Absolute rate coefficient of the OH + CH(3)C(O)OH reaction at T = 287-802 K. The two faces of pre-reactive H-bonding

The rate constants for the reaction OH + CH3C(O)OH --> products (1) were determined over the temperature range 287-802 K at 50 and 100 Torr of Ar or N2 bath gas using pulsed laser photolysis generation of OH by CH3C(O)OH photolysis at 193 nm coupled with OH detection by pulsed laser-induced fluorescence. The rate coefficient displays a complex temperature dependence with a sharp minimum at 530 K, indicating the competition between a reaction proceeding through a pre-reactive H-bonded complex to form CH3C(O)O + H2O, expected to prevail at low temperatures, and a direct methyl-H abstraction channel leading to CH2C(O)OH + H2O, which should dominate at high temperatures. The temperature dependence of the rate constant can be described adequately by k1(287-802 K) = 2.9 x 10(-9) exp{-6030 K/T} + 1.50 x 10(-13) exp{515 K/T} cm3 molecule(-1)(s-1), with a value of (8.5 +/- 0.9) x 10-13 cm3 molecule(-1)(s-1) at 298 K. The steep increase in rate constant in the range 550-800 K, which is reported for the first time, implies that direct abstraction of a methyl-H becomes the dominant pathway at temperatures greater than 550 K. However, the data indicates that up to about 800 K direct methyl-H abstraction remains adversely affected by the long-range H-bonding attraction between the approaching OH radical and the carboxyl -C(O)OH functionality.     

Recombination and chemical energy accommodation coefficients from chemical dynamics simulations: O/O2 mixtures reacting over a β-cristobalite (001) surface

A microkinetic model is developed to study the reactivity of an O/O(2) gas mixture over a β-cristobalite (001) surface. The thermal rate constants for the relevant elementary processes are either inferred from quasiclassical trajectory calculations or using some statistical approaches, resting on a recently developed interpolated multidimensional potential energy surface based on density functional theory. The kinetic model predicts a large molecular coverage at temperatures lower than 1000 K, in contrary to a large atomic coverage at higher temperatures. The computed atomic oxygen recombination coefficient, mainly involving atomic adsorption and Eley-Rideal recombination, is small and increases with temperature in the 700-1700 K range (0.01 < γ(O) < 0.02) in good agreement with experiments. In the same temperature range, the estimated chemical energy accommodation coefficient, the main contribution to which is the atomic adsorption process is almost constant and differs from unity (0.75 < β(O) < 0.80).     

超临界甲醇降解对苯二甲酸丁二酯的研究

作为一种综合性能优良的新型工程塑料,对苯二甲酸丁二酯(PBT)工程塑料及其各种合金在全球范围内已经广泛用于电子电气、汽车、机械及民用等各个领域,而中国是其中需求量最大的国家．     

Ignition time measurements for methylcylcohexane‐ and ethylcyclohexane‐air mixtures at elevated pressures

The ignition of methylcyclohexane (MCH)/air and ethylcyclohexane (ECH)/air mixtures has been studied in a shock tube at temperatures and pressures ranging from 881 to 1319 K and 10.8 to 69.5 atm, respectively, for equivalence ratios of 0.25, 0.5, and 1.0. Endwall OH* emission and sidewall pressure measurements were used to determine ignition delay times. The influence of temperature, pressure, and equivalence ratio on ignition has been characterized. Negative temperature coefficient behavior was observed for temperatures below 1000 K. These measurements greatly extend the database of kinetic targets for MCH and provide, to our knowledge, the first ignition measurements for ECH. The combination of the MCH measurements with previous shock tube and rapid compression machine measurements provides kinetic targets over a large temperature range, 680–1650 K, for the validation of kinetic mechanisms. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 82–91, 2009     

Multiple mechanical relaxations in ethylcyclohexane above the glass transition temperature

The mechanical response of ethylcyclohexane has been investigated at ultrasonic frequencies in a large temperature range from 300 K down to the glass transition region. The results indicate the existence of a secondary relaxation not yet reported for this system. The comparison with literature data leads to a rather complex dynamic behavior. In fact, this molecular liquid exhibits three different mechanical relaxations above the glass transition temperature: a main structural process and two additional processes, both having a possible intramolecular origin.     

Pronounced negative thermal expansion from a simple structure: cubic ScF(3)

Scandium trifluoride maintains a cubic ReO(3) type structure down to at least 10 K, although the pressure at which its cubic to rhombohedral phase transition occurs drops from >0.5 GPa at ～300 K to 0.1-0.2 GPa at 50 K. At low temperatures it shows strong negative thermal expansion (NTE) (60-110 K, α(l) ≈ -14 ppm K(-1)). On heating, its coefficient of thermal expansion (CTE) smoothly increases, leading to a room temperature CTE that is similar to that of ZrW(2)O(8) and positive thermal expansion above ～1100 K. While the cubic ReO(3) structure type is often used as a simple illustration of how negative thermal expansion can arise from the thermally induced rocking of rigid structural units, ScF(3) is the first material with this structure to provide a clear experimental illustration of this mechanism for NTE.     

Reaction of HO with hydroxyacetone (HOCH2C(O)CH3): rate coefficients (233-363 K) and mechanism

Absolute rate coefficients for the title reaction, HO + HOCH(2)C(O)CH(3)--> products (R1) were measured over the temperature range 233-363 K using the technique of pulsed laser photolytic generation of the HO radical coupled to detection by pulsed laser induced fluorescence. The rate coefficient displays a slight negative temperature dependence, which is described by: k(1)(233-363 K) = (2.15 +/- 0.30) x 10(-12) exp{(305 +/- 10)/T} cm(3) molecule(-1) s(-1), with a value of (5.95 +/- 0.50) x 10(-12) cm(3) molecule(-1) s(-1) at room temperature. The effects of the hydroxy-substituent and hydrogen bonding on the rate coefficient are discussed based on theoretical calculations. The present results, which extend the database on the title reaction to a range of temperatures, indicate that R1 is the dominant loss process for hydroxyacetone throughout the troposphere, resulting in formation of methylglyoxal at all atmospheric temperatures. As part of this work, the rate coefficient for reaction of O((3)P) with HOCH(2)C(O)CH(3) (R4) was measured at 358 K: k(4)(358 K) = (6.4 +/- 1.0) x 10(-14) cm(3) molecule(-1) s(-1) and the absorption cross section of HOCH(2)C(O)CH(3) at 184.9 nm was determined to be (5.4 +/- 0.1) x 10(-18) cm(2) molecule(-1).     

Reaction Mechanism and Kinetics for HCCO Radical with NO

The mechanism and dynamical properties for the reaction of HCCO radicals with NO were investigated theoretically. The minimum energy paths(MEP) of the reaction were calculated by using the density functional theory(DFT) at the B3LYP/6-311 G^** level, and the energies along the MEP were further refined at the QCISD(T)/6-311 G^** level. It is found that the reaction mechanism of the title reaction involves three channels, producing HCNO CO, HONC CO and HCN CO2 products, respectively. Channel 1 is the most favorable path. The rate constant for channel 1 were calculated over a temperature range of 800-2500 K by using the canonical variational transition-state theory(CVT). The rate constant for the main path is negatively dependent on temperature, which is a characteristic of radical reactions with negative activation energy, and the variational effect for the rate constant calculation is small in the whole temperature range.     

Computational study on the mechanism and rate constant for the C6H5 + C6H5NO reaction

The reaction mechanism of C6H5 + C6H5NO involving four product channels on the doublet-state potential energy surface has been studied at the B3LYP/6-31+G(d, p) level of theory. The first reaction channel occurs by barrierless association forming (C6H5)2NO (biphenyl nitroxide), which can undergo isomerization and decomposition. The second channel takes place by substitution reaction producing C12H10 (biphenyl) and NO. The third and fourth channels involve direct hydrogen abstraction reactions producing C6H4NO + C6H6 and C6H5NOH + C6H4, respectively. Bimolecular rate constants of the above four product channels have been calculated in the temperature range 300-2000 K by the microcanonical Rice-Ramsperger-Kassel-Marcus theory and/or variational transition-state theory. The result shows the dominant reactions are channel 1 at lower temperatures (T < 800 K) and channel 3 at higher temperatures (T > 800 K). The total rate constant at 7 Torr He is predicted to be k(t) = 3.94 x 10(21) T(-3.09) exp(-699/T) for 300-500 K, 2.09 x 10(20) T(-3.56) exp(2315/T) for 500-1000 K, and 1.51 x 10(2) T(3.30) exp(-3043/T) for 1000-2000 K (in units of cm3 mol(-1) s(-1)), agreeing reasonably with the experimental data within their reported errors. The heats of formation of key products including biphenyl nitroxide, hydroxyl phenyl amino radical, and N-hydroxyl carbazole have been estimated.     

Complex thermal chemistry of vinyltrimethylsilane on Si(100)-2x1

The surface chemistry of vinyltrimethylsilane (VTMS) on Si(100)-2x1 has been investigated using multiple internal reflection-Fourier transform infrared spectroscopy, Auger electron spectroscopy, and thermal desorption mass spectrometry. Molecular adsorption of VTMS at submonolayer coverages is dominating at cryogenic temperatures (100 K). Upon adsorption at room temperature, chemical reaction involving rehybridization of the double bond in VTMS occurs. Further annealing induces several reactions: molecular desorption from a monolayer by 400 K, formation and desorption of propylene by 500 K, decomposition leading to the release of silicon-containing products around 800 K, and, finally, surface decomposition leading to the production of silicon carbide and the release of hydrogen as H(2) at 800 K. This chemistry is markedly different from the previously reported behavior of VTMS on Si(111)-7x7 surfaces resulting in 100% conversion to silicon carbide. Thus, some information about the surface intermediates of the VTMS reaction with silicon surfaces can be deduced.     

A consensus view of the temperature dependence of the gas phase reaction: OH + HBr → H2O + Br

The temperature dependence of the rate coefficient for the reaction, OH + HBr has been reinvestigated at low temperatures (T = 48–224 K) by using uniform supersonic flow reactors with laser induced fluorescence detection. This paper presents two forms of global fits: k(T) = 1.11 × 10^?11 (T/298)^?0.91 cm³ s^?1 and k(T) = 1.06 × 10^?11 (T/298)^?1.09 cm³ s^?1, both of which accurately describe the temperature dependence of the rate coefficient for the title reaction within the temperature range 20–350 K. These fits indicate that at temperatures below 200 K, the rate coefficient for this reaction shows inverse temperature dependence, while above 200 K the reaction shows insignificant temperature dependence. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 339–344, 2002     

Ethene adsorption, dehydrogenation and reaction with Pd(110): Pd as a carbon 'sponge'

The interaction of ethene with the Pd(110) surface has been investigated, mainly with a view to understanding the dehydrogenation reactions of the molecule and mainly using a molecular beam reactor. Ethene adsorbs with a high probability over the temperature range 130 to 800 K with the low-coverage sticking probability dropping from 0.8 at 130 K to 0.35 at 800 K. The adsorption is of the precursor type, with a weakly held form of ethene being the intermediate between the gas phase and strong chemisorption. Dehydrogenation begins at approximately 300 K and is fast above 350 K. If adsorption is carried out at temperatures up to approximately 380 K, adsorption saturates after about 0.25 monolayer have adsorbed, but above approximately 450 K, adsorption continues at a high rate with continuous hydrogen evolution and C deposition onto the surface. It appears that, in the intermediate temperature range, the carbonaceous species formed is located in the top layer and thus interferes with adsorption, whereas the C goes subsurface above 450 K, the adsorption is almost unaffected, and the C signal is significantly attenuated in XPS. However, the deposited carbon can easily be removed again by reaction with oxygen, thus implying that the carbon remains in the selvedge, that is, in the immediate subsurface region probably consisting of a few atomic layers. No well-ordered structures are identified in either LEED or STM, though some evidence of a c(2 x 2) structure can be seen. The Pd surface, at least above 450 K, appears to act as a "sponge" for carbon atoms, and this effect is also seen for the adsorption of other hydrocarbons such as acetaldehyde and acetic acid.     

Oxidation of α-pinene by atmospheric oxygen in the supercritical CO<Subscript>2</Subscript>—ethyl acetate system in the presence of Co(II) complexes

The reactivity of monoterpene α-pinene in a flow reactor in the presence of cobalt catalyst in a complex supercritical solvent consisting of CO₂ and ethyl acetate is studied over the temperature range of 190–320°C and a pressure range of 110–125 atm. It was found that the main isomerization products include compounds with bicyclo[2.2.1]heptane and p-menthane backbones; the reaction is accompanied by partial racemization. The formation of oxidation products is observed in the presence of air, with epoxydation rather than allylic oxidation being the predominant process at the first stage. The oxidized products (campholenic aldehyde, verbenone, pinocamphone) are shown to be formed with a high enantioselectivity; the formation of acetoxylation products is observed at temperatures above 200°C.     

生物质热解和气化过程Cl及碱金属逸出行为的化学热力学平衡分析

减少生物质在热转化反应器中Cl与碱金属K和Na以气态组元逸出可有效遏制积灰、腐蚀等现象和减少污染气体排放。采用化学热力学平衡分析方法,在400K~1600K研究了秸秆、树皮、木屑、废木和橄榄渣五种生物质在过剩空气系数分别为0、0.4、0.8的热解和气化过程中Cl与碱金属K和Na的赋存形态变化及逸出特性。结果表明,Cl在热解和气化过程中主要是以KCl(s)、HCl(g)、KCl(g)、(KCl)2(g)和NaCl(g)化合物赋存并相互转化;在800K~1000K时,含Cl固态组元逐渐转化为气态组元;K和Na在900K时开始以气态组元逸出,且热解过程有少量KCN(g)和NaCN(g)逸出,而气化过程,温度大于1000K随过剩空气系数的增加,KCl(g)、K(g)和Na(g)等气态组元量逐渐减少,逐渐转化为NaCl(g)、KOH(g)和NaOH(g);减少Cl和碱金属K和Na逸出的理论最佳热解和气化温度分别为800K和900K。     

煤化学链燃烧Fe2O3载氧体的反应性研究

利用流化床反应器并以水蒸气作为气化-流化介质,研究了温度、反应时间、循环数对Fe₂O₃载氧体反应性的影响。实验表明,载氧体与煤气化产物的反应性随温度升高而增强,且温度越高,反应受化学反应控制时间越短。当温度高于900℃时,煤中碳转化为CO₂的比率大于90%,载氧体体现了很好的反应性,但反应温度低于850℃时,比率小于75%。反应温度900℃时,CO₂干基浓度随循环数而逐渐降低,CO、CH₄浓度增加,且CH₄浓度值大于CO。利用XRD、SEM分析了固体反应产物成分与微观形态结构。分析表明,Fe₂O₃的还原产物为Fe₃O₄,载氧体颗粒随循环数增加而逐渐烧结。     

Stereocontrolled aziridination of imines via a sulfonium ylide route and a mechanistic study

The reaction of N-diphenylphosphinoyl imines 1 with [3-(trimethylsilyl)allyl]dimethylsulfonium bromide (5) in the presence of NaH at room temperature predominantly gave trans-vinylaziridines 4. On the other hand, cis-vinylaziridines 4 were the main products when the preformed ylide prepared from the reaction of [3-(trimethylsilyl)allyl]diphenylsulfonium perchlorate (6) was reacted with the same imines 1 at low temperature. trans-Aziridines were also obtained when imines 1 and sulfinimines 9 were reacted with N,N-dimethylacetamide-2-dimethylsulfonium bromide (7) in the presence of a base, respectively. A mechanistic study showed that the stereochemistry of these reactions was controlled by the reactivity of the imines and ylides. A higher reactivity of imines and ylides favors the formation of cis-aziridines, whereas a lower reactivity leads to trans-products.     

HNCS与Cl原子的反应机理及电子密度拓扑分析

在QCISD(T)/6-311++G(d,p)和B3LYP/6-311++G(d,p)级别上研究了HNCS与Cl原子的反应机理. 并应用经典过渡态理论和正则变分过渡态理论结合小曲率隧道效应, 计算了200-2500 K温度范围内各反应通道的速率常数. 结果表明, HNCS与Cl原子反应存在3个反应通道. 当温度低于294 K时, 生成HCl+NCS的夺氢反应(a)是优势通道, 温度高于294 K时, 生成HNC(Cl)S的加成反应(c)为主反应通道, Cl进攻N的反应通道(b)因能垒较高而难以进行.     

An experimental study of the intersystem crossing and reactions of C2(X1Sigmag+) and C2(a3Pi(u)) with O2 and NO at very low temperature (24-300 K)

A low-temperature gas-phase kinetics study of the reactions and collisional relaxation processes involving C2(X1Sigma(g)+) and C2(a3Pi(u)) in collision with O2 and NO partners at temperatures from 300 to 24 K is reported. The experiments employed a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus to attain low temperatures. The C2 species were created using pulsed laser photolysis at 193 nm of mixtures containing C2Cl4 diluted in N2, Ar, or He carrier gas. C2(X1Sigma(g)+) molecules were detected via pulsed laser-induced fluorescence in the (D1Sigma(u)+ <-- X1Sigma(g)+) system, and C2(a3Pi(u)) molecules were detected via pulsed laser-induced fluorescence in the (d 3Pi(g) <-- a 3Pi(u)) system. Relaxation of 3C2 by intersystem crossing induced by oxygen was measured at temperatures below 200 K, and it was found that this process remains very efficient in the temperature range 50-200 K. Reactivity of C2(X1Sigma(g)+) with oxygen became very inefficient below room temperature. Using these two observations, it was found to be possible to obtain the C2(X1Sigma(g)+) state alone at low temperatures by addition of a suitable concentration of O2 and then study its reactivity with NO without any interference coming from the possible relaxation of C2(a3Pi(u)) to C2(X1Sigma(g)+) induced by this reagent. The rate coefficient for reaction of C2(X1Sigma(g)+) with NO was found to be essentially constant over the temperature range 36-300 K with an average value of (1.6 +/- 0.1) x 10(-10) cm3 molecule(-1) s(-1). Reactivity of C2(a3Pi(u)) with NO was found to possess a slight negative temperature dependence over the temperature range 50-300 K, which is in very good agreement with data obtained at higher temperatures.     

Rate constants for the reactions of CO3- and O3- with SO2 from 300 to 1440 K

Rate constants for the reactions of CO(3) (-) and O(3) (-) with SO(2) have been measured between 300 and 1440 K in a high temperature flowing afterglow apparatus. The CO(3) (-) rate constants near to the collision rate at low temperatures and fall by about a factor of 50 with temperature until a broad minimum is reached at 900-1300 K. The highest temperature point shows the increasing rate constant. Comparison to drift tube data taken in a helium buffer shows that total energy controls the reactivity, presumably because the reaction goes through a long lived complex even at 1440 K. The reaction of O(3) (-) with SO(2) was studied up to 1400 K. The rate constant is collisional until 700 K and then decreases with increasing temperature. Rate constants measured at 1300 and 1400 K appear to show an increase, but that observation is questionable since O(3) (-) could not be made cleanly. The O(3) (-) data at 1200 K and below show that total energy controls reactivity in that range.     

Detailed product analysis during the low temperature oxidation of n-butane

The products obtained from the low-temperature oxidation of n-butane in a jet-stirred reactor (JSR) have been analysed using two methods: gas chromatography analysis of the outlet gas and reflectron time-of-flight mass spectrometry. The mass spectrometer was combined with tunable synchrotron vacuum ultraviolet photoionization and coupled with a JSR via a molecular-beam sampling system. Experiments were performed under quasi-atmospheric pressure, for temperatures between 550 and 800 K, at a mean residence time of 6 s and with a stoichiometric n-butane/oxygen/argon mixture (composition = 4/26/70 in mol%). 36 reaction products have been quantified, including in addition to the usual oxidation products, acetic acid, hydrogen peroxide, C(1), C(2) and C(4) alkylhydroperoxides and C(4) ketohydroperoxides. Evidence of the possible formation of products (dihydrofuranes, furanones) derived from cyclic ethers has also been found. The performance of a detailed kinetic model of the literature has been assessed with the simulation of the formation of this extended range of species. These simulations have also allowed the analysis of possible pathways for the formation of some obtained products.