Ester decomposition as a two-center synchronous reaction

Experimental data on the molecular decomposition of esters with various structures into an olefin and the corresponding acid in the gas phase are analyzed in terms of the intersecting parabolas method. Enthalpies and kinetic parameters characterizing this decomposition have been calculated for 33 reactions. Ester decomposition is a concerted two-center reaction characterized by a very high classical potential barrier of thermoneutral reaction (148–206 kJ/mol). The totality of reactions examined is divided into eight classes. Activation energies and rate constants have been calculated for 38 reactions using the kinetic parameters obtained. The activation energies and rate constants of the reverse bimolecular reaction of acid addition to olefins have been calculated by the intersecting parabolas method. Factors in the activation energy of ester decomposition and formation reactions are discussed.     

The breakdown of vinyl ethers as a two-center synchronous reaction

The experimental data on the molecular decomposition of vinyl ethers of various structures to alkanes and the corresponding aldehydes or ketones in the gas phase were analyzed using the method of intersecting parabolas. The enthalpies and kinetic parameters of decomposition were calculated for 17 reactions. The breakdown of ethers is a two-center concerted reaction characterized by a very high classical potential barrier to the thermally neutral reaction (180–190 kJ/mol). The kinetic parameters (activation energies and rate constants) of back reactions of the formation of vinyl ethers in the addition of aldehydes or ketones to alkanes were calculated using the method of intersecting parabolas. The factors that influenced the activation energy of the decomposition and formation of ethers were discussed. Quantum-chemical calculations of several vinyl ether decomposition reactions were performed. Ether formation reactions were compared with the formation of unsaturated alcohols as competitive reactions, which can occur in the interaction of carbonyl compounds with alkenes.     

F-T合成校正综合动力学模型

考虑了烯烃、醇与酸的再吸附及其非本征效应(烯烃、醇与酸在催化剂孔道中的扩散作用、物理吸附及溶解度效应)对产物分布的影响,推导了基于详细反应机理的亚甲基插入的烷基机理F-T合成校正综合动力学模型.利用文献数据对动力学模型进行了回归,获得了与文献报道结果相一致的动力学参数.由校正动力学模型计算的烷烃、烯烃、醇与酸产物分布及烯烃比、醇烃比及酸烃比与实验数据较好地吻合.动力学计算结果表明,在铁锰催化剂上,烷烃、烯烃、醇与酸生成的反应是平行竞争反应,烯烃、醇与酸在催化剂表面的再吸附及二次反应导致产物分布偏离了ASF分布.动力学研究还表明,相同碳数的醇与酸产物在催化剂表面上再吸附及二次反应的机会比相同碳数的烯烃大.通过比较相同碳数的烯烃、醇与酸的分子体积及沸点,指出了在铁锰催化剂上,低碳数的烯烃、醇与酸的再吸附及二次反应对产物分布影响的非本征效应中,烯烃、醇与酸的扩散阻力不是主导效应.     

Decomposition reactions of azo compounds,as studied by quantum chemistry methods and the parabolic model

Decomposition reactions of azoalkanes of different structure were studied by quantum chemistry methods (MP2/6-311++G** calculations) and by the method of three intersecting parabolas (M3IP). The MP2 method was used to obtain the transition-state geometries, the bond lengths in the molecules under study, and the activation energies. Possible mechanisms of decomposition are discussed. Concerted decomposition of branched azoalkanes was shown to be the most probable mechanism of the process. The M3IP method was used to calculate the kinetic and thermodynamic parameters of concerted decomposition of azoalkanes and to determine and evaluate the main factors affecting the activation energy (E). The stabilization energy of the radical being formed in the decomposition reaction is one of the key factors determining the concerted mechanism. The kinetic parameters calculated by the two independent methods are in good agreement.

     

Olefin cross-metathesis of a vinyl-terminated self-assembled monolayer (SAM) on Au(111): electrochemical study using a ferrocenyl redox center

Self-assembled thiol monolayers bound to single-crystal Au(111) surfaces containing a terminal olefin have been prepared and used to monitor electrochemically the cross-metathesis (CM) between the surface and an olefin-terminated ferrocenyl (Fc) derivative from solution over time. Mixed SAM surfaces were prepared by first adsorbing a diluent for 2 days followed by the olefinic alkanethiol for known adsorption time intervals; three diluents of varying length were used. The oxidation peak areas from the voltammetry show the CM reaction yields a maximum amount of product at 100-150 min. Beyond this time, thiol desorption is apparent and the Fc oxidation peaks diminished. A kinetic simulation of the interfacial reactions involving CM and desorption reactions are described and aided in the interpretation of the voltammetric responses. The length of the diluent and the coverage of surface olefins were important factors in limiting undesirable self-CM reactions on the surface, and a model of the relationship between the diluent and surface concentration of olefin is described. This study shows that attention to monolayer formation and reaction conditions are important parameters when maximizing CM yields on surfaces.     

Reactivity of fluoroalkanes in reactions of coordinated molecular decomposition

Experimental results on the coordinated molecular decomposition of RF fluoroalkanes to olefin and HF are analyzed using the model of intersecting parabolas (IPM). The kinetic parameters are calculated to allow estimates of the activation energy (E) and rate constant (k) of these reactions, based on enthalpy and IPM algorithms. Parameters E and k are found for the first time for eight RF decomposition reactions. The factors that affect activation energy E of RF decomposition (the enthalpy of the reaction, the electronegativity of the atoms of reaction centers, and the dipole–dipole interaction of polar groups) are determined. The values of E and k for reverse reactions of addition are estimated.     

An analysis of the molecular concerted decomposition of nitroalkanes by the method of intersecting parabolas

The molecular concerted decomposition of nitroalkanes was analyzed using the model of two intersecting parabolas. Parameters describing the activation energy of decomposition as a function of the enthalpy of the reaction were obtained. These parameters were used to calculate the activation energies of unstudied reactions of the decomposition of RNO₂ and the activation energies and rate constants for reverse combination reactions between olefins and nitrous acid with the formation of the corresponding nitroalkanes. The concerted decomposition of nitroalkanes was also analyzed using another two-center model, in which decomposition was treated as two single-stage events, the transfer of H from the C-H bond to the O atom of the nitro group and the dissociation of the C-N bond accompanied by a shift of electrons in a five-membered reaction center. This approach and two elementary event models (a model of two intersecting parabolas and a model of the superposition of three parabolas) were used to calculate the spectrum of activation energies, in which each overall reaction event was characterized by an activation barrier of its own.     

625 nm低能量光子作用下WO3-x光催化烯烃异构化

烯烃异构化广泛用于合成药物、高强度材料和精细化学品.近年来,光催化烯烃异构化的发展解决了传统烯烃异构化的设备腐蚀、活性和选择性较差等问题.如Pd@TiO2和Pd配合物均可用于烯烃的光催化异构化,但仅对烯丙基苄基衍生物有效.K(o)nig等利用可见光、Co(acac)2和合适的配体实现了烯烃位置可控的异构化,但该反应仅限于末端烯烃.因此,需要开发一种简单高效的光催化烯烃异构化方法,一方面可以通过光催化烯烃异构化将末端烯烃或内烯烃转化为一个或多个位置异构化产物,将石油衍生物中存在的烯烃混合物转化为有价值的单一烯烃产品,另一方面能够大幅提高光催化烯烃异构化的效率.本文研究发现,在625 nm光照射下,氧缺陷WO3-x表现出较好的催化1-癸烯异构化活性.该催化剂不仅可以将各种链状和环状末端烯烃转化为相应的内烯烃异构体,而且能够将内烯烃混合物转化为单一末端烯烃产品.通过控制烯烃碳链的长度可以得到热力学和动力学异构化产物,当烯烃的碳数小于13时,主要得到动力学产物.在无光照时没有检测到产物,说明只有在光照下反应才能进行.WO3-x的氧缺陷使WO3-x的漫反射紫外可见(DR UV-vis)光谱在大于450 nm时出现了局域表面等离子体共振(LSPR)强吸收.不同波长光照下1-癸烯异构化反应转化率的变化趋势与WO3-x的DR UV-vis光谱一致,而且在625 nm红光照射下,1-癸烯的转化率最高达到99.4％,进一步说明反应是由光驱动的.625nm的光可以将WO3-x价带上的电子激发到缺陷能级,且该电子可以进一步被转移到WO3-x吸附的烯烃上(激发电子转移路径).为进一步研究氧缺陷对光催化烯烃异构化活性的影响,在300℃焙烧不同时间得到氧缺陷含量不等的WO3-x样品.X射线衍射谱、DR UV-vis光谱、X射线光电子能谱和电子顺磁共振光谱结果表明,随着焙烧时间的增加,催化剂中氧缺陷含量逐渐减少,其光催化1-癸烯异构化活性逐渐降低,证明催化剂中氧缺陷的存在可以提高其光催化性能.利用原位漫反射傅里叶变换红外光谱研究了氧缺陷对烯烃表面吸附和中间体形成的影响.结果 表明,WO3-x表面氧缺陷产生的不饱和W5+位点会与烯烃配位从而原位形成表面π配合物和π-烯丙基钨中间体.通过在反应体系中添加4-叔丁基邻苯二酚证明了反应是通过自由基机理进行的.综上所述,WO3-x与625 nm红光结合,实现了烯烃的光催化异构化.通过调节烯烃的碳链长度,可得到热力学和动力学产物,且该催化体系可以将石油衍生物中的内烯烃混合物转化为单一末端烯烃产品.WO3-x的氧缺陷既可以提高其光捕获能力,又可以用作光催化烯烃异构化的吸附和活化位点.本文提供了一种利用低能量光子进行烯烃高效选择性异构化的简便方法,该方法是传统烯烃异构化方法的补充.     

Reactivity of alkyl iodides in molecular decomposition reactions

The experimental results for the concerted molecular decomposition of alkyl iodides RI to olefin and HI were analyzed in terms of the intersecting parabolas model (IPM). The activation energies (E) and rate constants (k) of the earlier unstudied reactions of the concerted molecular decomposition of RI were calculated on the basis of the enthalpy of the reaction and IPM algorithms. The factors that influence on E of RI decay were established: the enthalpy of the reaction, the energy of stabilization of radical R*, the length and force constant of the C—I bond, and the size of the halogen atom. The values of E and k for the backward reactions of HI addition to olefins were estimated.     

Mechanism of homogeneous Ir(III) catalyzed regioselective arylation of olefins

The mechanism of hydroarylation of olefins by a homogeneous Ph-Ir(acac)(2)(L) catalyst is elucidated by first principles quantum mechanical methods (DFT), with particular emphasis on activation of the catalyst, catalytic cycle, and interpretation of experimental observations. On the basis of this mechanism, we suggest new catalysts expected to have improved activity. Initiation of the catalyst from the inert trans-form into the active cis-form occurs through a dissociative pathway with a calculated DeltaH(0 K)() = 35.1 kcal/mol and DeltaG(298 K)() = 26.1 kcal/mol. The catalytic cycle features two key steps, 1,2-olefin insertion and C-H activation via a novel mechanism, oxidative hydrogen migration. The olefin insertion is found to be rate determining, with a calculated DeltaH(0 K)() = 27.0 kcal/mol and DeltaG(298 K)() = 29.3 kcal/mol. The activation energy increases with increased electron density on the coordinating olefin, as well as increased electron-donating character in the ligand system. The regioselectivity is shown to depend on the electronic and steric characteristics of the olefin, with steric bulk and electron withdrawing character favoring linear product formation. Activation of the C-H bond occurs in a concerted fashion through a novel transition structure best described as an oxidative hydrogen migration. The character of the transition structure is seven coordinate Ir(V), with a full bond formed between the migrating hydrogen and iridium. Several experimental observations are investigated and explained: (a) The nature of L influences the rate of the reaction through a ground-state effect. (b) The lack of beta-hydride products is due to kinetic factors, although beta-hydride elimination is calculated to be facile, all further reactions are kinetically inaccessible. (c) Inhibition by excess olefin is caused by competitive binding of olefin and aryl starting materials during the catalytic cycle in a statistical fashion. On the basis of this insertion-oxidative hydrogen transfer mechanism we suggest that electron-withdrawing substituents on the acac ligands, such as trifluoromethyl groups, are good modifications for catalysts with higher activity.     

Decomposition processes of nickel hydroxide

The dehydration processes of nickel hydroxide were studied by means of thermogravimetry in a temperature range from 300 to 900 K. The kinetics of the low-temperature dehydroxylation (≈300–600 K) was studied under non-isothermal conditions. A model-free method was used to calculate the activation energy and to analyze the stepwise checking; the non-linear regression method was applied to calculate the kinetic parameters of multi-stage decomposition reactions. The features of the dehydroxylation kinetics for the multi-stage process are explained by the formation and decomposition of hydrogel and xerogel phases.     

A thermal decomposition study on cobalt(II) complexes of 1,2-di(imino-4′-antipyrinyl)ethane

The phenomenological, kinetic and mechanistic aspects of thermal decomposition of perchlorate, nitrate, chloride, bromide and iodide complexes of cobalt(II) with the Schiff base 1,2-di(imino-4′-antipyrinyl)ethane (GA) have been studied by TG and DTG analyses. The kinetic parameters like the activation energy, pre-exponential factor and entropy of activation were calculated. The decomposition reactions follow “random nucleation with one nucleus on each particle—Mampel model”.     

Palladium(0) complexes coordinated with substituted olefins and tertiary phosphine ligands

New palladium(0) complexes with a variety of coordinated olefins [Pd(olefin)(PMePh₂)₂] (II) (olefin = styrene, ethyl methacrylate, methyl methacrylate, methyl acrylate, methacrylonitrile, and dimethyl maleate), were prepared by the reactions of [PdEt₂(PMePh₂)₂] (I) with corresponding olefins in toluene. These complexes were characterized by means of elemental analysis, IR and ¹H NMR spectroscopy and the chemical reactions. The dissociation of the coordinated olefin from complex II in solution was confirmed by spectroscopic studies of [Pd(mma)(PMePh₂)₂] (mma = methyl methacrylate). From the variable temperature NMR study, kinetic parameters for the dissociation process were determined as E_a = 7 kcal/mol, and ΔS³ (293 K) = -30 cal/deg · mol. Some new hydrido complexes, [Pd(H)ClL₂] (IV) (L = PMePh₂, PEtPh₂ and PEt₂Ph), were prepared by the reactions of [Pd(olefin)L₂] with dry HCl.     

Dissolution properties of ammonium dinitramide in N-methyl pyrrolidone

The decomposition process of gadolinium hydroxide was studied by means of thermogravimetry in a temperature range from 300 to 900 K. The kinetics of low-temperature dehydroxylation (≈430–600 K) was studied under non-isothermal conditions. A model-free method was used to calculate the activation energy; a nonlinear regression method was applied to calculate the kinetic parameters of multi-stage decomposition reactions. The features of the dehydroxylation kinetics of the multi-stage process can be explained by the formation of GdOOH phase.     

A thermal decomposition study on cobalt(II) complexes of 1,2-bis(IMINO-4'-antipyrinyl)ethane

The phenomenological, kinetic and mechanistic aspects of thermal decomposition of perchlorate, nitrate, chloride, bromide and iodide complexes of cobalt(II) with the Schiff base 1,2-bis(imino-4'-antipyrinyl)ethane (GA) have been studied by TG and DTG analyses. The kinetic parameters like the activation energy, pre-exponential factor and entropy of activation were calculated. The decomposition reactions follow 'random nucleation with one nucleus on each particle - Mampel model'. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the mechanism of the initiation reaction in Grubbs-Hoveyda complexes

Grubbs-Hoveyda-type complexes with variable 4-R (complexes 1: 4-R = NEt(2), OiPr, H, F, NO(2)) and 5-R substituents (complexes 2: 5-R = NEt(2), OiPr, Me, F, NO(2)) at the 2-isopropoxy benzylidene ether ligand and with variable 4-R substituents (complexes 3: 4-R = H, NO(2)) at the 2-methoxy benzylidene ether ligand were synthesized and the respective Ru(II/III) redox potentials (ranging from ΔE = +0.46 to +1.04 V), and UV-vis spectra recorded. The initiation kinetics of complexes 1-3 with the olefins diethyl diallyl malonate (DEDAM), butyl vinyl ether (BuVE), 1-hexene, styrene, and 3,3-dimethylbut-1-ene were investigated using UV-vis spectroscopy. Electron-withdrawing groups at the benzylidene ether ligands were found to increase the initiation rates, while electron-donating groups lead to slower precatalyst activation; accordingly with DEDAM, the complex 1(NO(2)) initiates almost 100 times faster than 1(NEt(2)). The 4-R substituents (para to the benzylidene carbon) were found to have a stronger influence on physical and kinetic properties of complexes 1 and 2 than that of 5-R groups para to the ether oxygen. The DEDAM-induced initiation reactions of complexes 1 and 2 are classified as two-step reactions with an element of reversibility. The hyperbolic fit of the k(obs) vs [DEDAM] plots is interpreted according to a dissociative mechanism (D). Kinetic studies employing BuVE showed that the initiation reactions simultaneously follow two different mechanistic pathways, since the k(obs) vs [olefin] plots are best fitted to k(obs) = k(D)·k(4)/k(-D)·[olefin]/(1 + k(4)/k(-D)·[olefin]) + k(I)·[olefin]. The k(I)·[olefin] term dominates the initiation behavior of the sterically less demanding complexes 3 and was shown to correspond to an interchange mechanism with associative mode of activation (I(a)), leading to very fast precatalyst activation at high olefin concentrations. Equilibrium and rate constants for the reactions of complexes 1-3 with the bulky PCy(3) were determined. In general, sterically demanding olefins (DEDAM, styrene) and Grubbs-Hoveyda type complexes 1 and 2 preferentially initiate according to the dissociative pathway; for the less bulky olefins (BuVE, 1-hexene) and complexes 1 and 2 both D and I(a) are important. Activation parameters for BuVE reactions and complexes 1(NEt(2)), 1(H), and 1(NO(2)) were determined, and ΔS(?) was found to be negative (ΔS(?) = -113 to -167 J·K(-1)·mol(-1)) providing additional support for the I(a) catalyst activation.     

A general model for selectivity in olefin cross metathesis

In recent years, olefin cross metathesis (CM) has emerged as a powerful and convenient synthetic technique in organic chemistry; however, as a general synthetic method, CM has been limited by the lack of predictability in product selectivity and stereoselectivity. Investigations into olefin cross metathesis with several classes of olefins, including substituted and functionalized styrenes, secondary allylic alcohols, tertiary allylic alcohols, and olefins with alpha-quaternary centers, have led to a general model useful for the prediction of product selectivity and stereoselectivity in cross metathesis. As a general ranking of olefin reactivity in CM, olefins can be categorized by their relative abilities to undergo homodimerization via cross metathesis and the susceptibility of their homodimers toward secondary metathesis reactions. When an olefin of high reactivity is reacted with an olefin of lower reactivity (sterically bulky, electron-deficient, etc.), selective cross metathesis can be achieved using feedstock stoichiometries as low as 1:1. By employing a metathesis catalyst with the appropriate activity, selective cross metathesis reactions can be achieved with a wide variety of electron-rich, electron-deficient, and sterically bulky olefins. Application of this model has allowed for the prediction and development of selective cross metathesis reactions, culminating in unprecedented three-component intermolecular cross metathesis reactions.     

Kinetic parameters of the thermal decomposition processes of certain cobalt and titanium coprecipitates

Journal of Thermal Analysis and Calorimetry - The kinetic parameters of the thermal decomposition reactions of certain coprecipitates were calculated by a thermogravimetric method. The significance...     

Kinetics of thermal decomposition of aseptic packages

Kinetics of thermal decomposition of aseptic packages (e.g. Tetrapak cartons) and pyrolysis of this waste in a laboratory flow reactor was studied. Three different models for the calculation of the reaction rate and the determination of apparent kinetic parameters of thermal decomposition were used. The first method assumes a two stage thermal decomposition and the kinetic parameters were determined by fitting a derivative thermogravimetric (DTG) curve to experimentally determined thermogravimetric data of whole aseptic cartons. The second method uses kinetic parameters determined by fitting DTG curves to thermogravimetric data of individual components of aseptic packages. The last method was a multi-curve isoconversion method assuming a change of kinetic parameters with the increasing conversion. All types of the determined kinetic parameters were used in a mathematical model for thermal decomposition of mini briquettes made from aseptic packages at the temperature of 650°C. The model calculated also the heat conduction in the particles and it was verified by an independent set of experiments conducted in a laboratory screw type flow reactor.     

Toluene and benzyl decomposition mechanisms: elementary reactions and kinetic simulations

The high temperature decomposition kinetics of toluene and benzyl were investigated by combining a kinetic analysis with the ab initio/master equation study of new reaction channels. It was found that similarly to toluene, which decomposes to benzyl and phenyl losing atomic hydrogen and methyl, also benzyl decomposition proceeds through two channels with similar products. The first leads to the formation of fulvenallene and hydrogen and has already been investigated in detail in recent publications. In this work it is proposed that benzyl can decompose also through a second decomposition channel to form benzyne and methyl. The channel specific kinetic constants of benzyl decomposition were determined by integrating the RRKM/master equation over the C(7)H(7) potential energy surface. The energies of wells and saddle points were determined at the CCSD(T) level on B3LYP/6-31+G(d,p) structures. A kinetic mechanism was then formulated, which comprises the benzyl and toluene decomposition reactions together with a recently proposed fulvenallene decomposition mechanism, the decomposition kinetics of the fulvenallenyl radical, and some reactions describing the secondary chemistry originated by the decomposition products. The kinetic mechanism so obtained was used to simulate the production of H atoms measured in a wide pressure and temperature range using different experimental setups. The calculated and experimental data are in good agreement. Kinetic constants of the new reaction channels here examined are reported as a function of temperature at different pressures. The mechanism here proposed is not compatible with the assumption often used in literature kinetic mechanisms that benzyl decomposition can be effectively described through a lumped reaction whose products are the cyclopentadienyl radical and acetylene.