### 烷基过氧化氢中氢提取反应类大分子体系的反应能垒与速率常数的精确计算

1. a 四川大学化学学院 成都 610064;
b 四川大学化学工程学院 成都 610065
• 收稿日期:2018-01-12 出版日期:2018-04-15 发布日期:2018-02-26
• 通讯作者: 李泽荣 E-mail:lizerong@scu.edu.cn
• 基金资助:

项目受国家自然科学基金（No.91641120）资助.

### Accurate Calculation of the Energy Barriers and Rate Constants of the Large-size Molecular Reaction System for Abstraction from Alkyl Hydroperoxides

Chen Fangfanga, Sun Xiaohuia, Yao Qiana, Li Zeronga, Wang Jingbob, Li Xiangyuanb

1. a College of Chemistry, Sichuan University, Chengdu 610064;
b College of Chemical Engineering, Sichuan University, Chengdu 610065
• Received:2018-01-12 Online:2018-04-15 Published:2018-02-26
• Contact: 10.6023/A18010015 E-mail:lizerong@scu.edu.cn
• Supported by:

Project supported by the National Natural Science Foundation of China (No. 91641120).

The reaction class of a free radical with a molecule are non-elementary reactions with negative activation energies and they are usually proceeded through two reaction steps with the first step being a reactant complex formation. This class of reactions are widespread in the atmospheric chemistry and the mechanism of hydrocarbon fuel combustion, so they are extensively studied in the theoretical calculation and experimental studies. The reaction class of α-H abstraction from alkyl hydroperoxides (ROOH) by hydroxyl radicals, which are important in the mechanism of hydrocarbon fuel combustion, are chosen as the object of this study. The regularity of this reaction class are revealed by quantum chemical calculations and their kinetic parameters are accurately calculated. When the standard molar Gibbs free energy change of the formation of the reactant complex in the first step is equal to zero, the corresponding temperature is defined as the conversion temperature Tc in this study, and it is shown that a steady state approximation method are applicable for this kind of reaction system to calculate the overall reaction rate constants when the temperature is much higher than the Tc. Geometric optimization and frequency analysis for all species were conducted at the BHandHLYP/6-311G(d,p) level. Five reactions are chosen as the representative for the reaction class and their single point energies are calculated using the method of CCSD(T)/CBS and it is shown that the highest conversion temperature for the five reactions is 195.17 K, far below usual modeling lowest temperature of the hydrocarbon fuel combustion, and therefore, the steady state approximation method is reasonable. It is also shown that the reaction-center geometries of the transition states are conserved, and thus the isodesmic reaction method is applicable to this reaction class to correct the energy barriers and rate constants at low-level BHandHLYP method. The obtained energy barriers are compared with the results using high-level ab initio CCSD(T)/CBS method and it is shown that the maximum absolute deviation of reaction energy barriers can be reduced from 19.99 kJ·mol-1 before correction to 1.47 kJ·mol-1 after correction, indicating that the isodesmic reaction method are applicable for the accurate calculation of the kinetic parameters for large-size molecular systems with the negative activation energy reaction. Finally, energy barriers for 20 reactions in the class are calculated with the isodesmic reaction method, and then based on steady state approximation, the rate constants for the overall reactions are calculated using the transition state theory in combination with the isodesmic correction scheme. It is shown that the negative activation energy relationship for the reaction class only exists in the low temperature region.