Reaction of allylamine with hexylsilane

The reaction of hexylsilane with allylamine is accompanied by the liberation of hydrogen and formation of allylaminosilanes and compounds with the Si-Si bond. The hydrosilylation pathway virtually is not realized. The B3LYP/6-311G** calculations show that all the considered reactions are thermodynamically allowed.     

Mimicking the silicon surface: reactivity of silyl radical cations toward nucleophiles

Radical cations of selected low molecular-weight silicon model compounds were obtained by photoinduced electron transfer. These radical cations react readily with a variety of nucleophiles, regularly used in monolayer fabrication onto hydrogen-terminated silicon. From time-resolved kinetics, it was concluded that the reactions proceed via a bimolecular nucleophilic attack to the radical cation. A secondary kinetic isotope effect indicated that the central Si-H bond is not cleaved in the rate-determining step. Apart from substitution products, also hydrosilylation products were identified in the product mixtures. Observation of the substitution products, combined with the kinetic data, point to an bimolecular reaction mechanism involving Si-Si bond cleavage. The products of this nucleophilic substitution can initiate radical chain reactions leading to hydrosilylation products, which can independently also be initiated by dissociation of the radical cations. Application of these data to the attachment of organic monolayers onto hydrogen-terminated Si surfaces via hydrosilylation leads to the conclusion that the delocalized Si radical cation (a surface-localized hole) can initiate the hydrosilylation chain reaction at the Si surface. Comparison to monolayer experiments shows that this reaction only plays a significant role in the initiation, and not in the propagation steps of Si-C bond making monolayer formation.     

Intramolecular hydrogen bond in 3‐imino‐propenylamine isomers: AIM and NBO studies

The molecular structure and intramolecular hydrogen bond energy of 18 conformers of 3‐imino‐propenyl‐amine were investigated at MP2 and B3LYP levels of theory using the standard 6‐311++G** basis set. The atom in molecules or AIM theory of Bader, which is based on the topological properties of the electron density (ρ), was used additionally and the natural bond orbital (NBO) analysis was also carried out. Furthermore calculations for all possible conformations of 3‐imino‐propenyl‐amin in water solution were also carried out at B3LYP/6‐311++G** and MP2/6‐311++G** levels of theory. The calculated geometrical parameters and conformational analyses in gas phase and water solution show that the imine–amine conformers of this compound are more stable than the other conformers. B3LYP method predicts the IMA‐1 as global minimum. This stability is mainly due to the formation of a strong N? H···N intramolecular hydrogen bond, which is assisted by π‐electrons resonance, and this π‐electrons are established by NH2 functional group. Hydrogen bond energies for all conformers of 3‐imino‐propenyl‐amine were obtained from the related rotamers methods. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Decomposition of hexamethyldisilane on a hot tungsten filament and gas-phase reactions in a hot-wire chemical vapor deposition reactor

To study the effect of an Si-Si bond on gas-phase reaction chemistry in the hot-wire chemical vapor deposition (HWCVD) process with a single source alkylsilane molecule, soft ionization with a vacuum ultraviolet wavelength of 118 nm was used with time-of-flight mass spectrometry to examine the products from the primary decomposition of hexamethyldisilane (HMDS) on a heated tungsten (W) filament and from secondary gas-phase reactions in a HWCVD reactor. It is found that both Si-Si and Si-C bonds break when HMDS decomposes on the W filament. The dominance of the breakage of Si-Si over Si-C bond has been demonstrated. In the reactor, the abstraction of methyl and H atom, respectively, from the abundant HMDS molecules by the dominant primary trimethylsilyl radicals produces tetramethylsilane (TMS) and trimethylsilane (TriMS). Along with TMS and TriMS, various other alkyl-substituted silanes (m/z = 160, 204, 262) and silyl-substituted alkanes (m/z = 218, 276, 290) are also formed from radical combination reactions. With HMDS, an increasing number of Si-Si bonds are found in the gas-phase reaction products aside from the Si-C bond which has been shown to be the major bond connection in the products when TMS is used in the same reactor. Three methyl-substituted 1,3-disilacyclobutane species (m/z = 116, 130, 144) are present in the reactor with HMDS, suggesting a more active involvement from the reactive silene intermediates.     

Reactivity of alkyl versus silyl peroxides. The consequences of 1, 2-silicon bridging on the epoxidation of alkenes with silyl hydroperoxides and bis(trialkylsilyl)peroxides

The bond dissociation energies for a series of silyl peroxides have been calculated at the G2 and CBS-Q levels of theory. A comparison is made with the O-O BDE of the corresponding dialkyl peroxides, and the effect of the O-O bond strength on the activation barrier for oxygen atom transfer is discussed. The O-O bond dissociation enthalpies (DeltaH(298)) for bis (trimethylsilyl) peroxide (1) and trimethylsilyl hydroperoxide (2) are 54.8 and 53.1 kcal/mol, respectively at the G2 (MP2) and CBS-Q levels of theory. The O-O bond dissociation energies computed at G2 and G2(MP2) levels for bis(tert-butyl) peroxide and tert-butyl hydroperoxide are 45.2 and 48.3 kcal/mol, respectively. The barrier height for 1,2-methyl migration from silicon to oxygen in trimethylsilyl hydroperoxide is 47.9 kcal/mol (MP4//MP2/6-31G). The activation energy for the oxidation of trimethylamine to its N-oxide by bis(trimethylsilyl) peroxide is 28.2 kcal/mol (B3LYP/6-311+G(3df,2p)// B3LYP/6-31G(d)). 1,2-Silicon bridging in the transition state for oxygen atom transfer to a nucleophilic amine results in a significant reduction in the barrier height. The barrier for the epoxidation of E-2-butene with bis(dimethyl(trifluoromethyl))silyl peroxide is 25.8 kcal/mol; a reduction of 7.5 kcal/mol relative to epoxidation with 1. The activation energy calculated for the epoxidation of E-2-butene with F(3)SiOOSiF(3) is reduced to only 2.2 kcal/mol reflecting the inductive effect of the electronegative fluorine atoms.     

A theoretical study of reactivity and regioselectivity in the hydroxylation of adamantane by ferrate(VI)

The conversion of adamantane to adamantanols mediated by ferrate (FeO(4)(2)(-)), monoprotonated ferrate (HFeO(4)(-)), and diprotonated ferrate (H(2)FeO(4)) is discussed with the hybrid B3LYP density functional theory (DFT) method. Diprotonated ferrate is the best mediator for the activation of the C-H bonds of adamantane via two reaction pathways, in which 1-adamantanol is formed by the abstraction of a tertiary hydrogen atom (3 degrees ) and 2-adamantanol by the abstraction of a secondary hydrogen atom (2 degrees ). Each reaction pathway is initiated by a C-H bond cleavage via an H-atom abstraction that leads to a radical intermediate, followed by a C-O bond formation via an oxygen rebound step to lead to an adamantanol complex. The activation energies for the C-H cleavage step are 6.9 kcal/mol in the 1-adamantanol pathway and 8.4 kcal/mol in the 2-adamantanol pathway, respectively, at the B3LYP/6-311++G level of theory, whereas those of the second reaction step corresponding to the rebound step are relatively small. Thus, the rate-determining step in the two pathways is the C-H bond dissociation step, which is relevant to the regioselectivity for adamantane hydroxylation. The relative rate constant (3 degrees )/(2 degrees ) for the competing H-atom abstraction reactions is calculated to be 9.30 at 75 degrees C, which is fully consistent with an experimental value of 10.1.     

Heat of hydrogenation of 1,5-dehydroquadricyclane. A computational and experimental study of a highly pyramidalized alkene

The radical anion of the highly pyramidalized alkene 1,5-dehydroquadricyclane (1) was generated in the gas phase from the Squires reaction of 1,5-bis(trimethylsilyl)quadricyclane with F-/F2. The electron binding energy and proton affinity of 1*- were determined by bracketing experiments to be 0.6 +/- 0.1 eV and 386 +/- 5 kcal/mol, respectively. These values are in good agreement with values predicted by density functional theory (B3LYP/6-31+G*) and ab initio (CASPT2/6-31+G*) calculations. The experimental heat of hydrogenation of 1, obtained from a thermochemical cycle, was found to be 91 +/- 9 kcal/mol. This value of deltaH(H2) leads to values of 67 +/- 9 kcal/mol for the olefin strain energy (OSE) of 1, 172 +/- 9 kcal/mol for its heat of formation, and 23 +/- 9 kcal/mol for its pi bond dissociation enthalpy. Since the retro-Diels-Alder reaction of neutral 1 is computed to be highly exothermic, the finding that 1*- apparently does not undergo a retro-Diels-Alder reaction is of particular interest. The B3LYP/6-31+G* optimized geometry of 1 suggests that the bonding in this alkene is partially delocalized, presumably because the highly pyramidalized double bond in 1 interacts with the distal cyclopropane bonds in a manner that eventually leads to a retro-Diels-Alder reaction. The good agreement of the B3LYP and (2/2)CASPT2 values for the heat of hydrogenation and OSE of 1 with the experimentally derived values provides indirect evidence for the correctness of the B3LYP prediction that the equilibrium geometry of 1 lies part way along the reaction coordinate to the transition structure for the retro-Diels-Alder reaction.     

Mechanisms of the α,α-diphenylprolinol trimethylsilyl ether-catalyzed enantioselective aza-Michael reaction

Depending on the nature of the aza-Michael donor, the C-N bond formation in the α,α-diphenylprolinol trimethylsilyl ether-catalyzed aza-Michael reactions was found to proceed via either (i) an iminol intermediate in a stepwise reaction, or (ii) a concerted reaction. This is contrary to the commonly proposed iminium mechanism for organocatalyst-catalyzed aza-Michael reactions. The iminol intermediate is formed from the reaction between the catalyst and the amine (Michael donor). These proposed mechanisms are able to account for the experimentally observed product enantioselectivity.     

The mechanism of 1,2-addition of disilene and silene: hydrogen halide addition

The mechanism of 1,2-addition reactions of HF and HCl to Si=Si, Si=C, and C=C bonds has been investigated by ab initio quantum chemical methods. Geometries and relative energies of the stationary points and all the transition states were determined by using the MP2/6-311++G(d,p), B3LYP/6-311++G(d,p), and CBS-Q levels of theory. The investigated reactions can be characterized by two main thermodynamic profiles. The type in which the reagent molecule attacks a carbon atom is moderately exothermic with a high activation barrier. The second type in which a hydrogen halide attacks a silicon is strongly exothermic with a low activation energy. At the early stage of all the reactions a weakly bonded initial complex is found which indicates that the initial step of all the reactions is an electrophilic attack of hydrogen halide. The geometry and charge distribution of the transition state of the reactions indicate two main types of mechanism. If silicon is attacked, the halogen-silicon bond formation precedes the H-Y bond breaking. If, however, carbon is attacked, the first step is always an ionic dissociation of the hydrogen halide and a carbenium ion formation, which is stabilized by the C-Y bond formation in the final step of the reaction. The reaction diagrams and proposed mechanisms explain the experimentally found regioselectivity well.     

Effect of trimethylsilyl substitution on the structure and properties of phthalocyanine: A density functional theory study

The geometries of four isomers of the trimethylsilyl substituted phthalocyanine (Pc)— I , II , III , and IV —have been optimized at the B3LYP/3‐21G level of density functional theory. Normal‐mode vibrational analyses have been performed and their standard thermodynamic functions, molar fractions, and electronic absorption spectra calculated. Single‐point energies have been calculated at the B3LYP/6‐311G* level for all isomers to evaluate the heats of formation from an isodesmic reaction. It is found that substitution has little influence on the geometry and electronic structures of the Pc framework. The corresponding geometric parameters in various isomers are close. According to the B3LYP/6‐311G*//B3LYP/3‐21G results, substitution at the peripheral position of the isoindole with an inner hydrogen is most favorable. The energies increase in the order of IV < II < III < I , and the energy difference between IV and I is 5.75 kJ/mol. The molar fractions of IV , II , III , and I are 0.80, 0.17, 0.02, and 0.02 and the heats of formation are 2009.96, 2010.10, 2015.85, and 2016.52 kJ/mol, respectively. This indicates that nonperipheral substituted Pcs have higher energy and little production because they are not stable under the considered conditions. The electronic spectra of the substituted Pcs calculated using the ZINDO method have two strong Q absorption bands around 700 nm and one B band around 300 nm that are slightly shifted compared with those in Pc. The ratios of the oscillator strength of the B band to the Q bands are much lowered by trimethylsilyl substitution. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Toward selective functionalisation of oligosilanes: borane-catalysed dehydrogenative coupling of silanes with thiols

Among established methods for transforming Si-H bonds, carbonyl hydrosilylation and heterodehydrogenative coupling with alcohols catalysed by B(C6F5)3 are shown to provide exceptionally clean routes to the derivatisation of tetra-substituted disilanes such as [Ph2SiH]2, giving no products resulting from Si-Si bond cleavage. Even higher activity is observed for the borane-catalysed dehydrogenative coupling of silanes with alkyl- and arylthiols, the first examples of such Si-S bond formation in the absence of a transition metal catalyst. Clean, quantitative syntheses of a range of thiosilanes are reported, and the lability of the Si-S linkage toward subsequent alcoholysis is investigated. The crystal structure of 2,3-disila-2,2,3,3-tetramethyl-1,4-benzodioxane is presented.     

Agreement between experiment and hybrid DFT calculations for O--H bond dissociation enthalpies in manganese complexes

Information on the accuracy of DFT functionals for redox reactions in transition metal systems is rather limited. To analyze the performance of some popular functionals for redox reactions in manganese systems, calculated O--H bond dissociation enthalpies for Mn-ligands in six different complexes are compared to experimental results. In this benchmark, B3LYP performs well with a mean absolute error of 3.0 kcal/mol. B98 gives similar results to B3LYP (error of 3.8 kcal/mol). B3LYP* gives lower O--H bond strengths than B3LYP and has a mean error of 5.0 kcal/mol. Compared to B98 and B3LYP, B3LYP* has an error trend for the manganese ligands that is more similar to the error for a free water molecule. The nonhybrid functional BLYP consistently and significantly underestimates the O--H bond strengths by approximately 20 kcal/mol. HCTH407 has a rather large mean error of 9.4 kcal/mol and shows no consistent trend. The results support the use of hybrid functionals and the present computational method for large model systems containing manganese. An example is the oxygen evolving complex in photosystem II where hybrid functionals predict the appearance of a Mn(IV)-oxyl radical before the O--O bond formation step.     

Dispersion energy enforced dimerization of a cyclic disilylated plumbylene

By reaction of 1,4-dipotassio-1,1,4,4-tetrakis(trimethylsilyl)tetramethyltetrasilane with PbBr(2) in the presence of triethylphosphine a base adduct of a cyclic disilylated plumbylene could be obtained. Phosphine abstraction with B(C(6)F(5))(3) led to formation of a base-free plumbylene dimer, which features an unexpected single donor-acceptor PbPb bond. The results of density functional computations at the M06-2X and B3LYP level of theory indicate that the dominating interactions which hold the plumbylene subunits together and which define its actual molecular structure are attracting van der Waals forces between the two large and polarizable plumbylene subunits.     

聚苯乙烯热降解机理的理论研究

采用密度泛函理论B3LYP/6-311G（d）方法,对聚苯乙烯（PS）热降解反应机理进行了研究。PS热降解的主要产物是苯乙烯,其次是甲苯、α-甲基苯乙烯、乙苯和二聚体等芳烃化合物。PS热降解反应主要包括主链C-C键均裂、β-断裂、氢转移和自由基终止等反应。针对以上各类反应进行了路径设计和理论计算分析,对参与反应的分子的几何结构进行了优化和频率计算,获得了各热降解路径的标准动力学和热力学参数。计算结果表明,苯乙烯主要由自由基的链端β-断裂反应形成;二聚体主要由分子内1,3氢转移的反应形成;α-甲基苯乙烯由分子内的1,2氢转移后进行β-断裂形成;甲苯由苯甲基自由基夺取主链上的氢原子形成;乙苯由苯乙基自由基夺取氢原子形成。动力学分析表明,苯乙烯形成所需要的能垒低于其他产物形成所需要的能垒,故苯乙烯为主要的热降解产物;这与相关实验结果基本一致。     

Acid/base catalysis by pure water: the aldol reaction

[reaction: see text] The mechanism of aldol reactions in pure water has been studied with density functional calculations (B3LYP/6-311++G(3d,3p)//B3LYP/6-31G(d)). The reaction is a three-step process that involves: (1) water autoionization generates catalytic hydroxide and hydronium ions, (2) hydroxide and hydronium ions rapidly convert donor aldehyde or ketone into enol, and (3) C-C bond formation and proton transfer occur to give the aldol product. This study provides a general basis for understanding acid/base catalysis by pure water.     

A-T碱基对单羟基自由基加成产物的单电子氧化还原性质

采用密度泛函理论在B3LYP/DZP++//B3LYP/6-31 ++G(d,p)水平上研究A-T碱基对的单羟基加成产物的氧化还原性质.计算表明,所有8种加成复合物都表现出显著的氧化性,但其还原性却很弱.加成复合物AC2-T、AC4-T、AC5-T的俘获电子诱发T碱基N3位上的H原子向A碱基的N1位迁移,产生这种氢迁移的根源在于A碱基俘获电子后电子密度较大,有利于在A碱基上形成新的N-H键.     

Hydrosilylation vs. [2 + 2]-cycloaddition: A theoretical study with iron and ruthenium complexes

Recently an exciting new mechanism of hydrosilylation had been found in experiments with the ruthenium-silylene complex [Cp^∗(i-Pr₃P)Ru(H)₂Si(H)Ph · OEt₂][B(C₆F₅)₄] by Glaser and Tilley. The mechanism of the hydrosilylation and possible alternative pathways are investigated with quantum chemical methods utilizing the B3LYP method, a double zeta pseudopotential basis set for iron and ruthenium and the 6-31G^∗ basis set for all other elements. Starting from the model complex [Cp(H₃P)Ru(H)₂Si(H)Ph]⁺ the coordination of ethene at the silicon atom leads preferably to the hydrosilylation of a terminal Si-H-bond. The analysis of the electron density distribution of the catalytic active complex shows surprising bond features between Ru and Si. The Ru-Si bond is bridged by two hydrogen atoms.The [2 + 2]-cycloaddition of the alkene to the Ru-Si-bond, which would be a reasonable alternative reaction pathway, was not observed. It is necessary to make drastic changes in the ligand environment of the transition metal-silicone complex to observe cycloaddition reactions. With complexes of the type (OC)₄MSi(H)Ph (M = Ru, Fe) the cycloaddition could be a serious alternative to the hydrosilylation.     

MPW1K Performs Much Better than B3LYP in DFT Calculations on Reactions that Proceed by Proton-Coupled Electron Transfer (PCET)

DFT calculations have been performed with the B3LYP and MPW1K functional on the hydrogen atom abstraction reactions of ethenoxyl with ethenol and of phenoxyl with both phenol and alpha-naphthol. Comparison with the results of G3 calculations shows that B3LYP seriously underestimates the barrier heights for the reaction of ethenoxyl with ethenol by both proton-coupled electron transfer (PCET) and hydrogen atom transfer (HAT) mechanisms. The MPW1K functional also underestimates the barrier heights, but by much less than B3LYP. Similarly, comparison with the results of experiments on the reaction of phenoxyl radical with alpha-naphthol indicates that the barrier height for the preferred PCET mechanism is calculated more accurately by MPW1K than by B3LYP. These findings indicate that the MPW1K functional is much better suited than B3LYP for calculations on hydrogen abstraction reactions by both HAT and PCET mechanisms.     

Density functional study of the conformations and intramolecular proton transfer in thiohydroxamic acids

The conformational preferences of thiohydroxamic acids (N-hydroxythioamides) are investigated by the density functional B3LYP/6-311++G(3df,3pd)//B3LYP/6-31G(d) method in this work. Unlike hydroxamic acids, the thione and thiol forms are found to be equally stable in the gas phase, and the reaction pathways for the interconversion between the thione and thiol forms have been deduced to involve rotation about the C[double bond, length as m-dash]N bond of the thiol tautomer in the rate-determining step. The effect of aqueous solvation on the reactions has also been investigated. It is found that inclusion of a few explicit water molecules in an implicit solvent calculation is necessary in order to accurately account for hydrogen bonding effects. Thiohydroxamic acids, like their hydroxamic acid analogues, are found to be N-acids, both in the gas phase and in aqueous solution.     

吸热型碳氢燃料正癸烷热裂解机理、热沉及产物分布的理论研究

采用密度泛函理论(DFT)的B3LYP方法在6-311G(d,p)基组水平上对正癸烷裂解过程中涉及的反应物、产物及过渡态进行了几何构型优化和振动频率计算,运用B3LYP/aug-cc-pVTZ方法计算单点能并构建势能剖面图。利用TheRate程序包及Eckart校正模型计算了各反应速率常数k。采用统计热力学原理求得不同温度下的热容C_p,m^θ及熵S_{298 K}^θ,并通过设计等键反应获得了各物种的标准生成焓△_fH_{298 K}^θ。用Chemkin II程序模拟预测了产物分布,理论计算了热沉值,并讨论了温度、压力对产物分布和热沉的影响。结果表明,C-C键断裂过程是反应的初始步骤,且抽氢反应较β键断裂反应更易进行。裂解起始温度为500 ℃,反应主要发生在600~700 ℃,其主要产物为氢气、甲烷、乙烯、乙烷、丙烯和1,3-丁二烯,且产物分布随温度不同而变化。模拟计算获得正癸烷在温度600 ℃、压力2.5 MPa条件下的总热沉值为2.334 MJ/kg,对应的热裂解转化率为25.9%,该热沉值可以满足速率为5~6马赫数的飞行器的冷却要求。