| The mechanism and kinetics of the oxidation degradation reactions of cresols (o-cresol, m-cresol and p-cresol) in the troposphere are investigated using density functional theory (DFT) and the transition state theory (TST).The research includes as follows:Ⅰ. The mechanism and kinetics for addition and hydrogen abstraction reaction of cresols with OH radicalReaction mechanisms for the OH-initiated oxidation of cresols in the troposphere were investigated using B3LYP hybrid density functional method with the 6-311++G(2df,2pd) basis set. Single-point energy calculations with the same basis set were performed using the coupled cluster method with single, double, and perturbative triple configurations, CCSD(T). The equilibrium geometries, energies, and thermodynamics properties of all the stationary points along the addition reaction pathway and hydrogen abstraction reaction pathway were calculated. The rate constants and the branching ratios of each pathway were evaluated using classical transition state theory (TST) in the temperature range of 200 to 360K, to simulated temperatures in all parts of the troposphere. The calculated rate constants are in good agreement with available experimental values. The addition reaction and hydrogen abstraction reaction are both thermodynamically irreversible.1. The ortho (at position ortho- to the hydroxyl group) addition pathway was found dominant for the reaction of o-cresol and OH radical, accounting for 95.3-75.0% of the overall products from 200 K to 360K. The 6-adduct is the most stable among all adduct products, followed by 2-adduct. They existed hydrogen bonds and their energies are -100.46 and -93.50 kJ·mol-1 at CCSD(T) level, respectively. The calculated rate constants value is in the range 8.88×10-126.56×10-11cm3 molecule-1 s-1. The branching ratio of hydrogen abstraction reaction accounts for 2.6-12.8% in the temperature range of 200 K to 360K.2. The ortho (at position ortho- to the hydroxyl group) addition pathway was found dominant for the reaction of m-cresol and OH radical, accounting for 95.787.5% of the overall products from 200 K to 360K. The 1-adduct is the most stable among all adduct products, followed by 2-adduct and 6-adduct. They existed hydrogen bonds and their energies are -97.21, -93.07 and -89.02 kJ·mol-1at CCSD(T) level, respectively. The calculated rate constants value is in the range in 1.44×10-116.37×10-10cm3 molecule-1 s-1. The branching ratio of hydrogen abstraction reaction accounts for 0.00.6% in the temperature range of 200 K to 360K.3. The ortho (at position ortho- to the hydroxyl group) addition pathway was found dominant for the reaction of p-cresol and OH radical, accounting for 99.03-91.78% of the overall products from 200 K to 360K. The 1-adduct is the most stable among all adduct products, followed by 2-adduct. They existed hydrogen bonds and their energies are -95.52 and -95.22kJ·mol-1 at CCSD(T) level, respectively. The calculated rate constants value is in the range in 1.09×10-11-3.27×10-10 cm3 molecule-1 s-1. The branching ratio of hydrogen abstraction reaction accounts for 0.00.1.1% in the temperature range of 200 K to 360K.This might be due to the fact that the OH has electrophilic character and it is expected to preferentially add to an ortho position. Hydrogen abstraction pathways of OH-initiated oxidation of o-cresol are also important reaction pathway in the troposphere. However, the hydrogen abstraction pathways of OH-initiated oxidation of m-cresol and p-cresol are negative reaction pathway in the troposphere.Ⅱ. The addition and hydrogen abstraction for the main cresol-OH·adducts with O2 The reaction mechanism and kinetics of adducts with O2 are investigated using DFT method. There are cis- and trans- addition reaction pathways, and hydrogen abstraction pathway. The equilibrium geometries, energies, and thermodynamics properties of all the stationary points along the addition reaction pathway and hydrogen abstraction reaction pathway were calculated. The rate constants and the branching ratios of each pathway were evaluated using classical transition state theory (TST) in the temperature range of 200K to 360K, to simulated temperatures in all parts of the troposphere.1. The main products are 2-adduct and 6-adduct for the reaction o-cresol with OH radical. The Cis-2O5 and CIS-2O1 addition pathways were found dominant for the ]reaction of 2-adduct and O2, accounting for 53.139.7% and 40.037.7% of the overall products from 200 K to 360K, respectively. The Cis-2O3-add is the most stable among all adduct products because it existed hydrogen bond. The calculated rate constants value is in the range in 2.68×10-161.52×10-15 cm3 molecule-1 s-1 in the temperature range of 200K to 360K. The branching ratio of hydrogen abstraction reaction accounts for 4.911.8% in the temperature range of 200 K to 360K.The Cis-6O3 addition pathway was found dominant for the reaction of 6-adduct and O2, accounting for 70.048.6% of the overall products from 200 K to 360K, respectively. The Cis-6O5-add is the most stable among all adduct products because it existed hydrogen bond. The calculated rate constants value is in the range in 7.51×10-177.41×10-16 cm3 molecule-1 s-1 in the temperature range of 200K to 360K.2. The main products are 2-adduct and 6-adduct for the reaction m-cresol with OH radical. The Cis-2M5 addition pathway was found dominant for the reaction of 2-adduct and O2, accounting for 78.7-39.3% of the overall products from 200 K to 360K, respectively. The Cis-2M5 is the most stable among all adduct products because it existed hydrogen bond. The calculated rate constants value is in the range in 7.98×10-161.91×10-15 cm3 molecule-1 s-1 in the temperature range of 200K to 360K. The branching ratios of cis-, trans- and hydrogen abstraction reactions are 88.261.6%, 10.030.5% and 1.87.9%, respectively.The Cis-6M3 addition pathway was found dominant for the reaction of 6-adduct and O2, accounting for 75.835.9% of the overall products from 200 K to 360K, respectively. The Cis-6M3-add is the most stable among all adduct products because it existed hydrogen bond. The calculated rate constants value is in the range in 6.80×10-163.43×10-15 cm3 molecule-1 s-1 in the temperature range of 200K to 360K. The branching ratios of cis-, trans- and hydrogen reactions are 88.9-57.2%, 8.1-34.5% and 3.0-8.3%, respectively.3. The main product is 2-adduct and 6-adduct for the reaction p-cresol with OH radical. The Cis-2P5 addition pathway was found dominant for the reaction of 2-adduct and O2, accounting for 92.4-64.4% of the overall products from 200 K to 360K. The Cis-2P3-add is the most stable among all adduct products because it existed hydrogen bond. The calculated rate constants value is in the range in 6.92×10-16 1.92×10-15 cm3 molecule-1 s-1 in the temperature range of 200K to 360K. The branching ratios of cis-, trans- and hydrogen reactions are 97.9-81.6%, 1.2-11.8% and 0.9-6.6%, respectively. The main pathway is cis- addition for the reaction of the main adducts with O2, in the temperature range of 200 to 360 K. The branching ratio of cis- addition is negatively dependent on temperature, and the branching ratio of trans- addition is positively dependent on temperature, while the branching ratio of hydrogen abstraction is no more than 10%。The aromatic peroxy is the main produc of the reaction of cresol-OH adducts and O2. The methyl-catechol, methyl-resorcinol and methyl-hydroquinon are formed from the hydrogen abstraction reaction of cresol-OH adduct.The detailed mechanism for the oxidation degradation of cresols is well known through the theoretical study. It can help us to understand the SOA formation from the oxidation degradation of o-cresols in urban atmospheres . |
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