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烷基自由基反应的从头算研究:CH₃与C₂H₅的结合与歧化反应以及化学活化C₃H₈的分解

Ab initio studies of alkyl radical reactions: Combination and disproportionation reactions of CH3 with C2H5, and the decomposition of chemically activated C3H8.

作者信息

Zhu R S, Xu Z F, Lin M C

机构信息

Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.

出版信息

J Chem Phys. 2004 Apr 8;120(14):6566-73. doi: 10.1063/1.1665370.

Abstract

This paper reports the first quantitative ab initio prediction of the disproportionation/combination ratio of alkyl+alkyl reactions using CH3+C2H5 as an example. The reaction has been investigated by the modified Gaussian-2 method with variational transition state or Rice-Ramsperger-Kassel-Marcus calculations for several channels producing (1) CH4+CH2CH2, (2) C3H8, (3) CH4CH3CH, (4) H2+CH3CHCH2, (5) H2+CH3CCH3, and (6) C2H6+CH2 by H-abstraction and association/decomposition mechanisms through singlet and triplet potential energy paths. Significantly, the disproportionation reaction (1) producing CH4+C2H4 was found to occur primarily by the lowest energy path via a loose hydrogen-bonding singlet molecular complex, H3CHC2H4, with a 3.5 kcal/mol binding energy and a small decomposition barrier (1.9 kcal/mol), instead of a direct H-abstraction process. Bimolecular reaction rate constants for the formation of the above products have been calculated in the temperature range 300-3000 K. At 1 atm, formation of C3H8 is dominant below 1200 K. Over 1200 K, the disproportionation reaction becomes competitive. The sum of products (3)-(6) accounts for less than 0.3% below 1500 K and it reaches around 1%-4% above 2000 K. The predicted rate constant for the disproportionation reaction with multiple reflections above the complex well, k1=5.04 x T(0.41) exp(429/T) at 200-600 K and k1=1.96 x 10(-20) T(2.45) exp(1470/T) cm3 molecule(-1) s(-1) at 600-3000 K, agrees closely with experimental values. Similarly, the predicted high-pressure rate constants for the combination reaction forming C3H8 and its reverse dissociation reaction in the temperature range 300-3000 K, k2(infinity)=2.41 x 10(-10) T(-0.34) exp(259/T) cm3 molecule(-1) s(-1) and k(-2)(infinity)=8.89 x 10(22) T(-1.67)exp(-46 037/T) s(-1), respectively, are also in good agreement with available experimental data.

摘要

本文以CH₃⁺C₂H₅为例,首次报告了烷基+烷基反应歧化/化合比的定量从头算预测。通过改进的高斯-2方法,利用变分过渡态或Rice-Ramsperger-Kassel-Marcus计算,研究了该反应的几个通道,这些通道通过单重态和三重态势能路径,通过氢原子夺取和缔合/分解机制产生(1)CH₄+CH₂CH₂、(2)C₃H₈、(3)CH₄CH₃CH、(4)H₂+CH₃CHCH₂、(5)H₂+CH₃CCH₃和(6)C₂H₆+CH₂。值得注意的是,发现生成CH₄+C₂H₄的歧化反应(1)主要通过最低能量路径发生,该路径经由一个具有3.5千卡/摩尔结合能和小分解势垒(1.9千卡/摩尔)的松散氢键单重态分子复合物H₃CHC₂H₄,而不是直接的氢原子夺取过程。已计算了上述产物形成的双分子反应速率常数,温度范围为300 - 3000K。在1个大气压下,低于1200K时C₃H₈的形成占主导。超过1200K,歧化反应变得具有竞争力。产物(3)-(6)的总和在1500K以下占比不到0.3%,在2000K以上达到约1% - 4%。对于在复合物阱上方有多次反射的歧化反应,预测的速率常数在200 - 600K时k1 = 5.04×T(0.41) exp(429/T),在600 - 3000K时k1 = 1.96×10⁻²⁰ T(2.45) exp(1470/T) cm³ molecule⁻¹ s⁻¹,与实验值密切吻合。同样,在300 - 3000K温度范围内,预测的形成C₃H₈的化合反应及其逆解离反应的高压速率常数,分别为k2(∞)=2.41×10⁻¹⁰ T⁻⁰.³⁴ exp(259/T) cm³ molecule⁻¹ s⁻¹和k⁻²(∞)=8.89×10²² T⁻¹.⁶⁷ exp(⁻46 037/T) s⁻¹,也与现有实验数据高度吻合。

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