ab initio molecular orbital and density functional analysis of acetylene + O2 reactions with CHEMKIN evaluation
ab initio molecular orbital and density functional analysis of acetylene + O2 reactions with CHEMKIN evaluation
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DOI：
10.1002/1097-4601(2000)32:10
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发表时间：
2000
影响因子：
1.5
通讯作者：
C. Sheng;J. Bozzelli
中科院分区：
文献类型：
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作者：
C. Sheng;J. Bozzelli
许多研究人员指出，乙炔与氧气的直接反应需要包括在详细的反应机理中，以模拟观察到的火焰速度和诱导时间。四个途径的乙炔氧化链增长的引发被认为是和速率常数进行比较的值中使用的机制：13 O2 + HCCH到三重态加合物和在三重态表面上的反应23 O2 + HCCH到三重态加合物，三重态加合物在反应环境中通过碰撞转化为单重态加合物，单重态加合物31 O2 + HCCH进一步反应生成单重态加合物4 HCCH异构化生成亚乙烯基，然后与3 O2发生亚乙烯基插入反应分析了O2（3 O2）在三重态表面上加成反应氧化乙炔的基本反应途径。采用从头算分子轨道和密度泛函方法计算了该体系中反应物、过渡态和产物的热力学性质。乙炔在三重态表面上的氧化反应是通过将分子氧O2（3 O2）加到碳原子上而引发的，从而形成三重态过氧乙烯双自由基。主要产物为两个甲酰自由基或乙二醛自由基加氢原子，反应途径包括三个过渡态：O2（3 π）与乙炔加成（TS 1），过氧自由基在同位碳上加成形成二环氧乙烷（TS 2），以及在三元环中的OO键断裂（TS 3）。单点QCISD（T）和B3 LYP计算与大基组进行了尝试验证的重要过渡态的势垒高度。产物形成的第二途径是通过与浴气碰撞将三重态过氧乙烯双自由基自旋转化为单线态。单线态过氧-亚乙基双自由基快速闭环形成四元环，然后断裂过氧键形成乙二醛，乙二醛进一步解离成两个甲酰基或乙二醛自由基加氢原子。通过这一途径的总正向速率常数估计为kf = 2.21 × 107T1.46e−33.1（kcal/mol）/RT。文献中的另外两个途径，HCCH + O2（1Δ）和乙炔的压力依赖性异构化为亚乙烯基，然后亚乙烯基与O2反应（3 Δ），也进行了完整性评估。对四种拟议途径中的每一种进行CHEMKIN建模，并在0.013 atm和1 atm下在35毫秒内评价这些反应的浓度分布。三重态表面上的贯通反应被评价为不重要。三重态加合物的形成与转化（通过碰撞）到单线态和亚乙烯基路径显示类似的和较低的速率比那些使用的机制，分别。我们对Benson的HCCH + O2（1Δ）途径的实现表明需要包括：（i）逆反应，（ii）初始加合物进一步反应的障碍，以及（iii）O2（1Δ）加成障碍的进一步评估。三重态加合物转化为单重态和亚乙烯基的氧化反应都是乙炔氧化反应的引发途径。© 2000 John Wiley & Sons，Inc. Int J Chem Kinet 32：623-641，2000
A number of researchers have indicated that a direct reaction of acetylene with oxygen needs to be included in detailed reaction mechanisms in order to model observed flame speeds and induction times. Four pathways for the initiation of acetylene oxidation to chain propagation are considered and the rate constants are compared with values used in the mechanisms: 1 3O2 + HCCH to triplet adduct and reaction on the triplet surface 2 3O2 + HCCH to triplet adduct, conversion of triplet adduct to singlet adduct via collision in the reaction environment, with further reaction of the singlet adduct 3 1O2 + HCCH to singlet adduct 4 Isomerization of HCCH to vinylidene and then vinylidene insertion reaction with 3O2 Elementary reaction pathways for oxidation of acetylene by addition reaction of O2(3Σ) on the triplet surface are analyzed. ab initio molecular orbital and density functional calculations are employed to estimate the thermodynamic properties of the reactants, transition states, and products in this system. Acetylene oxidation reaction over the triplet surface is initiated by addition of molecular oxygen, O2(3Σ), to a carbon atom, forming a triplet peroxy-ethylene biradical. The reaction path to major products, either two formyl radicals or glyoxal radical plus hydrogen atom, involves reaction through three transition states: O2(3Σ) addition to acetylene (TS1), peroxy radical addition at the ipso-carbon to form a dioxirane (TS2), and cleavage of OO bond in a three-member ring (TS3). Single-point QCISD(T) and B3LYP calculations with large basis sets were performed to try to verify barrier heights on important transition states. A second pathway to product formation is through spin conversion of the triplet peroxy-ethylene biradical to the singlet by collision with bath gas. Rapid ring closure of the singlet peroxy-ethylene biradical to form a four-member ring is followed by breaking of the peroxy bond to form glyoxal, which further dissociates to either two formyl radicals or a glyoxal radical plus hydrogen atom. The overall forward rate constant through this pathway is estimated to be kf = 2.21 × 107T1.46e−33.1(kcal/mol)/RT. Two additional pathways from the literature, HCCH + O2(1Δ) and pressure-dependent isomerization of acetylene to vinylidene and then vinylidene reaction with O2(3Σ), are also evaluated for completeness. CHEMKIN modeling on each of the four proposed pathways is performed and concentration profiles from these reactions are evaluated at 0.013 atm and 1 atm over 35 milliseconds. Through reaction on the triplet surface is evaluated to be not important. Formation of the triplet adduct with conversion (via collision) to a singlet and the vinylidene paths show similar and lower rates than those used in mechanisms, respectively. Our implementation of the HCCH + O2(1Δ) pathway of Benson suggests the need to include: (i) reverse reaction, (ii) barriers to further reaction of the initial adduct plus (iii) further evaluation of the O2(1Δ) addition barrier. The pathways from triplet adduct with conversion to singlet and from vinylidene are both recommended for initiation of acetylene oxidation. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 623–641, 2000