The reaction of the hydroxyl radical with acetylene

The reaction of OH with acetylene was studied in a discharge flow system at room temperature. OH was generated by the reaction of atomic hydrogen with NO₂ and was monitored throughout the reaction using ESR spectroscopy. Mass-spectrometric analysis of the reaction products yielded the following results: (1) less than 3 molecules of OH were consumed, and less than 2 molecules of H₂O were formed for every molecule of acetylene that reacted; (2) CO was identified as the major carbon-containing product; (3) NO, formed in the generation of OH, reacted with a reaction intermediate to give among other products N₂O. These observations placed severe limitations on the choice of a reaction mechanism. A mechanism containing the reaction OH + C₂H₂ → HC₂O + H₂ better accounted for the experimental results than one involving the abstraction reaction OH + C₂H₂ → C₂H + H₂O. The rate constant for the initial reaction was measured as 1.9 ± 0.6 × 10^?13 cm³ molecule^?1 sec^?1.     

烷基自由基β位裂解反应类反应势垒与速率常数的精确计算

提出反应类等键方法并用于高温燃烧机理中一类重要反应——烷基自由基β位裂解反应的反应势垒和速率常数的精确校正计算. 通过10种不同从头算水平对类反应中5个代表反应的反应势垒的计算发现, 用反应类等键反应方法和直接从头算方法获得的5 个代表反应的反应势垒最大绝对偏差的平均值分别为5.32 和16.16 kJ·mol^-1, 表明反应类等键反应方法计算的反应势垒对不同水平从头算方法的依赖性小, 可在较低从头算水平计算得到精确的反应势垒, 解决大分子体系反应势垒的精确计算问题. 此外应用反应类等键反应方法在BHandHLYP/cc-pVDZ 从头算水平计算了3 个代表反应的速率常数, 并与文献报道的实验值进行了比较, 其在500-2000 K温度区间内计算速率常数与实验速率常数中较大值与较小值的比值k_max/k_min的平均值为1.67, 最大值也仅有2.49. 表明应用反应类等键反应方法在较低从头算水平即可对同类反应的速率常数进行精确计算.最后在BHandHLYP/cc-pVDZ从头算水平用反应类等键反应方法计算了13个烷基自由基β位裂解反应的速率常数.     

Grignard Reaction as a Result of Tunneling of the Triplet Branch of the Reaction Through a Singlet Barrier

Quantum-chemical calculations were carried out to study the mechanism of the Grignard reaction Mg + CH₃F. The reaction between Mg, CH₃F gives rise to a triplet complex which has a lower energythan the corresponding singlet complex at MgFCH₃ angles ranging from 60 to 139°. The transition from thesinglet branch of the reaction to the nonequilibrium triplet branch results in decomposition of CH₃F with formation of various final products. A linear Grignard reagent CH₃-Mg-F is formed by simultaneous homolytic CH₃-F dissociation and ionic covalent interaction of the methyl group and the fluorine atom with the magnesium cation arising in the course of the reaction. Normal mode frequencies for the Mg + CH₃F complex at various MgFCH₃ are were calculated. The calculation results nicely agree with experimental data.     

Studies in the formation of poly(oxazolidones). II. Selectivity of catalysts and kinetics in the synthesis of 2-oxazolidone from butyl isocyanate and phenylglycidyl ether

The reaction of butyl isocyanate with phenylglycidyl ether was selected as a model reaction for the synthesis of aliphatic isocyanate-based poly(2-oxazolidones). The selectivity of different metal halides and aluminum trichloride/triphenylphosphine oxide (AlCl₃ TPPO) and aluminum hexamethylphosphoramide (AlCl₃ HMPA) complexes were investigated for oxazolidone formation. Both FTIR and mass spectrographic methods were employed for characterization of the reaction products. The kinetics of the model reaction was studied using AlCl₃ TPPO in o-dichlorobenzene at 120 and 140°C.     

Effect of the nature of the active component and support on the activity of catalysts for the hydrolysis of sodium borohydride

The effect of the nature of an active component and a support on the rate of hydrolysis of aqueous sodium borohydride solutions was studied. It was found that the activity of supported catalysts, which were reduced in a reaction medium of sodium borohydride, decreased in the order Rh > Pt ≈ Ru ? Pd regardless of the nature of the support (γ-Al₂O₃, a Sibunit carbon material, or TiO₂). The catalysts based on TiO₂ exhibited the highest activity. As found by UV-vis diffuse reflectance spectroscopy, the composition and structure of the supported precursor of an active component depend on the nature of the support. It is likely that rhodium clusters with different reaction properties were formed on various supports under the action of a reaction medium.     

Exploring the Reaction Paths on the Potential Energy Surfaces of the S1 and T1 States in Methylenecyclopropane

The reaction paths of methylenecyclopropane 1 on the potential energy surfaces (PESs) of the lowest triplet (T₁) state and the lowest excited singlet (S₁) state, as well as that of the ground state (S₀), were explored by using the nudged elastic band method at the MRMP2//MCSCF/6‐31++G(d,p) and DFT(B3LYP)/6‐31++G(d,p) levels of theory. After vertical excitation of 1, three transition states on the PES of the lowest triplet state and one transition state on the S₁ PES were found along the reaction path to produce a carbene, cyclobutylidene 2. All of these transition states are lower in energy than the S₁ state produced by vertical excitation at the S₀ energy minimum in 1. Fast transition is predicted to occur from the T₁ state or from the S₁ state to the S₀ state due to strong spin‐orbit coupling or nonadiabatic coupling in the geometrical vicinity of 2. On the MRMP2 S₀ PES, the energy barriers of 5.0, 10.3 and 13.5 kcal mol^?1 were obtained for C migration reaction (backward reaction), 1,2‐H migration reaction to cyclobutene 3, and 1,3‐H migration reaction to bicyclopropane 4, respectively, started at 2. The introduction of phenyl groups makes the energy barriers smaller due to the π conjugation between the carbene center and phenyl groups.     

Kinetics of the initiation step of the thermal decomposition of SiCl4

The initiation reaction of the thermal decomposition of silicon tetrachloride was studied behind reflected shock waves at temperatures between 1550 K and 2370 K and pressures between 1 and 1.5 bar. Atomic resonance absorption spectrometry (ARAS) was applied for time-resolved measurements of H atoms at the L_α-line in SiCl₄/H₂/Ar systems. Additional experiments were performed in the SiCl₄/Ar system following the absorption of SiCl₄ at the L_α-line. Rate coefficients for the reaction (RI) were determined to be: The choice between two possible alternatives of the first decomposition step, namely elimination of either Cl₂ or Cl, has been made in favor of the second reaction on the basis of kinetic and energetic considerations. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 415–420, 1997.     

Kinetic model of the oxidation of ZrSi<Subscript>2</Subscript> powders

The oxidation kinetics of Zr-disilicide (ZrSi₂) powders up to temperatures of 1550°C were studied in flowing air using non-isothermal and isothermal thermogravimetric (TG) analysis. During the oxidation process two main thermal events were detected. The first stage of the oxidation reaction leads to the formation of elemental silicon as an intermediate reaction product. Upon further temperature increase the newly formed silicon is oxidized. Completely oxidized ZrSi₂ samples consist of ZrSiO₄, amorphous and crystalline SiO₂ as well as some residual ZrO₂. The experimental TG data were analysed with a model-fitting kinetic method. The gas-solid reaction is complex and can best be fitted with a multi-step reaction scheme consisting of branching reactions based on 3D diffusion mechanisms and a fractal order reaction.     

Rheological study on the cure kinetics of two-component addition cured silicone rubber

The cure kinetics for two-component silicone rubber formed by addition reaction was studied by the rheological method. The influence of reaction temperature (T) on the cure kinetics was explored in detail. It was observed that the data of gel time (t _gel, i.e. the time when the reaction reaches the gel point) or a specific reaction time (t _nc) (defined as the reaction time before which time the influence of confinement of network on the diffusion of reaction components can be neglected) versus T obey certain functional relationship, which was well explained by the cure kinetics model of thermoset network. The cure kinetics for the two-component silicone rubber can be well fitted by the Kamal-Sourour(autocatalyst) reaction model rather than Kissinger model. When the reaction time was before or equal to t _nc, the reaction order obtained by the Kamal-Sourour reaction model was 2, which was consistent with the reaction order inferred from the two components chemical reaction when the diffusion of reaction components was not influenced by the formed cross-linked polymer network. When the reaction time was larger than t _nc, such as to the end of reaction (t _e), the influence of confinement of network on the diffusion of reaction components cannot be neglected, and the reaction order obtained by the Kamal-Sourour reaction model was larger than 2. It was concluded that the confinement effect of network had a greater influence on the cure kinetics of the silicone rubber. The reaction rate constants (k _r) under different temperatures were also determined by Kamal-Sourour reaction model. The activation energy (E) for the two-component silicone rubber was also calculated from the results of lnt _gel, lnt _nc, and lnk _r versus 1/T, respectively. The three values of E were close, which indicated that above analyses were self-consistent.     

Computational study of the substitution effect on the mechanism for phospha-Wittig reaction

The phospha-Wittig reaction HP=PH₃ + O=CHX → HP=CHX + O=PH₃ (X = H, F, Cl, Me, OMe, NMe₂, CMe₃) was examined using the density functional theory calculations. All of the structures were completely optimized at the B3LYP/6-311++G** level of theory. The reactivities of various O=CHX were examined by estimating their activation energies. The main finding of this work is that the configuration mixing model can successfully predict the relative ordering of the activation energy and reaction enthalpies of the phospha-Wittig reaction. It was demonstrated that O=CHX with more electro-releasing substituents will possess a smaller singlet–triplet splitting. This will facilitate the phospha-Wittig reaction and will result in a larger exothermicity.     

Transition metal substituted polyoxometalates and their application in the direct hydroxylation of benzene to phenol with hydrogen peroxide

A series of transition metal substituted polyoxometalates with a Keggin structure were prepared and utilized for the hydroxylation of benzene to phenol. Among the compounds tested, [(CH₃)₄N]₄PMo₁₁VO₄₀ exhibits the highest phenol yield (13.0%) and selectivity (90.6%) in acetic acid/acetonitrile. Vanadium peroxo is the active site of the reaction, and ammonium also plays an important role. The influence of various reaction parameters, such as solvent, reaction time, reaction temperature, and amount of hydrogen peroxide used were investigated to obtain the optimal reaction conditions.     

Fluctuating Storage of the Active Phase in a Mn-Na2WO4/SiO2 Catalyst for the Oxidative Coupling of Methane

Structural dynamics of a Mn-Na₂WO₄/SiO₂ catalyst were detected directly under reaction conditions during the oxidative coupling of methane via in situ XRD and operando Raman spectroscopy. A new concept of fluctuating storage and release of an active phase in heterogeneous catalysis is proposed that involves the transient generation of active sodium oxide species via a reversible reaction of Na₂WO₄ with Mn₇SiO₁₂. The process is enabled by phase transitions and melting at the high reaction temperatures that are typically applied.     

Physico-Chemistry of the Limestone Sulphation Process

Aspects of the mechanism of the overall reaction between CaCO₃/CaO and SO₂/SO₃ under oxidizing conditions are discussed. The limestone and lime sulphation processes were carried out in a thermobalance under conditions relevant to atmospheric fluidized bed combustion. Sulphated samples, prepared in the form of cross-section particles, were examined in a scanning electron microscope by energy-dispersive X-ray and back-scattered electron imaging. Photomicrographs are presented. The reaction proceeded from the outer surface of the particles and along the pores. Surface textural changes during the reaction were considered. The layer of products was identified as controlling both the rate and extent of limestone/lime sulphation. In the products, two sulphur-bearing solids (CaSO₄ and CaS) were identified. The presence of CaS, which may cause difficulties in practice, is attributed to CaSO₃ disproportionation. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Effects of Intramolecular Hydrogen Bonding on the Reaction of Phenols with Epoxide in the Presence of Nano Calcium Carbonate

The reaction of phenols and dihydroxybenzenes with epoxide in the presence of nano CaCO₃ was studied. Catechol could react with epoxide and gave monochlorohydrin derivative; other dihydroxybenzenes and monomeric phenols had no reaction under the same conditions. The reaction of catechol with epoxide did not occur when nano CaCO₃ was replaced by a normal one. These were attributed to the strong interaction between nano CaCO₃ and the substrate as catechol possessed intrahydrogen bond and excess active hydrogen, which can induce the intramolecular proton transfer via the intramolecular hydrogen bond and promote the reaction of hydroxyl and epoxide. This is an example revealing the unique role of the hydrogen bond played in chemical reactions.     

A comparative study of the effects of decomposition rate control and mechanical grinding on the thermal decomposition of aluminum hydroxide

Summary Two different processes of the thermal decomposition of synthetic bayerite, i.e., the non-isothermal decomposition of mechanically ground sample in flowing N₂ and the controlled rate thermal decomposition of crystalline bayerite under vacuum, were investigated comparatively. In comparison with the conventional non-isothermal decomposition of crystalline bayerite in flowing N₂, the reaction temperature of the thermal decomposition was lowered by the individual effects of mechanical grinding of the sample and the reaction rate control. These decomposition processes indicated similar behavior characterized by the restricted changes of the specific surface area during the course of decomposition reaction and the formation of an amorphous alumina as the decomposition product. Different thermal behaviors were observed for those amorphous Al₂O₃ produced by the respective decomposition processes.     

Bulk hydrosilylation reaction of poly(dimethylsiloxane) chains catalyzed by a platinum salt: Effect of the initial concentration of reactive groups on the final extent of reaction

Model silicone networks obtained by curing linear poly(dimethylsiloxane) (PDMS) chains with end‐vinyl groups, (B₂), with a polyfunctional silane‐terminated crosslinker of functionality f, (A_f), through a hydrosilylation reaction have been widely used. In these networks, the principal characteristics of their ultimate molecular structure are strongly affected by the final extent of reaction reached during the crosslinking reaction. This work analyzes the effect of the initial concentration of the reactive end groups on the maximum attainable extent of reaction under normal bulk crosslinking conditions. This was accomplished by examining the reaction between linear B₂ PDMS chains with difunctional and trifunctional silanes. The experimental results were fitted by an exponential equation to have an empirical equation able to predict the maximum extent of reaction to be obtained as a function of the initial concentration of reactive groups. Molecular parameters relevant to this study, such as the degree of polymerization, the weight‐average molecular weight for the A₂ + B₂ system, or the weight fraction of solubles for the A₃ + B₂ system, were calculated with a mean field theory (recursive approach). © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1099–1106, 2003     

Kinetics of the alkaline hydrolysis of isoproturon in CTAB and NaLS micelles

Kinetics of the alkaline hydrolysis of isoproturon has been studied in the absence and presence of cetyltrimethylammonium bromide (CTAB) and sodium lauryl sulfate (NaLS) micelles. CTAB micelles were found to enhance the rate of reaction, while NaLS micelles inhibited the reaction rate. The reaction obeyed first‐order kinetics in [isoproturon] and was linearly dependent on [NaOH] at lower concentration. The rate of reaction became independent at higher [NaOH]. At lower [NaOH] the reaction proceeded via formation of hydroxide ion addition complex, while at higher [NaOH] the reaction occurred via deprotonation of ? NH? , leading to the formation of isocyanate. The values of k_w, k_m, and K_s were determined by considering the pseudophase ion exchange model. The activation parameters have also been reported. The effect of added salts (NaCl and KNO₃) on the reaction rate has also been studied. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 39–45, 2007     

Studies in the formation of poly(oxazolidone)s. III. Catalysis and kinetics of the model oxazolidone formation from cyclohexyl isocyanate and phenylglycidyl ether

The reaction of cyclohexyl isocyanate with phenylglycidyl ether was selected as model reaction for the synthesis of cycloaliphatic isocyanate-based poly(2-oxazolidone)s. The selectivity of AlCl₃ and AlCl₃-triphenylphosphine oxide (AlCl₃–TPPO) and AlCl₃-hexamethylphosphoric triamide (AlCl₃–HMPA) complexes was studied for 2-oxazolidone formation. The reaction products were identified by means of the melting point, ¹H-NMR, and IR spectroscopy. The kinetics of the model reaction was studied using AlCl₃-TPPO in o-dichlorobenzene at 120 and 140°C.     

An MNDO study of the reaction of methanol with fulminic acid and acetonitrile oxide

The reaction pathway of fulminic acid (HCNO) and acetonitrile oxide (CH₃CNO) with methanol as a nucleophile (RCNO + CH₃OH → RC(OCH₃)?NOH) and the formation of H-bonded complex with methanol have been studied using the MNDO method. MNDO-SCF calculations were performed with complete geometry optimization using the Davidon–Fletcher–Powell method. The reaction pathways were studied by varying all the bond lengths, the bond angles and the twist angles, using the distance C₃? O₂(R) between the carbon of the 1,3-dipoles and the oxygen of the methanol molecule as the reaction coordinate. The reaction is exothermic and proceeds in two steps. The first step is the formation of a five-centered hydrogen-bonded complex (INT ) and is the rate-determining step of the reaction. The second step involves the rearrangement of the H-bonded complex to the product, and this step requires a very small amount of activation energy. Thus, there is an intermediate on the reaction pathway, and therefore, the reaction is stepwise. Acetonitrile oxide is less reactive (activation energy 34.59 kcal/mol) relative to fulminic acid (activation energy 28.91 kcal/mol).     

Reaction Mechanism on the Synergistic Photodegradation of Dye X3B and Photoreduction of Dichromate in the Presence of Polyoxometalate

Photodegradation of a textile dye X3B and photoreduction of dichromate (Cr(VI)) in an acidic aqueous solution were studied under 320 nm cut-off UV light irradiation in the presence of two polyoxometalates (POM), H₃PW₁₂O₄₀ (PW), and H₄SiW₁₂O₄₀ (SiW). The reactions in POM-X3B-Cr(VI) system were faster than those in POM-X3B, POM-Cr(VI), and X3B-Cr(VI) systems. For all reactions, PW was more photoactive than SiW. The reaction rates were proportional to the initial concentration of each component. The effects of N₂, O₂, and air were small but regular, indicating Cr(VI) photoreduction by a reduced POM. Quenching experiments with H₂O₂ and ethanol revealed that X3B photodegradation mainly occurred through hydroxyl radical (OH). It was proposed that the production of OH and a reduced POM⁻ by the reaction between H₂O and excited POM^* was the rate determining step, with which all evidence could be well interpreted. Different effects of POM concentration in a two- or three-component system on the reaction rates suggested that the reaction between H₂O and excited POM^* was reversible.