrier energy, reaction heat, equilibrium constant and rate constant are calculated. The results indicate that this reaction is exothermic, spontaneous and high temperature above 900 K is favourable for the isomerization. The prediction is in agreement with experiment.     

Development and applications of a low-temperature differential thermal analyzer (77 < T,K < 330)

A differential thermal analyzer for the temperature range 77 to 330 K is described and some applications, determination of the impurity content, the triple point temperature and the enthalpy of fusion of a substance, are given. The temperature and energy calibration of the apparatus are done with diphenyloxide, hexafluorobenzene and 1,3-difluorobenzene. The energy calibration coefficient can be expressed as a linear relation ofT ³.n-Heptane, cyclohexane and 1,2-dichloroethane are investigated.     

煤燃烧过程中砷与氮氧化物的反应机理

应用量子化学密度泛函理论B3LYP方法,研究了砷与氮氧化物(N_2O、NO_2和NO)的反应机理。全参数优化了各反应物、中间体、过渡态和产物的几何构型,通过频率分析证实中间体和过渡态的真实性,并通过内禀反应坐标(IRC)计算以进一步确定过渡态。为了得到更精确的能量信息,在B2PLYP水平下计算各结构的单点能,并通过动力学参数深入分析其反应机理。结果表明,砷与三种氮氧化物(N_2O、NO_2和NO)的反应能垒分别为78.45、2.58、155.85 k J/mol。在298-1800 K,各反应速率随温度的升高而增大。由于砷与NO_2的反应能垒较低,其反应速率大于1012cm3/(mol·s),说明该反应容易发生且速率极快。砷与N_2O和NO的反应,在298-900 K,反应速率随温度的升高明显增加;当温度进一步升高,其增加的趋势有所减缓。     

Thermal and ion-induced surface reactions of 1,1-difluoroethylene on Si(111)7 x 7 and vitreous SiO2

Thermal and ion-induced reactions of 1,1-difluoroethylene (1,1-C2H2F2 or iso-DFE) on Si(111)7 x 7 and vitreous SiO2 surfaces have been investigated by vibrational electron energy loss spectroscopy and thermal desorption spectrometry. Like ethylene, iso-DFE predominantly chemisorbs via a [2 + 2] cycloaddition mechanism onto the 7 x 7 surface as a di-sigma-bonded difluoroethane-1,2-diyl adstructure, which undergoes H abstraction and defluorination, producing hydrocarbon fragments and SiF(x) (x = 1-3) upon annealing to >700 K. Ion irradiation of Si(111)7 x 7 in iso-DFE at 50 eV impact energy appears to substantially enhance the production of hydrocarbon fragments and SiF(x)(), leading to stronger SiF4 desorption products over an extended temperature range (400-900 K). The observed SiC and SiF(x) produced on the 7 x 7 surface by ion irradiation in iso-DFE are found to be similar to those obtained by ion irradiation in the fluoromethane homologues, CF4 and CH2F2. The production of higher relative concentrations for the larger SiF(x) and C2-containing fragments is evidently favored on the 7 x 7 surface. On a vitreous SiO2 surface, ion irradiation in iso-DFE, unlike that in CF4 and CH2F2, appears to produce less SiF(x) than that on the 7 x 7 surface, which indicates that surface O does not interact strongly with the C2-containing fragments. The presence or absence of a C=C bond and the relative F-to-C ratio of the sputtering gas could therefore produce important effects on the resulting surface products obtained by low-energy ion irradiation.     

Configurational isomerization ofcis- andtrans-1,2-, 1,3-and 1,4-dimethylcyclohexanes on Rh/SiO2 catalyst

Configurational isomerization ofcis- andtrans-1,2-, 1,3-, and 1,4-dimethylcyclohexanes has been studied on Rh/SiO₂ catalyst in the range of 373–523 K and a hydrogen partial pressure range of 13.3–100 kPa. Temperature has a small effect on the isomerization rate while this rate increases with increasing hydrogen pressure. Isomers containing axial-equatorial methyl groups were converted at higher rates than those containing diequatorial methyl groups. The difference between Rh and Pt catalysts can be interpreted by assuming that on Pt the π-bonded surface intermediates play more important role than on Rh. Dedicated to Professor Pál Tétényi on the occasion of his 70th birthday     

Potential energy surface for the CCl4 + H --> CCl3 + ClH reaction: kinetics and dynamics study

An analytical potential energy surface for the gas-phase CCl4 + H --> CCl3 + ClH reaction was constructed with suitable functional forms to represent vibrational modes. This surface is completely symmetric with respect to the permutation of the four chlorine atoms and is calibrated with respect to experimental thermal rate constants available over the temperature range 297-904 K. On this surface, the thermal rate constants were calculated using variational transition-state theory with semiclassical transmission coefficients over a wider temperature range 300-2500 K, therefore obtaining kinetics information at higher temperatures than are experimentally available. This surface was also used to analyze dynamical features, such as tunneling and reaction-path curvature. In the first case, the influence of the tunneling factor is very small since a heavy chlorine atom has to pass through the barrier. In the second, it was found that vibrational excitation of the Cl-H stretching mode can be expected in the exit channel.