A computational investigation of the reactions of methylene, chlorocarbene, and dichlorocarbene with cyclopropane

The reactions of CH(2), CHCl, and CCl(2) with cyclopropane, 1, have been examined computationally. In all cases the lowest energy reaction between the carbene and 1 is predicted to be C-H insertion. In the reaction of CH(2) with 1, the transition state for C-C insertion leading to cyclobutane is 1.7 kcal/mol higher in enthalpy than the transition state for C-H insertion at the G3B3 level. A pathway higher in energy than C-H insertion in the reactions of CHCl and CCl(2) with 1 involves two-bond cleavages generating ethylene along with chloro and dichloroethylene, respectively.     

Ti^＋离子和C2H4分子自旋禁阻反应中C—H键活化机理的理论研究

用密度泛函B3LYP方法,研究了二重态和四重态势能面自旋禁阻反应Ti^＋（^4F,3d^24s^1）＋C2H4→TiC2H2^＋（^2A2）＋H2的微观机理．通过自旋．轨道耦合的计算讨论了势能面交叉点和可能的自旋翻转过程．中间体IM1-^4B2处,四重态和二重态间的旋-轨耦合值为59．3cm^-1．自旋多重度必将发生变化,从四重态系间穿越到二重态势能面形成共价型复合物IM1-^2A1,同时导致四重态势能面的势垒明显降低．到插入中间体IM2后,二重态势能面上有两条不同的反应路径,即分步和协同路径,后者在二重态势能面上得到放热产物TiC2H2^＋（^2A2）＋H2具有较低的活化势垒,4．52kcal／mol,其主反应路径为：Ti^＋＋C2H4→^4IC→IM1—^4B2→4.2ISC→IM1—^2A1→[^2TSins]→IM2-^2A”→[^2TSMCTS]→IM5→TiC2H2^＋（^2A2）＋H2．     

U+与CO2气相反应的密度泛函理论研究

运用密度泛函理论(DFT)中的B3LYP方法,U原子用含相对论有效原子实势(ECP)校正的基组(SDD),C、O原子采用6-311+G(d)基组,对气相中U+和CO2的反应进行了理论研究.通过研究二重和四重自旋态的反应势能面(PESs),优化得到了两条反应路径的反应物、中间体、过渡态和产物的结构.用"两态反应"(TSR)分析反应机理,结果表明体系的优先选择路径为高自旋态进入和低自旋态离开反应,发生在四重态和二重态的自旋多重度的改变使得整个反应系统能以一个低能反应途径进行.     

A theoretical study of alcohol oxidation by ferrate

The conversion of methanol to formaldehyde mediated by ferrate (FeO(4)2-), monoprotonated ferrate (HFeO4-), and diprotonated ferrate (H2FeO4) is discussed with the hybrid B3LYP density functional theory (DFT) method. Diprotonated ferrate is the best mediator for the activation of the O-H and C-H bonds of methanol via two entrance reaction channels: (1) an addition-elimination mechanism that involves coordination of methanol to diprotonated ferrate; (2) a direct abstraction mechanism that involves H atom abstraction from the O-H or C-H bond of methanol. Within the framework of the polarizable continuum model (PCM), the energetic profiles of these reaction mechanisms in aqueous solution are calculated and investigated. In the addition-elimination mechanism, the O-H and C-H bonds of ligating methanol are cleaved by an oxo or hydroxo ligand, and therefore the way to the formation of formaldehyde is branched into four reaction pathways. The most favorable reaction pathway in the addition-elimination mechanism is initiated by an O-H cleavage via a four-centered transition state that leads to intermediate containing an Fe-O bond, followed by a C-H cleavage via a five-centered transition state to lead to formaldehyde complex. In the direct abstraction mechanism, the oxidation reaction can be initiated by a direct H atom abstraction from either the O-H or C-H bond, and it is branched into three pathways for the formation of formaldehyde. The most favorable reaction pathway in the direct abstraction mechanism is initiated by C-H activation that leads to organometallic intermediate containing an Fe-C bond, followed by a concerted H atom transfer from the OH group of methanol to an oxo ligand of ferrate. The first steps in both mechanisms are all competitive in energy, but due to the significant energetical stability of the organometallic intermediate, the most likely initial reaction in methanol oxidation by ferrate is the direct C-H bond cleavage.     

CS与NO分子反应势能面的理论研究

应用量子化学从头计算和密度泛函理论(DFT)对CS分子和NO分子的反应机理进行了研究. 在B3LYP/6- 311G**和CCSD(T)/6-311G**水平上计算了CS分子与NO分子反应的二重态和四重态反应势能面. 计算结果表明, 二重态反应势能面中, CS分子的C端和NO的N端连接是主要的反应方式. 反应物先经过过渡态TS1, 形成具有直线结构的中间体1 (CSNO). 中间体1经过一系列异构化得到主要产物P₁ (CO＋SN). 此反应是放热反应, 反应热为－183.75 kJ/mol . 而四重态由于反应入口势垒过高, 是不重要的.     

Two states and two more in the mechanisms of hydroxylation and epoxidation by cytochrome P450

Past studies have shown that oxidation reactions by P450 Compound I (Cpd I) can be described by two competing quartet and doublet spin states, which possess three unpaired electrons, hence tri-radicals. One electron excitation from the delta orbital to sigma* xy generates two states that possess five unpaired electrons, so-called penta-radicals, in sextet and quartet situations, and which were shown by theory to lie only approximately 12-14 kcal/mol higher in energy than the tri-radical ground states (ref 7). The present study focuses on the C-H hydroxylation and C=C epoxidation of propene by these penta-radical states. It is shown that the initial energy differences, between the penta-radical and tri-radical states, diminish along the reaction pathway, due to the favorable and cumulative exchange stabilization of the more open-shell species. Furthermore, theory suggests that hydrogen bonding to the thiolate ligand, and general polarity of the environment, reduce these gaps further, thereby making the penta-radical states accessible to ground-state reactivity. The interconversion between the tri-radical and penta-radical states along the reaction coordinate will depend on the dynamics of spin-flips and energy barriers between the states. Especially interesting should be the region of the reaction intermediates; for both epoxidation and hydroxylation, this region is typified by a dense manifold of spin states and electromeric states (that differ by the oxidation state of iron), such that the total reactivity would be expected to reflect the interplay of these states, giving rise to multistate reactivity.     

Theoretical study on the gas‐phase reaction mechanism between rhodium monoxide and methane for methanol production

The gas‐phase reaction mechanism between methane and rhodium monoxide for the formation of methanol, syngas, formaldehyde, water, and methyl radical have been studied in detail on the doublet and quartet state potential energy surfaces at the CCSD(T)/6‐311+G(2d, 2p), SDD//B3LYP/6‐311+G(2d, 2p), SDD level. Over the 300–1100 K temperature range, the branching ratio for the Rh(⁴F) + CH₃OH channel is 97.5–100%, whereas the branching ratio for the D‐CH₂ORh + H₂ channel is 0.0–2.5%, and the branching ratio for the D‐CH₂ORh + H₂ channel is so small to be ruled out. The minimum energy reaction pathway for the main product methanol formation involving two spin inversions prefers to both start and terminate on the ground quartet state, where the ground doublet intermediate CH₃RhOH is energetically preferred, and its formation rate constant over the 300–1100 K temperature range is fitted by k_CH3RhOH = 7.03 × 10⁶ exp(?69.484/RT) dm³ mol^?1 s^?1. On the other hand, the main products shall be Rh + CH₃OH in the reactions of RhO + CH₄, CH₂ORh + H₂, Rh + CO +2H₂, and RhCH₂ + H₂O, whereas the main products shall be CH₂ORh + H₂ in the reaction of Rh + CH₃OH. Meanwhile, the doublet intermediates H₂RhOCH₂ and CH₃RhOH are predicted to be energetically favored in the reactions of Rh + CH₃OH and CH₂ORh + H₂ and in the reaction of RhCH₂ + H₂O, respectively. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

Mechanism of metal chloride-promoted Mukaiyama aldol reactions

Ab initio calculations (MP2/6-311+G**//B3LYP/6-31G*) were employed to investigate the mechanism of metal chloride-promoted Mukaiyama aldol reaction between trihydrosilyl enol ether and formaldehyde. The metal chlorides considered include TiCl4, BCl3, AlCl3, and GaCl3. In contrast to the concerted pathway of the uncatalyzed aldol reaction, the Lewis acid-promoted reactions favor a stepwise mechanism. Three possible stepwise pathways were located. The lowest energy pathway corresponds to a simultaneous C-C bond formation and a chlorine atom shift in the first (rate-determining) step. This process is calculated to have a low activation barrier of 12 kJ mol-1 for the TiCl4-promoted reaction. The alternative [2+2] cycloaddition and direct carbon-carbon bond formation pathways are energetically competitive. BCl3, AlCl3, and GaCl3 are predicted to be efficient catalysts for the silicon-directed aldol reaction as they strongly activate the formaldehyde electrophile. Formation of a stable pretransition state intermolecular pi-pi complex between enol silane and the activated formaldehyde (CH2=O...MCln) is a key driving force for the facile metal chloride-promoted reactions.     

气相VO（∑＋）与CH3OH（1^A′）分子自旋禁阻反应C—H，O—H键活化机理的理论研究

采用密度泛函理论（DFT）B3LYP与cCsD方法研究了二重态和四重态势能面自旋禁阻反应VO（∑’）活化cH30H（1^A′）分子c—H,0—H键的微观机理．通过自旋一轨道耦合的计算讨论了势能面交叉点和可能的自旋翻转过程．在MEcP处,四重态和二重态问的旋轨耦合常数为131．14cm^-1．自旋多重度发生改变,从四重态系间穿越到二重态势能面形成中间体2^IM1,导致反应势能面的势垒明显降低．     

Methane activation by chromium oxide cations in the gas phase: a theoretical study

Density Functional Theory, in its B3LYP formulation, was used to explore quantitative details of the potential energy hypersurfaces for the C-H bond activation reaction of methane by chromium dioxide cation. Both doublet ground and quartet excited states of the cation were considered, and all the minima and transition states localized along the paths leading to the formation of the experimentally observed products were characterized. All the calculated paths involve spin inversions that decrease the barrier heights of the involved transition states but do not play a significant role. Reaction pathways were also studied employing the nonhybrid BP86 functional, the reparametrized B3LYP* functional, and the CCSD(T) approach. Because other examples in the literature indicate that sequential ligation enhances the reactivity of bare transition metals cations, the state-selective reactivity of the chromium monoxide cation with respect to methane was also investigated and compared with that of the bare cation.     

Theoretical investigation of the Fe+-catalyzed oxidation of acetylene by N2O

The gas-phase Fe(+)-mediated oxidation of acetylene by N2O on both sextet and quartet potential energy surfaces (PESs) is theoretically investigated using density functional theory. Geometries and energies of all the stationary points involved in the catalytic reaction are located. For the catalytic cycles, the crucial step is the initial N2O reduction by Fe(+) to form FeO(+), in which a direct O-abstraction mechanism is located on the sextet PES, whereas the quartet pathway favors a N-O insertion mechanism. Spin inversion moves the energy barrier for this process downward to a position below the ground-state entrance channel. The second step of the catalytic cycles involves two mechanisms corresponding to direct hydrogen abstraction and cyclization. The former mechanism accounts for the ethynol formation with the upmost activation barrier below the entrance channel by about 5 kcal/mol. The other mechanism involves a "metallaoxacyclobutene" structure, followed by four possible pathways, i.e., direct dissociation, C-C insertion, C-to-O hydrogen shift, and/or C-to-C hydrogen shift. Among these pathways, strong exothermicities as well as energetically low location of the intermediates suggest oxidation to ketene and carbon monoxide along the C-to-C hydrogen shift pathway is the most favorable. Reduction of the CO loss partner FeCH2(+) by another N2O molecule constitutes the third step of the catalytic cycles, which contains direct abstraction of O from N2O giving OFeCH2(+), intramolecular rearrangement to form Fe(+)-OCH2, and nonreactive dissociation. This reaction is also energetically favored considering the energy acquired from the initial reactants.     

CH_3SCH_3在Fe~+作用下的脱烷基化反应机理的理论研究

采用密度泛函理论的B3LYP方法,在6-311++G(d,p)和DGDZVP基组水平上研究了CH3SCH3在Fe+作用下的脱烷基化的四重态和六重态微观反应机理,全参数优化了反应势能面上各驻点的几何构型,振动分析和内禀反应坐标(IRC)分析结果证实了中间体和过渡态的真实性.找到了三条可能的反应通道,对结果的分析表明:对于六重态的反应体系,二甲硫醚的脱甲烷化反应主要经历了四个基本步骤,即先驱复合物、C—S活化、(-H转移和非反应性的分裂.对于四重态的Fe+/CH3SCH3反应体系,含有C—S和C—H插入反应的两个路径都可以导致脱甲烷反应的发生,其中C—S插入反应路径的能垒较低,是主要反应通道.     

Theoretical study of low-spin, high-spin, and intermediate-spin states of [Fe(III)(pap)2](+) (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato). Mechanism of light-induced excited spin state trapping

The mechanism of light-induced excited spin state trapping (LIESST) of [FeIII(pap)2]+ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato) was discussed on the basis of potential energy surfaces (PESs) of several important spin states, where the PESs were evaluated with the DFT(B3LYP) method. The PES of the quartet spin state crosses those of the doublet and sextet spin states around its minimum. This means that the spin transition occurs from the quartet spin state to either the doublet spin state or the sextet spin state around the PES minimum of the quartet spin state. The PES minimum of the sextet spin state is slightly less stable than that of the doublet spin state by 0.18 eV (4.2 kcal/mol). This small energy difference is favorable for the LIESST. The doublet-sextet spin crossover point is 0.41 eV (9.6 kcal/mol) above the PES minimum of the sextet spin state. Because of this considerably large activation barrier, the thermal spin transition and the tunneling process do not occur easily. In the doublet spin state, the ligand to ligand charge transfer (LLCT) transition is calculated to be 2.16 eV with the TD-DFT(B3LYP) method, in which the pi orbital of the phenoxy moiety and the pi* orbital of the imine moiety in the pap ligand participate. This transition energy is moderately smaller than the visible light of 550 nm used experimentally. In the sextet spin state, the ligand to metal charge transfer (LMCT) transition is calculated to be at 2.36 eV, which is moderately higher than the visible light (550 nm). These results indicate that the irradiation of the visible light induces the LIESST to generate the sextet spin state but the reverse-LIESST is also somewhat induced by the visible light, indicating that the complete spin conversion from the doublet spin state to the sextet one does not occur, as reported experimentally.     

Theoretical study of C-H and C-F activation in CH4-nFn (n =1-4) molecules by platinum

CASSCF followed by MRMP2 calculations have been carried out to analyze the reactions of a naked platinum atom with the fluorocarbon compounds CH(4-n)Fn (n = 1-4). For each of these interactions the potential-energy surfaces which correlate with the triplet ground state and the first excited singlet state of the free fragments were investigated for representative states evolving from different approaching modes of the reactants. For all the fluorinated fragments activation of the C-H and C-F bonds by the metal is strongly determined by the low-multiplicity channels arising from the first excited asymptote. Although stable products are predicted for insertion of the metallic atom into both the C-H and the C-F bonds of the different fluorocarbon compounds, comparison between the calculated energy barriers for reactions taking place in the same fluorinated molecule suggests in all cases a kinetic preference for the C-H bond oxidative addition to the platinum atom.     

Preparation and properties of metallacyclobutanes of nickel and palladium

Bis(phosphine)-3,3-dimethylnickela- and palladacyclobutanes have been prepared by intramolecular C-H insertion reaction of the corresponding dineopentyl metal complexes. Nickelacyclobutane complexes decompose when heated thereby undergoing competitive carbon-carbon bond cleavage to give isobutene and ethylene, with reductive elimination affording 1,1-dimethylcyclopropane and skeletal isomeri-zation of the metallacyclic ring yielding 3-methyl-1-butene, whereas the palladium analog gave no significant amounts of CC bond cleavage products.Added phos-phine was seen to have an effect on CC bond scission of nickelacyclobutane complexes. Nickelacyclobutane complexes in solution are thought to be in equilibrium with olefin-coordinated nickel-carbene complex on the basis of available experimental evidence from hydrogenolysis, carbene-trap reactions with olefins and reaction with carbon monoxide     

Quantum mechanical/molecular mechanical investigation of the mechanism of C-H hydroxylation of camphor by cytochrome P450cam: theory supports a two-state rebound mechanism

The stereospecific cytochrome P450-catalyzed hydroxylation of the C(5)-H((5-exo)) bond in camphor has been studied theoretically by a combined quantum mechanical/molecular mechanical (QM/MM) approach. Density functional theory is employed to treat the electronic structure of the active site (40-100 atoms), while the protein and solvent environment (ca. 24,000 atoms) is described by the CHARMM force field. The calculated energy profile of the hydrogen-abstraction oxygen-rebound mechanism indicates that the reaction takes place in two spin states (doublet and quartet), as has been suggested earlier on the basis of calculations on simpler models ("two-state reactivity"). While the reaction on the doublet potential energy surface is nonsynchronous, yet effectively concerted, the quartet pathway is truly stepwise, including formation of a distinct intermediate substrate radical and a hydroxo-iron complex. Comparative calculations in the gas phase demonstrate the effect of the protein environment on the geometry and relative stability of intermediates (in terms of spin states and redox electromers) through steric constraints and electronic polarization.     

一种非血红素四氮杂轮烯配合物[Fe(Ⅲ)TMTAA]催化过氧化氢歧化反应机理的理论研究

采用密度泛函理论B3LYP方法计算了一种非血红素四氮杂轮烯配合物[Fe(Ⅲ)TMTAA]催化H2O2歧化的反应机理.对二重态、四重态和六重态势能面上各驻点进行了全优化,发现反应易于沿四重态势能面发生.整个反应分两阶段进行,第一阶段通过氧氧均裂形成中间体IM6和第一个水,第二阶段经两次氢转移形成第二个水.反应决速步骤为O—O均裂步骤,能垒为63.9kJ·mol-1,相对于自由H2O2均裂所需能垒226.7kJ·mol-1有较大的降低.这表明标题配合物可有效地降低标题反应的能垒,有可能作为一种潜在的过氧化氢仿酶.     

Reaction pathways of Sc+ (3D, 1D) and Fe+ (6D, 4F) with acetone in the gas phase: metal ion oxidation and acetone deethanization

The reactions of Sc⁺ (³D, ¹D) and Fe⁺ (⁶D, ⁴ F) with acetone have been investigated in both high‐ and low‐spin states using density functional theory. Our calculations have indicated that oxidation of Sc⁺ by acetone can take place by (1) metal‐mediated H migration, (2) direct methyl‐H shift and/or (3) C = O insertion. The most energetically favorable pathway is metal‐mediated H migration followed by intramolecular ScO⁺ rotation and dissociation. For the deethanization of acetone mediated by Fe⁺, the reaction occurs on either the quartet or sextet surfaces through five elementary steps, i.e. encounter complexation, C–C bond activation, methyl migration, C–C coupling and non‐reactive dissociation. The rate‐determining step along the quartet‐state potential‐energy surface (PES) is similar to that in the case of Ni⁺ (² F, 3d⁹), namely the methyl‐migration step. For the sextet‐state PES, however, the energy barrier for methyl migration is lower than that for C–C bond activation, and the rate‐determining step is C–C coupling. In general, the low‐spin‐state pathways are lower in energy than the high‐spin‐state pathways; therefore, the reaction pathways for the oxidation of Sc⁺ and the Fe⁺‐mediated deethanization of acetone mostly involve the low‐spin states. Copyright © 2012 John Wiley & Sons, Ltd.     

Theoretical analysis of the mechanism of palladium(II) acetate-catalyzed oxidative Heck coupling of electron-deficient arenes with alkenes: effects of the pyridine-type ancillary ligand and origins of the meta-regioselectivity

A systematic theoretical study is carried out on the mechanism for Pd(II)-catalyzed oxidative cross-coupling between electron-deficient arenes and alkenes. Two types of reaction pathways involving either a sequence of initial arene C-H activation followed by alkene activation, or the reverse sequence of initial alkene C-H activation followed by arene activation are evaluated. Several types of C-H activation mechanisms are discussed including oxidative addition, σ-bond metathesis, concerted metalation/deprotonation, and Heck-type alkene insertion. It is proposed that the most favored reaction pathway should involve an initial concerted metalation/deprotonation step for arene C-H activation by (L)Pd(OAc)(2) (L denotes pyridine type ancillary ligand) to generate a (L)(HOAc)Pd(II)-aryl intermediate, followed by substitution of the ancillary pyridine ligand by alkene substrate and direct insertion of alkene double bond into Pd(II)-aryl bond. The rate- and regio-determining step of the catalytic cycle is concerted metalation/deprotonation of arene C-H bond featuring a six-membered ring transition state. Other mechanism alternatives possess much higher activation barriers, and thus are kinetically less competitive. Possible competing homocoupling pathways have also been shown to be kinetically unfavorable. On the basis of the proposed reaction pathway, the regioselectivity predicted for a number of monosubstituted benzenes is in excellent agreement with experimental observations, thus, lending further support for our proposed mechanism. Additionally, the origins of the regioselectivity of C-H bond activation is elucidated to be caused by a major steric repulsion effect of the ancillary pyridine type ligand with ligands on palladium center and a minor electronic effect of the preinstalled substituent on the benzene ring on the cleaving C-H bond. This would finally lead to the formation of a mixture of meta and para C-H activation products with meta products dominating while no ortho products were detected. Finally, the multiple roles of the ancillary pyridine type ligand have been discussed. These insights are valuable for our understanding and further development of more efficient and selective transition metal-catalyzed oxidative C-H/C-H coupling reactions.     

Competition between C-C and C-H insertion in prototype transition metal-hydrocarbon reactions

The competition between C-C and C-H insertion in model transition-metal reactions with cyclopropane and propene (C3H6) was studied as a function of total energy. Insertion of neutral transition metal atoms M (= Y, Zr, Nb, and Mo*) into the C-C bonds of cyclopropane led to formation of MCH2 + C2H4, whereas C-H insertion produced MC3H4 + H2. The measured product branching ratios verify the relative potential energy barrier heights for C-C and C-H insertion predicted by ab initio calculations.