Criegee中间体CH_3CHOO与OH自由基反应机理的理论研究

采用CCSD(T)//B3LYP/6-311+G(d,p)方法研究了Criegee中间体CH_3CHOO与OH自由基反应的微观机理.结果表明,上述反应存在抽氢、加成-分解和氧化3类反应通道,其中,syn-CH3CHOO+OH以抽β-H为优势通道,表观活化能为-4.88 k J/mol;anti-CH_3CHOO+OH则以加成-分解反应为优势通道,表观活化能为-13.25 k J/mol.在加成-分解和氧化反应通道中,anti-构象的能垒均低于syn-构象,而抽氢反应则是syn-(β-H)的能垒低于anti-构象.速率常数计算表明,anti-构象的加成-分解反应通道具有显著的负温度效应;syn-和anti-构象的氧化通道具有显著的正温度效应.3类反应具有显著不同的温度效应,说明通过改变温度可显著调节3类反应的相对速率.     

Criegee中间体RCHOO(R=H,CH_3)与NO_2反应机理及大气中HNO_3的形成

采用CCSD(T)/aug-cc-p VTZ//M06-2X/cc-p VDZ方法研究了RCHOO(R=H,CH_3)+NO_2反应的微观机理,并讨论了甲基取代对反应活性的影响.结果表明,该反应存在加成-分解、氧化-氢转移和直接抽氢3类反应机理,其中氧化-氢转移反应的主要产物为RCO+HNO_3,表观活化能仅为40.51 k J/mol,且释放出235.04 k J/mol的能量,是反应的优势通道;而生成RCHO+NO_3自由基的加成-分解通道较难进行.甲基取代使α-C电正性增大,端氧负性增加,这有利于加成-分解和氧化-氢转移反应的进行,但syn-CH_3CHOO中显著的位阻因素反而使加成-分解反应活性有所降低.     

CH_3CH_2O与HO_2抽氢反应与主通道速率常数的理论研究

在B3LYP//6-311++G(2df,2p)水平完成了对CH_3CH_2O与HO_2反应各驻点物种的几何构型优化,并在相同水平上对相关物种进行了频率分析和内禀反应坐标(IRC)计算.为得到较准确的单点能信息,同时采用CCSD(T)/cc-pVTZ方法对反应途径中各驻点进行单点能校正.标题反应的单、三重态势能剖面图揭示,反应在单、三重态势能面上均有3条抽氢通道.其中单重态通道分别生成1CH3CHO+H_2O_2(R1),CH_3CH_2OH+1 O_2(R2)和1CH_2CH_2O+H_2O_2(R3),三重态通道分别生成3CH3CHO+H_2O_2(R4),CH_3CH_2OH+3O_2(R5)和3CH_2CH_2O+H_2O_2(R6);势垒高度揭示三重态在动力学和热力学上比单重态更具优势.200K~1 200K区间内标题反应速率常数计算结果表明,除通道R5的速率常数几乎不随温度变化外,所有其他通道的速率常数均随温度升高而增大(即速率呈现正温度系数效应),同时发现抽氢通道R5的分支比始终大于94%,因而是绝对优势通道.     

CF_2ClC(O)OCH_2CH_3+OH的反应机理及动力学性质的理论研究

利用双水平直接动力学方法,在MCG3-MPWB//M06-2X/aug-cc-pVDZ水平上研究了CF_2ClC(0)OCH_2CH_3+OH的微观反应机理.得到了反应物CF_2ClC(O)OCH_2CH_3的5种稳定构象(RCl~RC5),并对每一构象考察了发生在-CH_3-和-CH_2-基团上的所有可能氢提取反应通道.利用改进的变分过渡态理论(ICVT)结合小曲率隧道效应校正(SCT)计算了各反应通道的速率常数,分析了各构象反应位点选择性.结果表明,对于构象RCl和RC2,低温时氢提取反应主要发生在-CH_2-基团上;而对于构象RC3RC4和RC5,发生在-CH_3基团上的氢提取反应通道在整个温度区间内占绝对优势.根据Boltzmann配分函数计算总包反应速率常数,在298 K温度下计算的体系总包反应速率常数与实验值相符,进而给出200~1000 K温度范围内拟合了速率常数的三参数Arrhenius表达式:k_(overall)=5.45×10~(25)T~(4.54)exp(-685/T).     

NCO自由基与O和N反应的理论研究

采用密度泛函和量子化学从头算方法, 对NCO自由基和O, N原子反应的势能面进行了理论研究, 讨论了主要的反应通道. 这两种自由基反应的机理比较类似, 初始都有两种进攻方式. NCO与O的主反应通道是O原子从N端无势垒加合, 经过一低垒过渡态, 得到稳定产物P1(CO+NO), 而对NCO与N反应得到了一完整反应通道和无垒加合产物.     

CH_4与NO_2反应的微观机理及动力学性质的理论研究

采用量子化学计算方法,研究了CH_4+NO_2反应直接氢抽提反应通道的机理和速率常数.该反应有3条反应通道分别生成CH_3+HNO_2,CH_3+trans-HONO和CH_3+cis-HONO.计算结果表明采用变分过渡态理论加小曲率隧道效应校正计算得到反应速率常数和已有的实验值很吻合.在整个研究温度区间,O原子提取H原子生成CH_3+cis-HONO是反应的主要通道.     

CH_3自由基与C_2H_5CN高温反应机理的理论研究

用G3(MP2)//B3PW91/6-311G(d,p)双级别方法研究了CH_3自由基与C_2H5_CN的反应机理和动力学性质.计算表明反应存在抽氢、加成-消除和取代3种机理7条反应通道.用CVT方法计算了所有反应通道在1 000K~3 000K温度范围内的速率常数,结果表明计算值与实验值符合得很好.     

HO_2+NO气相反应机理及主通道速率常数的理论研究

采用CCSD(T)/aug-cc-p VTZ//MP2/aug-cc-p VTZ方法构建了HO_2+NO反应的单、三重态反应势能剖面.研究结果表明,标题反应在单、三重态均存在3条反应通道,分别经抽氧、加成和抽氢反应生成产物NO_2+OH、HNO_3和HNO+O_2.其中抽氧、加成反应的优势通道发生在单重态势能面上(通道R1和R2),而抽氢的优势通道则发生在三重态势能面上(通道R6).利用经典过渡态理论(TST)并结合Wigner矫正模型在200K~1500K温度范围内计算了优势通道R1,R2和R6的速率常数kTST/W.结果显示,在计算温度范围内抽氧、加成反应的通道R1和R2始终存在竞争,且抽氢反应通道R6在高温区500K~1500K竞争作用也逐渐显现,但抽氧通道R1分支比始终大于81%,具有绝对优势.     

反式2-甲基-2-丁烯酸甲酯与臭氧反应机理的计算研究

采用G3B3方法构建反式2-甲基-2-丁烯酸甲酯与O3反应体系以及后续Criegee自由基有、无水分子参与下异构化反应的势能面剖面.结果表明,反式2-甲基-2-丁烯酸甲酯与O3反应首先生成一个稳定的五元环中间体,此中间体按断键位置不同后续裂解反应存在两条路径,分别生成产物P1(CH3CHOO+CH3OC(O)C(CH3)O)和P2(CH3CHO+CH3OC(O)C(CH3)OO).利用经典过渡态理论(TST)并结合Wigner矫正模型计算了200-1200 K温度区间内标题反应的速率常数kTST/W.计算结果显示,294 K时,该反应速率常数为7.55×10-18cm3molecule-1s-1,与Bernard等对类似反应所测实验值非常接近.生成的Criegee自由基(CH3CHOO和CH3OC(O)C(CH3)OO)可分别与水分子发生α-加成及β-氢迁移反应,其中Criegee自由基与水的α-加成反应较其与水的β-氢迁移反应具有优势.另外与无水分子参与CH3CHOO和CH3OC(O)C(CH3)OO异构化反应相比,水分子的参与使得异构化反应较为容易进行.     

一种判断羧酸衍生物加成—消除反应活性的方法

羧酸衍生物与亲核试剂反应的历程已经研究清楚,首先是亲核试剂进攻羰基碳原子发生亲核加成,然后进行消除,属于加成-消除反应。其活性次序为:RCOX>(RCO)_2O>RCO_2R′>RCONHR′,此活性次序可用电子效应加以解释。由于与羰基直接相连的—(X|¨)、—(O|¨)COR、—(O|¨)R′、—(N|¨)HR′等基团均具有供     

Theoretical investigations on removal reactions of ethenol by H atom

Ethenol is a recently identified combustion intermediate. However, its chemistry remains unclear. In present work, the removal reactions of ethenol by H atom are investigated. The geometries of all species involved in the reaction are optimized at B3LYP/6-311++G(d,p), and their single point energies are extrapolated to the infinite-basis-set limit at the level CCSD(T). Energies are also calculated at G3B3, CBS-APNO, and CCSD(T)/6-311++G(3df, 2p) for comparison. A total of six elementary reactions, including four abstractions and two additions, with explicit transition states are investigated. The results show that the reactions are selective: for abstractions, the hydrogen atom, linked to the oxygen atom, is the most reactive; while for additions, the preferred carbon site is the head "CH(2)═". The rate constants are estimated in the temperature range 300-3000 K according to the conventional transition state theory with the Eckart tunneling model. The dominant channels are the two additions in the whole temperature range. The abstractions can be competitive at high temperature but still do not dominate. The calculated rate constants for the reverse reaction of (R6), syn-CH(2)═CHOH + H ? CH(3)·CHOH, are consistent with the available literature values. Finally, the Fukui functions are calculated to analyze the site reactivity.     

Transient infrared spectra of CH3SOO and CH3SO observed with a step-scan Fourier-transform spectrometer

A step-scan Fourier-transform spectrometer coupled with a multipass absorption cell was employed to monitor time-resolved infrared absorption of transient species produced upon irradiation at 248 nm of a flowing mixture of CH(3)SSCH(3) and O(2) at 260 K. Two transient bands observed with origins at 1397±1 and 1110±3 cm(-1) are tentatively assigned to the antisymmetric CH(3)-deformation and O-O stretching modes of syn-CH(3)SOO, respectively; the observed band contour indicates that the less stable anti-CH(3)SOO conformer likely contributes to these absorption bands. A band with an origin at 1071±1 cm(-1), observed at a slightly later period, is assigned to the S=O stretching mode of CH(3)SO, likely produced via secondary reactions of CH(3)SOO. These bands fit satisfactorily with vibrational wavenumbers and rotational contours simulated based on rotational parameters of syn-CH(3)SOO, anti-CH(3)SOO, and CH(3)SO predicted with density-functional theories B3LYP/aug-cc-pVTZ and B3P86/aug-cc-pVTZ. Two additional bands near 1170 and 1120 cm(-1) observed at a later period are tentatively assigned to CH(3)S(O)OSCH(3) and CH(3)S(O)S(O)CH(3), respectively; both species are likely produced from self-reaction of CH(3)SOO. The production of SO(2) via secondary reactions was also observed and possible reaction mechanism is discussed.     

H atom attack on propene

The reaction of propene (CH(3)CH═CH(2)) with hydrogen atoms has been investigated in a heated single-pulsed shock tube at temperatures between 902 and 1200 K and pressures of 1.5-3.4 bar. Stable products from H atom addition and H abstraction have been identified and quantified by gas chromatography/flame ionization/mass spectrometry. The reaction for the H addition channel involving methyl displacement from propene has been determined relative to methyl displacement from 1,3,5-trimethylbenzene (135TMB), leading to a reaction rate, k(H + propene) → H(2)C═CH(2) + CH(3)) = 4.8 × 10(13) exp(-2081/T) cm(3)/(mol s). The rate constant for the abstraction of the allylic hydrogen atom is determined to be k(H + propene → CH(2)CH═CH(2) + H(2)) = 6.4 × 10(13) exp(-4168/T) cm(3)/(mol s). The reaction of H + propene has also been directly studied relative to the reaction of H + propyne, and the relationship is found to be log[k(H + propyne → acetylene + CH(3))/k(H + propene → ethylene + CH(3))] = (-0.461 ± 0.041)(1000/T) + (0.44 ± 0.04). The results showed that the rate constant for the methyl displacement reaction with propene is a factor of 1.05 ± 0.1 larger than that for propyne near 1000 K. The present results are compared with relevant earlier data on related compounds.     

Direct dynamics study on the hydrogen abstraction reactions N2H4 + R-->N2H3 + RH (R=NH2, CH3)

We present a direct ab initio dynamics study on the hydrogen abstraction reactions N(2)H(4)+R-->N(2)H(3)+RH (R=NH(2),CH(3)), which are predicted to have six possible reaction channels for NH(2) abstraction and four for CH(3) abstraction caused by the different N(2)H(4) isomers and various attacking orientations of foreign radical to N(2)H(4). The structures and frequencies at the stationary points and the points along the minimum energy paths (MEPs) of all reaction channels are obtained at the UMP2(full)6-31+G(d,p) level of theory. Energetic information of stationary points and the points along the MEPs is further refined by means of MC-QCISD method. The rate constants of these channels are calculated using the improved canonical variational transition-state theory with the small-curvature tunneling correction (ICVT/SCT) method. The calculated results show that the favorable reaction channels are channels (n1) and (n4) as well as (c1) and (c3) (refer to Scheme 1) in the whole temperature range. The total ICVT/SCT rate constants of all channels for the two reactions at the MC-QCISDUMP2(full)6-31+G(d,p) level are both in good agreement with the available experimental data, and corresponding three-parameter expressions of k(ICVTSCT) in 220-3000 K are fitted as 6.46 x 10(-15)(T298)(3.60) exp(-386T) cm(3) mol(-1) s(-1) for NH(2) abstraction and 1.04 x 10(-14)(T298)(4.00) exp(-2037T) cm(3) mol(-1) s(-1) for CH(3) abstraction. Additionally, the long range interaction between the H atom of X-H bond in foreign radicals and the lone pair on the nonreactive N atom of the transition states is further discussed to explain the various transition-state numbers of the two similar hydrogen abstraction reactions.     

Selective surface decoration of corannulene

The reaction of corannulene (C(20)H(10)) with 1,2-C(2)H(4)Hal(2) (Hal = Cl or Br) in the presence of AlCl(3) affords stable nonplanar carbocations C(20)H(10)CH(2)CH(2)Hal(+) (Hal = Cl (1) and Br (2)) with an -CH(2)CH(2)Hal moiety attached to the interior carbon atom of the bowl. In the analogous reaction with 1-bromo-2-chloroethane, the selective (up to 98%) abstraction of chloride is observed with the formation of cation 2. The molecular structures of bowl-shaped carbocations 1 and 2 crystallized as salts with AlCl(4)(-) counterions are revealed by single-crystal X-ray diffraction. The reaction of 2 with methanol or ethanol provides further decoration of the nonplanar polyarene upon the nucleophilic addition of alkoxy groups to the exterior carbon atom of the corannulene moiety. The (1)H NMR investigation of the corresponding products, C(20)H(10)(CH(2)CH(2)Br)(OCH(2)R) (R = H (3) and CH(3) (4)), shows the formation of intramolecular H···O and H···Br hydrogen bonds.     

Reactivity of substituted charged phenyl radicals toward components of nucleic acids

Reactions of differently substituted phenyl radicals with components of nucleic acids have been investigated in the gas phase. A positively charged group located meta with respect to the radical site was employed to allow manipulation of the radicals in a Fourier-transform ion cyclotron resonance mass spectrometer. All of these electrophilic radicals react with sugars via exclusive hydrogen atom abstraction, with adenine and uracil almost exclusively via addition (likely at the C8 and C5 carbons, respectively), and with the nucleoside thymidine by hydrogen atom abstraction and addition at C5 in the base moiety (followed by elimination of (*)CH(3)). These findings parallel the reactivity of the phenyl radical with components of nucleic acids in solution, except that the selectivity for addition is different. Like HO(*), the electrophilic charged phenyl radicals appear to favor addition to the C5-end of the C5-C6 double bond of thymine and thymidine, whereas the phenyl radical preferentially adds to C6. The charged phenyl radicals do not predominantly add to thymine, as the neutral phenyl radical and HO(*), but mainly react by hydrogen atom abstraction from the methyl group (some addition to C5 in the base followed by loss of (*)CH(3) also occurs). Adenine appears to be the preferred target among the nucleobases, while uracil is the least favored. A systematic increase in the electrophilicity of the radicals by modification of the radicals' structures was found to facilitate all reactions, but the addition even more than hydrogen atom abstraction. Therefore, the least reactive radicals are most selective toward hydrogen atom abstraction, while the most reactive radicals also efficiently add to the base. Traditional enthalpy arguments do not rationalize the rate variations. Instead, the rates reflect the radicals' electron affinities used as a measure for their ability to polarize the transition state of each reaction.     

Theoretical dynamic studies on the reactions of CH3C(O)CH3-nCl(n) (n = 0-3) with the chlorine atom

The theoretical investigations were performed on the reaction mechanisms for the title reactions CH(3)C(O)CH(3) + Cl --> products (R1), CH(3)C(O)CH(2)Cl + Cl --> products (R2), CH(3)C(O)CHCl(2) + Cl --> products (R3), and CH(3)C(O)CCl(3) + Cl --> products (R4) by ab initio direct dynamics approach. Two different reaction channels have been found: abstract of the H atom from methyl (--CH(3)) group or chloromethyl (--CH(3-n)Cl(n)) group of chloroacetone and addition of a Cl atom to the carbon atom of the carbonyl group of chloroacetone followed by methyl or chloromethyl eliminations. Because of the higher potential energy barrier, the contribution of addition-elimination reaction pathway to the total rate constants is very small and thus this pathway is insignificant in atmospheric conditions. The rate constants for the H-abstraction reaction channels are evaluated by using canonical variational transition state theory incorporating with the small-curvature tunneling correction. Theoretical overall rate constants are in good agreement with the available experimental values and decrease in the order of k(1) > k(2) > k(3) > k(4). The results indicate that for halogenated acetones the substitution of halogen atom (F or Cl) leads to the decrease in the C--H bond reactivity and more decrease of reactivity is caused by F-substitution.     

Hydrogen atom abstraction reactions from tertiary amines by benzyloxyl and cumyloxyl radicals: influence of structure on the rate-determining formation of a hydrogen-bonded prereaction complex

A time-resolved kinetic study on the hydrogen atom abstraction reactions from a series of tertiary amines by the cumyloxyl (CumO(?)) and benzyloxyl (BnO(?)) radicals was carried out. With the sterically hindered triisobutylamine, comparable hydrogen atom abstraction rate constants (k(H)) were measured for the two radicals (k(H)(BnO(?))/k(H)(CumO(?)) = 2.8), and the reactions were described as direct hydrogen atom abstractions. With the other amines, increases in k(H)(BnO(?))/k(H)(CumO(?)) ratios of 13 to 2027 times were observed. k(H) approaches the diffusion limit in the reactions between BnO(?) and unhindered cyclic and bicyiclic amines, whereas a decrease in reactivity is observed with acyclic amines and with the hindered cyclic amine 1,2,2,6,6-pentamethylpiperidine. These results provide additional support to our hypothesis that the reaction proceeds through the rate-determining formation of a C-H/N hydrogen-bonded prereaction complex between the benzyloxyl α-C-H and the nitrogen lone pair wherein hydrogen atom abstraction occurs, and demonstrate the important role of amine structure on the overall reaction mechanism. Additional mechanistic information in support of this picture is obtained from the study of the reactions of the amines with a deuterated benzyloxyl radical (PhCD(2)O(?), BnO(?)-d(2)) and the 3,5-di-tert-butylbenzyloxyl radical.     

Hydrogen atom reactivity toward aqueous tert-butyl alcohol

Through a combination of pulse radiolysis, purification, and analysis techniques, the rate constant for the H + (CH(3))(3)COH → H(2) + (?)CH(2)C(CH(3))(2)OH reaction in aqueous solution is definitively determined to be (1.0 ± 0.15) × 10(5) M(-1) s(-1), which is about half of the tabulated number and 10 times lower than the more recently suggested revision. Our value fits on the Polanyi-type, rate-enthalpy linear correlation ln(k/n) = (0.80 ± 0.05)ΔH + (3.2 ± 0.8) that is found for the analogous reactions of other aqueous aliphatic alcohols with n equivalent abstractable H atoms. The existence of such a correlation and its large slope are interpreted as an indication of the mechanistic similarity of the H atom abstraction from α- and β-carbon atoms in alcohols occurring through the late, product-like transition state. tert-Butyl alcohol is commonly contaminated by much more reactive secondary and primary alcohols (2-propanol, 2-butanol, ethanol, and methanol), whose content can be sufficient for nearly quantitative scavenging of the H atoms, skewing the H atom reactivity pattern, and explaining the disparity of the literature data on the H + (CH(3))(3)COH rate constant. The ubiquitous use of tert-butyl alcohol in pulse radiolysis for investigating H atom reactivity and the results of this work suggest that many other previously reported rate constants for the H atom, particularly the smaller ones, may be in jeopardy.     

Kinetic solvent effects on hydrogen abstraction reactions from carbon by the cumyloxyl radical. The importance of solvent hydrogen-bond interactions with the substrate and the abstracting radical

A kinetic study of the hydrogen atom abstraction reactions from propanal (PA) and 2,2-dimethylpropanal (DMPA) by the cumyloxyl radical (CumO?) has been carried out in different solvents (benzene, PhCl, MeCN, t-BuOH, MeOH, and TFE). The corresponding reactions of the benzyloxyl radical (BnO?) have been studied in MeCN. The reaction of CumO? with 1,4-cyclohexadiene (CHD) also has been investigated in TFE solution. With CHD a 3-fold increase in rate constant (k(H)) has been observed on going from benzene, PhCl, and MeCN to TFE. This represents the first observation of a sizable kinetic solvent effect for hydrogen atom abstraction reactions from hydrocarbons by alkoxyl radicals and indicates that strong HBD solvents influence the hydrogen abstraction reactivity of CumO?. With PA and DMPA a significant decrease in k(H) has been observed on going from benzene and PhCl to MeOH and TFE, indicative of hydrogen-bond interactions between the carbonyl lone pair and the solvent in the transition state. The similar k(H) values observed for the reactions of the aldehydes in MeOH and TFE point toward differential hydrogen bond interactions of the latter solvent with the substrate and the radical in the transition state. The small reactivity ratios observed for the reactions of CumO? and BnO? with PA and DMPA (k(H)(BnO?)/k(H)(CumO?) = 1.2 and 1.6, respectively) indicate that with these substrates alkoxyl radical sterics play a minor role.