C2H+H2CO: a new route for formaldehyde removal

The title unknown reaction is theoretically studied at various levels to probe the interaction mechanism between the ethynyl radical (HC triple bond C) and formaldehyde (H(2)C double bond O). The most feasible pathway is a barrier-free direct H-abstraction process leading to acetylene and formyl radical (C(2)H(2)+HCO) via a weakly bound complex, and then the product can take secondary dissociation to the final product C(2)H(2)+CO+H. The C-addition channel leading to propynal plus H-atom (HCCCHO+H) has the barrier of only 3.6, 2.9, and 2.1 kcal/mol at the CCSD(T)/6-311+G(3df,2p)MP2//6-311G(d,p)+ZPVE, CCSD(T)/6-311+G(3df,2p)//QCISD/6-311G(d,p)+ZPVE, and G3//MP2 levels, respectively [CCSD(T)--coupled cluster with single, double, and triple excitations; ZPVE--zero-point vibrational energy; QCISD--quadratic configuration interaction with single and double excitations; G3//MP2-Gaussian-3 based on Moller-Plesset geometry]. The O addition also leading to propynal plus H atom needs to overcome a higher barrier of 5.3, 8.7, and 3.0 kcalmol at the three corresponding levels. The title no-barrier reaction presents a new efficient route to remove the pollutant H(2)CO, and should be included in the combustion models of hydrocarbons. It may also represent the fastest radical-H(2)CO reaction among the available theoretical data. Moreover, it could play an important role in the interstellar chemistry where the zero- or minute-barrier reactions are generally favored. Discussions are also made on the possible formation of the intriguing propynal in space via the title reaction on ice surface.     

Mechanistic and kinetic study of the O + CH2OH reaction

The mechanism for the O + CH2OH reaction was investigated by various ab initio quantum chemistry methods. For the chemical activation mechanism, that is, the addition/elimination path, the couple-cluster methods including CCSD and CCSD(T) were employed with the cc-pVXZ (X = D, T, Q, 5) basis sets. For the abstraction channels, multireference methods including CASSCF, CASPT2, and MRCISD were used with the cc-pVDZ and cc-pVTZ basis sets. It has been shown that the production of H + HCOOH is the major channel in the chemical activation mechanism. The minor channels include HCO + H2O and OH + CH2O. The hydrogen abstraction by an O atom from the CH2OH radical produces either OH + CH2O or OH + HCOH. Moreover, the two abstraction reactions are essentially barrierless processes. The rate constants for the association of O with CH2OH have been calculated using the flexible transition state theory. A weak negative temperature dependence of the rate constants is found in the range 250-1000 K. Furthermore, it is estimated that the abstraction processes also play an important role in the O + CH2OH reaction. Additionally, the falloff behavior for the OCH2OH --> H + HCOOH reaction has been investigated. The present theoretical results are compared to the experimental measurements to understand the mechanism and kinetic behavior of the O + CH2OH reaction and the unimolecular reaction of the OCH2OH radical.     

Criegee中间体RCHOO(R=H,CH_3)与NCO的反应机理

采用CCSD(T)/aug-cc-p VTZ//B3LYP/6-311+G(2df,2p)方法对Criegee中间体RCHOO(R=H,CH_3)与NCO反应的机理进行了研究,利用经典过渡态理论(TST)并结合Eckart校正模型计算了标题反应在298~500 K范围内优势通道的速率常数.结果表明,上述反应包含亲核加成、氧化和抽氢3类机理,其中每类又包括NCO中N和O分别进攻的两种形式.亲核加成反应中O端进攻为优势通道,氧化和抽氢反应则是N端进攻为优势通道;甲基取代使CH_3CHOO反应活性高于CH2OO;anti-CH_3CHOO的加成及氧化反应活性高于syn-CH_3CHOO,而抽氢反应则是syn-CH_3CHOO的活性高于anti-CH_3CHOO.anti-构象对总速率常数的贡献大于syn-构象,且总速率常数具有显著的负温度效应.     

用分子束研究.F(^2P)与CH3F和CH3Cl的化学反应

近年来, 我们在研究含氟烯烃和烷烃的红外激光诱导氧化和氯化反应的基础上, 深入研究了红外激光诱导卤代烷烃的脱卤化氢并生成: CF2卡宾和:CFCF3卡宾的反应[1-4]以往的研究往往是根据反应产物推论反应机理, 认为在反应过程中存在着卡宾中间体,但在实验中未能直接检测到. Kakimoto[5,6]曾报道过在流动体系中测到了.F+CH3F和.F+CH3Cl反应中:CHF和:CHCl的激光荧光激发谱, 但没有讨论卡宾形成的机理.Hirota[7]在讨论.F+CH3F反应时, 认为:CHF可能由攫氢过程产生而对于.F+CH3Cl反应同时生成:CHF和:CHCl未做说明. 本实验中用扩散分子束代替了流动反应体系, 从而大大减少了产物和反应物气体分子间的猝灭过程, 获得了信噪比大而清晰的图谱, 由此确证了:CHF和:CHCl的存在, 说明了.F+CH3Cl反应中自由基攫氢过程和偶合反应过程共存的反应历程. 这一结论对红外激光诱导一碳卤代宾化学反应机理研究有重要参考意义.     

Atomic radical—molecule reactions F + CH<Subscript>3</Subscript>C≡CH: mechanistic study

The reaction of atomic radical F with propyne has been studied theoretically using ab initio quantum chemistry methods and transition state theory. The potential energy surface was calculated at the CCSD(T)/aug-cc-pVDZ (single-point) level using the UMP2/6-311++G(d,p) optimized structures. Two reaction mechanisms including the addition–isomerization–elimination reaction mechanism and the directed hydrogen abstraction reaction mechanism are considered. For the hydrogen abstraction reactions, i.e., the most probable evolution pathway in the title reaction, the HF formation occurs via direct abstraction mechanism dominantly and the H atom picked up by the atomic radical F should come mostly from the methyl group of normal propyne. On the other hand, for the addition–isomerization–elimination mechanism, the most feasible pathway should be the atomic radical F attacking on the C≡C triple bond in propyne (CH₃C≡CH) to form a weakly-bound adduct A1 with no barrier, followed by F addition to the C≡C triple bond to form the low-lying intermediate isomer 5. Subsequently, isomer 5 directly dissociates to P3 H₂CCCHF + H via transition state TS₅/P3. The other reaction pathways on the doublet PES are less competitive due to thermodynamical or kinetic factors. Furthermore, based on the analysis of the kinetics of all channels through which the addition and abstraction reaction proceed, we expect that the competitive power of reaction channels may vary with experimental conditions for the title reaction. The present work will provide useful information for understanding the processes of atomic radical F reaction with other unsaturated hydrocarbons. This material is available from author via E-mail.     

A crossed beam and ab initio investigation on the formation of vinyl boron monoxide (C2H3BO; X1A') via reaction of boron monoxide (11BO; X2Σ+) with ethylene (C2H4; X1A(g))

The reaction dynamics of the boron monoxide radical ((11)BO; X(2)Σ(+)) with ethylene (C(2)H(4); X(1)A(g)) were investigated at a nominal collision energy of 12.2 kJ mol(-1) employing the crossed molecular beam technique and supported by ab initio and statistical (RRKM) calculations. The reaction is governed by indirect scattering dynamics with the boron monoxide radical attacking the carbon-carbon double bond of the ethylene molecule without entrance barrier with the boron atom. This addition leads to a doublet radical intermediate (O(11)BH(2)CCH(2)), which either undergoes unimolecular decomposition through hydrogen atom emission from the C1 atom via a tight transition state located about 13 kJ mol(-1) above the separated products or isomerizes via a hydrogen shift to the O(11)BHCCH(3) radical, which also can lose a hydrogen atom from the C1 atom. Both processes lead eventually to the formation of the vinyl boron monoxide molecule (C(2)H(3)BO; X(1)A'). The overall reaction was determined to be exoergic by about 40 kJ mol(-1). The reaction dynamics are also compared to the isoelectronic ethylene (C(2)H(4); X(1)A(g)) - cyano radical (CN; X(2)Σ(+)) system studied earlier.     

Density functional theory investigation of the reactions of isodihalomethanes (CH(2)X-X where X = Cl, Br, or I) with ethylene: substituent effects on the carbenoid behavior of the CH(2)X-X species

We investigated the chemical reactions of isodihalomethane (CH(2)X-X) and CH(2)X radical species (where X = Cl, Br, or I) with ethylene and the isomerization reactions of CH(2)X-X using density functional theory calculations. The CH(2)X-X species readily reacts with ethylene to give the cyclopropane product and an X(2) product via a one-step reaction with barrier heights of approximately 2.9 kcal/mol for CH(2)I-I, 6.8 kcal/mol for CH(2)Br-Br, and 8.9 kcal/mol for CH(2)Cl-Cl. The CH(2)X reactions with ethylene proceed via a two-step reaction mechanism to give a cyclopropane product and X atom product with much larger barriers to reaction. This suggests that photocyclopropanation reactions using ultraviolet excitation of dihalomethanes most likely occurs via the isodihalomethane species and not the CH(2)X species. The isomerization reactions of CH(2)X-X had barrier heights of approximately 14.4 kcal/mol for CH(2)I-I, 11.8 kcal/mol for CH(2)Br-Br, and 9.1 kcal/mol for CH(2)Cl-Cl. We compare our results for the CH(2)X-X carbenoids to results from previous calculations of the Simmons-Smith-type carbenoids (XCH(2)ZnX) and Li-type carbenoids (LiCH(2)X) and discuss their differences and similarities as methylene transfer agents.     

Theoretical study of the C6H3 potential energy surface and rate constants and product branching ratios of the C2H(2Sigma+) + C4H2(1Sigma(g)+) and C4H(2Sigma+) + C2H2(1Sigma(g)+) reactions

Ab initio CCSD(T)cc-pVTZ//B3LYP6-311G(**) and CCSD(T)/complete basis set (CBS) calculations of stationary points on the C(6)H(3) potential energy surface have been performed to investigate the reaction mechanism of C(2)H with diacetylene and C(4)H with acetylene. Totally, 25 different C(6)H(3) isomers and 40 transition states are located and all possible bimolecular decomposition products are also characterized. 1,2,3- and 1,2,4-tridehydrobenzene and H(2)CCCCCCH isomers are found to be the most stable thermodynamically residing 77.2, 75.1, and 75.7 kcal/mol lower in energy than C(2)H + C(4)H(2), respectively, at the CCSD(T)/CBS level of theory. The results show that the most favorable C(2)H + C(4)H(2) entrance channel is C(2)H addition to a terminal carbon of C(4)H(2) producing HCCCHCCCH, 70.2 kcal/mol below the reactants. This adduct loses a hydrogen atom from the nonterminal position to give the HCCCCCCH (triacetylene) product exothermic by 29.7 kcal/mol via an exit barrier of 5.3 kcal/mol. Based on Rice-Ramsperger-Kassel-Marcus calculations under single-collision conditions, triacetylene+H are concluded to be the only reaction products, with more than 98% of them formed directly from HCCCHCCCH. The C(2)H + C(4)H(2) reaction rate constants calculated by employing canonical variational transition state theory are found to be similar to those for the related C(2)H + C(2)H(2) reaction in the order of magnitude of 10(-10) cm(3) molecule(-1) s(-1) for T = 298-63 K, and to show a negative temperature dependence at low T. A general mechanism for the growth of polyyne chains involving C(2)H + H(C[triple bond]C)(n)H --> H(C[triple bond]C)(n+1)H + H reactions has been suggested based on a comparison of the reactions of ethynyl radical with acetylene and diacetylene. The C(4)H + C(2)H(2) reaction is also predicted to readily produce triacetylene + H via barrierless C(4)H addition to acetylene, followed by H elimination.     

Formation of the 2,4-pentadiynyl-1 radical (H2CCCCCH, X2B1) in the crossed beams reaction of dicarbon molecules with methylacetylene

The chemical dynamics to synthesize the 2,4-pentadiynyl-1 radical, HCCCCCH(2)(X(2)B(1)), via the neutral-neutral reaction of dicarbon with methylacetylene, was examined in a crossed molecular beams experiment at a collision energy of 37.6 kJ mol(-1). The laboratory angular distribution and time-of-flight spectra of the 2,4-pentadiynyl-1 radical and its fragmentation patterns were recorded at m/z = 63-60 and m/z = 51-48. Our findings suggest that the reaction dynamics are indirect and dictated by an initial attack of the dicarbon molecule to the pi electron density of the methylacetylene molecule to form cyclic collision complexes. The latter ultimately rearranged via ring opening to methyldiacetylene, CH(3)-C triple bond C-C triple bond C-H. This structure decomposed via atomic hydrogen emission to the 2,4-pentadiynyl-1 radical; here, the hydrogen atom was found to be emitted almost parallel to the total angular momentum as suggested by the experimentally observed sideways scattering. The overall reaction was strongly exoergic by 182 +/- 10 kJ mol(-1). The identification of the resonance-stabilized free 2,4-pentadiynyl-1 radical represents a solid background for the title reaction to be included into more refined reaction networks modeling the chemistry of circumstellar envelopes and also of sooting combustion flames.     

AIM Study on the Reaction of CH2SH Radical with Fluorine Atom

The reaction of CH2SH radical with fluorine atom was studied at the levels of B3LYP/6-311G(d,p) and MP2(Full)/6-311G(d,p). The computational results show that the reaction has three channels and proceeds by the addition of fluorine atoms on carbon or sulfur sites of CH2SH, forming initial intermediates. The calculated results show that the channel in which fluorine attaches to the carbon atom to form CH2S and HF, is the most likely reaction pathway. Topological analysis of electron density was carried out for the three channels. The change trends of the chemical bonds on the reaction paths were discussed. The energy transition states and the structure transition regions (states) of the three channels were found. The calculated results show that the structure transition regions are broad in unobvious exothermic reactions or unobvious endothermic reactions, and are narrow in obvious exothermic reactions or obvious endothermic reactions.     

Chemo- and periselectivity in the addition of [OsO2(CH2)2] to ethylene: a theoretical study

Quantum chemical calculations by using density functional theory at the B3LYP level have been carried out to elucidate the reaction course for the addition of ethylene to [OsO2(CH2)2] (1). The calculations predict that the kinetically most favorable reaction proceeds with an activation barrier of 8.1 kcal mol(-1) via [3+2] addition across the O=Os=CH2 moiety. This reaction is -42.4 kcal mol(-1) exothermic. Alternatively, the [3+2] addition to the H2C=Os=CH2 fragment of 1 leads to the most stable addition product 4 (-72.7 kcal mol(-1)), yet this process has a higher activation barrier (13.0 kcal mol(-1)). The [3+2] addition to the O=Os=O fragment yielding 2 is kinetically (27.5 kcal mol(-1)) and thermodynamically (-7.0 kcal mol(-1)) the least favorable [3+2] reaction. The formal [2+2] addition to the Os=O and Os=CH2 double bonds proceeds by initial rearrangement of 1 to the metallaoxirane 1 a. The rearrangement 1-->1 a and the following [2+2] additions have significantly higher activation barriers (>30 kcal mol(-1)) than the [3+2] reactions. Another isomer of 1 is the dioxoosmacyclopropane 1 b, which is 56.2 kcal mol(-1) lower in energy than 1. The activation barrier for the 1-->1 b isomerization is 15.7 kcal mol(-1). The calculations predict that there are no energetically favorable addition reactions of ethylene with 1 b. The isomeric form 1 c containing a peroxo group is too high in energy to be relevant for the reaction course. The accuracy of the B3LYP results is corroborated by high level post-HF CCSD(T) calculations for a subset of species.     

Crossed molecular beam studies of the reactions of allyl radicals, C3H5(X2A2), with methylacetylene (CH3CCH(X1A1)), allene (H2CCCH2(X1A1)), and their isotopomers

The chemical dynamics of the reaction of allyl radicals, C(3)H(5)(X(2)A(2)), with two C(3)H(4) isomers, methylacetylene (CH(3)CCH(X(1)A(1))) and allene (H(2)CCCH(2)(X(1)A(1))) together with their (partially) deuterated counterparts, were unraveled under single-collision conditions at collision energies of about 125 kJ mol(-1) utilizing a crossed molecular beam setup. The experiments indicate that the reactions are indirect via complex formation and proceed via an addition of the allyl radical with its terminal carbon atom to the terminal carbon atom of the allene and of methylacetylene (alpha-carbon atom) to form the intermediates H(2)CCHCH(2)CH(2)CCH(2) and H(2)CCHCH(2)CHCCH(3), respectively. The lifetimes of these intermediates are similar to their rotational periods but too short for a complete energy randomization to occur. Experiments with D4-allene and D4-methylacetylene verify explicitly that the allyl group stays intact: no hydrogen emission was observed but only the release of deuterium atoms from the perdeuterated reactants. Further isotopic substitution experiments with D3-methylacetylene combined with the nonstatistical nature of the reaction suggest that the intermediates decompose via hydrogen atom elimination to 1,3,5-hexatriene, H(2)CCHCH(2)CHCCH(2), and 1-hexen-4-yne, H(2)CCHCH(2)CCCH(3), respectively, via tight exit transition states located about 10-15 kJ mol(-1) above the separated products. The overall reactions were found to be endoergic by 98 +/- 4 kJ mol(-1) and have characteristic threshold energies to reaction between 105 and 110 kJ mol(-1). Implications of these findings to combustion and interstellar chemistry are discussed.     

Theoretical study of the reactions CF3CH2OCHF2 + OH/Cl and its product radicals and parent ether(CH3CH2OCH3) with OH

A dual-level direct dynamic method is employed to study the reaction mechanisms of CF3CH2OCHF2 (HFE-245fa2; HFE-245mf) with the OH radicals and Cl atoms. Two hydrogen abstraction channels and two displacement processes are found for each reaction. For further study, the reaction mechanisms of its products (CF3CH2OCF2 and CF3CHOCHF2) and parent ether CH3CH2OCH3 with OH radical are investigated theoretically. The geometries and frequencies of all the stationary points and the minimum energy paths (MEPs) are calculated at the B3LYP/6-311G(d,p) level. The energetic information along the MEPs is further refined at the G3(MP2) level of theory. For reactions CF3CH2OCHF2 + OH/Cl, the calculation indicates that the hydrogen abstraction from --CH2-- group is the dominant reaction channel, and the displacement processes may be negligible because of the high barriers. The standard enthalpies of formation for the reactant CF3CH2OCHF2, and two products CF3CH2OCHF2 and CF3CHOCHF2 are evaluated via group-balanced isodesmic reactions. The rate constants of reactions CF3CH2OCHF2 + OH/Cl and CH3CH2OCH3 + OH are estimated by using the variational transition state theory over a wide range of temperature (200-2000 K). The agreement between the theoretical and experimental rate constants is good in the measured temperature range. From the comparison between the rate constants of the reactions CF3CH2OCHF2 and CH3CH2OCH3 with OH, it is shown that the fluorine substitution decreases the reactivity of the C--H bond.     

C2H与HO2双自由基反应的密度泛函理论研究

应用量子化学从头算和密度泛函理论(DFT)对C₂H与HO₂双自由基的单重态反应进行了研究.在UB3LYP/6-311G水平上优化了反应通道上各驻点(反应物、中间体、过渡态和产物)的几何构型.在CCSD(T)/6-311G^{**水平上计算了各物种的单点能,并对总能量进行了零点能校正.研究结果表明,反应物中自由基C₂H的边端C进攻自由基HO₂的边端O是主要的进攻方式.首先形成了中间体1(HCCOOH),由此经过不同的反应通道可以得到主要产物P1,次要产物P2,P3和P5.生成P1的反应热为-814.40kJ/mol.自由基C₂H的中间C进攻自由基HO₂的边端O是次要的进攻方式,可以得到产物P4和P6.根据势能面分析,所有反应均是放热反应.}     

Direct ab initio dynamics study of the reaction of C2(A3Πu) with CH4

The reaction of C₂(A³Π_u) with CH₄ has been investigated over a wide temperature range 200–3,000 K by direct ab initio dynamics method at the BMC‐CCSD//BB1K/6‐311+G(2d,2p) level of theory. The optimized geometries and frequencies of the stationary points are calculated at the BB1K/6‐311+G(2d,2p) level, and then the energy profiles of the reactions are refined using the BMC‐CCSD method. The activation barrier height for H‐abstraction reaction was calculated to be 4.44 kcal/mol in temperature range (337–605 K), and the electron transfer behavior was also analyzed by quasi‐restricted molecular orbital method in detail. The canonical variational transition‐state theory (CVT) with the small curvature tunneling (SCT) correction method is used to calculate the rate constants over a wide temperature range 200–3,000 K. The theoretical results shows that variational effect is to some extent large in lower temperature range, and small curvature and tunneling effect play important roles to the H‐atom abstraction only at lower temperatures. The CVT/SCT rate constants are in good agreement with the available experimental results. Our theoretical study is expected to provide a direct insight into the reaction mechanism and may be useful for estimating the kinetics of the title reaction over a wide temperature range where no experimental data are available so far. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Theoretical study of the reaction mechanism of HCN+ and CH4 of relevance to Titan's ion chemistry

Titan is the largest satellite of Saturn. In its atmosphere, CH4 is the most abundant neutral after nitrogen. In this paper, the complex doublet potential-energy surface related to the reaction between HCN+ and CH4 is investigated at the B3LYP/6-311G(d,p), CCSD(T)/6-311G++(3df,2pd)(single-point), and QCISD/6-311G(d,p) computational levels. A total of seven products are located on the PES. The initial association of HCN+ with CH4 is found to be a prereaction complex 1 (HCNHCH3(+)) without barrier. Starting from 1, the most feasible pathway is the direct H-abstraction process (the internal C-H bond dissociation) leading to the product P1 (HCNH++CH3). By C-C addition, prereaction complex 1 can form intermediate 2 (HNCHCH3(+)) and then lead to the product P2 (CH3CNH++H). The rate-controlling step of this process is only 25.6 kcal/mol. It makes the Path P2 (1) R --> 1 --> TS1/2 --> 2 --> TS2/P2 --> P2 another possible way for the reaction. P3 (HCNCH3(+) + H), P5 (cNCHCH2(+) + H2), and P6 (NCCH3(+) + H2) are exothermic products, but they have higher barriers (more than 40.0 kcal/mol); P4 (H + HCN + CH3(+)) and P7 (H + H2 + HCCNH+) are endothermic products. They should be discovered under different experimental or interstellar conditions. The present study may be helpful for investigating the analogous ion-molecule reaction in Titan's atmosphere.     

Reaction dynamics on the formation of styrene: a crossed molecular beam study of the reaction of phenyl radicals with ethylene

The reaction dynamics of phenyl radicals (C6H5) with ethylene (C2H4) and D4-ethylene (C2D4) were investigated at two collision energies of 83.6 and 105.3 kJ mol-1 utilizing a crossed molecular beam setup. The experiments suggested that the reaction followed indirect scattering dynamics via complex formation and was initiated by an addition of the phenyl radical to the carbon-carbon double bond of the ethylene molecule forming a C6H5CH2CH2 radical intermediate. Under single collision conditions, this short-lived transient species was found to undergo unimolecular decomposition via atomic hydrogen loss through a tight exit transitions state to synthesize the styrene molecule (C6H5C2H3). Experiments with D4-ethylene verified that in the corresponding reaction with ethylene the hydrogen atom was truly emitted from the ethylene unit but not from the phenyl moiety. The overall reaction to form styrene plus atomic hydrogen from the reactants was found to be exoergic by 25 +/- 12 kJ mol(-1). This study provides solid evidence that in combustion flames the styrene molecule, a crucial precursor to form polycyclic aromatic hydrocarbons (PAHs), can be formed within a single neutral-neutral collision, a long-standing theoretical prediction which has remained to be confirmed by laboratory experiments under well-defined single collision conditions for the last 50 years.     

Radical-molecule reactions HCO/HOC + C2H2: mechanistic study

A detailed computational study is performed on the unknown radical-molecule reactions between HCO/HOC and acetylene (C2H2) at the CCSD(T)/6-311G(2d,p)//B3LYP/6-311G(d,p)+ZPVE, Gaussian-3//B3LYP/6-31G(d), and Gaussian-3//MP2(full)/6-31G(d) levels. For the HCO + C2H2 reaction, the most favorable pathway is direct C-addition forming the intermediate HC=CHCH=O followed by a 1,3-H-shift leading to H2C=CHC=O, which finally dissociates to the product C2H3 + CO. The overall reaction barrier is 13.8, 10.5, and 11.3 kcal/mol, respectively, at the three levels. The quasi-direct H-donation process to produce C2H3 + CO with barriers of 14.0, 14.1, and 14.1 kcal/mol is less competitive. Thus only at higher temperatures could the HCO + C2H2 reaction play a role. In contrast, the HOC + C2H2 reaction can barrierlessly generate C2H3 + CO via the quasi-direct H-donation mechanism proceeding via a prereactive complex with OH...C2 hydrogen bonding. This is suggestive of the potential importance of the HOC + C2H2 reaction in both combustion and interstellar processes. However, the direct C-addition channel is much less competitive. For both reactions, the possible formation of the intriguing interstellar molecules propadiene and propynal is also discussed. The present theoretical study represents the first attempt to probe the reaction mechanism between HOC and pi-systems. Future laboratory investigations on both reactions (particularly HOC + C2H2) are recommended.     

A mechanistic study of the 2-thienylmethyl + HO2 radical recombination reaction

Radical recombination reactions are important in the combustion of fuel oils. Shale oil contains alkylated heteroaromatic species, the simplest example of which is the 2-thienylmethyl radical. The ab initio potential energy surface for the reaction of the 2-thienylmethyl radical with the HO(2) radical has been examined. Seventeen product channels corresponding to either addition/elimination or direct hydrogen abstraction have been characterized for the first time. Direct hydrogen abstract from HO(2) proceeds via a weakly bound van der Waals complex, which leads to 2-methylthiophene, 2-methylene-2,3-dihydrothiophene, or 2-methylene-2,5-dihydrothiophene depending upon the 2-thienylmethyl radical reaction site. The addition pathway for the two radical reactants is barrierless with the formation of three adducts, as distinguished by HO(2) reaction at three different sites on the 2-thienylmethyl radical. The addition is exothermic by 37-55 kcal mol(-1) relative to the entrance channel, and these excess energies are available to promote further decomposition or rearrangement of the adducts, leading to nascent products such as H, OH, H(2)O, and CH(2)O. The reaction surfaces are characterized by relatively low barriers (most lower than 10 kcal mol(-1)). Upon the basis of a careful analysis of the overall barrier heights and reaction exothermicities, the formations of O(2), OH, and H(2)O are likely to be important pathways in the radical recombination reactions of 2-thienylmethyl + HO(2).