Theoretical Study on the Mechanisms of Typical Volatile Organic Compounds Reacting with OH and NO3 Radicals
Author:Ma Yong Mei
Supervisor:su ke he
The volatile organic compounds（VOCs）from anthropogenic contributions have been confirmed to have great influence on atmospheric chemistry,environmental effect and human health in the urban area.However,these compounds can be easily transformed in the chemical oxidation reactions with hydroxyl radicals（OH）,nitrate radicals（NO3）and ozone（O3）in the atmosphere.The oxidation process investigations of the volatile organic compounds in atmosphere are of great significance for exploring the formation of secondary organic aerosol（SOA）and second pollutants.Reaction mechanisms of the typical aromatic hydrocarbon toluene with NO3 and vinyl ether with OH have been investigated with the First Principles combined with statistical thermodynamics and chemical kinetics.1.Atmospheric oxidation of toluene initialed by NO3 radicalThe reaction potential energy profiles were explored with density functional theory（DFT）B3LYP/6-311G（d,p）and M06-2X/6-311G（d,p）and refined with accurate model chemistry G3（MP2）.The changes of enthalpy and the Gibbs free energy for the elementary reactions were developed with the standard statistical thermodynamics.The high pressure limit thermal rate constants were evaluated using the canonical variational transition-state theory（CVT）with inclusion of small-curvature-tunneling（SCT）for the reactions having a pronounced（or tight）transition state and,those using the Rice-Ramsperger-Kassel-Marcus theory（RRKM）approach coupled with Eckart tunneling correction（RRKM/Eckart）for the reactions having a loose transition state.All the forward and reverse rate constants as a function of temperature were fitted into three Parameter Arrhenius expressions with the correlation coefficients being larger than 0.9.The main results are summarized as follows.Hydrogen abstraction and addition/elimination reactions of toluene and NO3 radical were found.For the hydrogen abstraction reaction,it is found that in addition to the side chain H-abstraction,the ring H-abstraction reactions are also possible.The side chain H-abstraction is dominant and irreversible below 700 K while the ring H-abstractions are competitive and reversible above 800 K.For the addition/elimination reactions,four different NO3-toluene adduct isomeric radicals can be easily formed with the highest energy barrier of only 5.3kJ·mol-1 by attacking the ipso,ortho,para and meta positions.The four NO3-toluene adducts can further dissociate in three distinct pathways.The way in forming an epoxide structure has the lowest barrier and is the dominant reaction in eliminating NO2 molecule.It found that both of the forward and reverse reactions are posible for the NO3 addition reactions in200-700 K with the forward reaction being dominant while the forward and reverse reactions are competitive in 800-1500 K.However,the reverse reactions of IM1 and IM2 become dominant 1500 K.The over-all reaction rate obtained in this work is in very good agreement with the available experimental result at 300 K.For the subsequent reactions of the four toluene-NO3 adducts with NO2,twenty toluene-NO3-NO2 isomeric adducts are found with barrierless associations.In forming these,the antarafacial（or trans-）toluene-NO3-NO2 adducts are more stable than the correspondence cis-ones and the addition reactions are all exothermic and nearly irreversible with significant negative Gibbs free energies.It is also found that NO2 is able to abstract the active H atom（attaching the C atom that has attached the-NO3 group）on the benzene ring.However,the rate constants of the addition reaction are significantly higher than those of the hydrogen abstractions.Therefore,the additions are the dominant reactions.Further elimination reactions of the toluene-NO3-NO2 adducts are also explored and found to form the corresponding nitrobenzene and HNO3.The barriers of the trans-toluene-NO3-NO2 adducts are lower than those of the cis-toluene-NO3-NO2,and the rate constants of the former are larger than those of the latter ones.In the reactions of the four toluene-NO3 adducts with O2,the O2 molecule can combined onto the adducts to produce ten cis and ten trans toluene-NO3-O2 peroxy radicals via the corresponding transition state.Most of the cis additions have lower energy barriers and their products have slightly higher stability.All the reactions are spontaneous,exothermic and irreversible.These radicals may proceed elimination and isomerization reactions.In the eliminations,the barriers in eliminating HNO3 of the cis-toluene-NO3-O2 radicals are much higher than those of the trans peroxy ones leading to the much smaller rate constants,and making them negligible in the reaction system.For the isomerization reactions of the toluene-NO3-O2 peroxy radicals,the cyclization reactions which form three kinds of bicyclic radicals with transition states are mainly explored.One of which includes an O-O bond in a six-membered and in a four-membered ring.The rest have an O-O bond bridging the bicyclic radicals including a five-membered and a seven-membered ring.Among those,the seven-membered ring is a delocalized allylΠradical and is found the main isomerization path,having the rate constant significantly larger than the others.It is also investigated that the the O2 molecule may abstract the avtive H on the benzene ring to form nitrooxy-toluene and HO2radical.However,the rate constants are smaller than those of the adductions.Comparing the effective reaction rates of toluene-NO3 adducts with NO2 and with O2,it is found that the reaction rates with O2 are faster than those with NO2.The overall reaction rates of the toluene-NO3 adducts with NO2 and with O2 are larger than those of toluene with NO3radical.This implies that the further reactions with NO2 and O2 will proceed once toluene and NO3 radical forms adducts.2.The reaction of vinyl ethers with OH radicalThe energy profiles for both methyl vinyl ether（MVE）and ethyl vinyl ether（EVE）reacting with OH radical were investigated theoretically with QCISD（T）/aug-cc-pvtz//M06-2X/6-311++g（d,p）method.The high pressure limit rate constants were evaluated with the CVT/SCT or the RRKM/Eckart method in the temperature region 200-2000 K.The main conclusions are drawn as follows.Five hydrogen abstraction and two addition/elimination reaction channels of cis-MVE+OH were obtained.The results show that all the hydrogen abstractions are feasible with energy barriers less than 36.7 kJ·mol-1,within which the most favorable reaction is the two out-of-plane H atoms in the-CH3 group being abstracted with a very small energy barrier of 4.2 kJ·mol-1.For the addition of cis-MVE+OH,a pre-complex is formed at the entrance.The dissociation of pre-complex is bifurcated into two distinct pathways,OH adding to the terminal carbon and to the central carbon via the corresponding transitions states with the formation of stable intermediates,respectively.The direct decomposition and isomerization channels for IM1 and IM2 were found with moderate energy barriers.The overall rate constants obtained in this work are in very good agreement with the available experimental detections at 299,352 and 427 K.The atmospheric half-life of MVE simulated with a typical initial concentration of an experiment is estimated to be 5.51 h and is consistent with the observed data 5.18 h.The calculated reaction rate indicates that the reaction of MVE and OH is essentially a fast reaction and is kinetics controlled.The consumption rate of MVE decreases with increasing temperature in 300-700 K and increases in 800-2000 K showing that the addition/elimination reactions are dominant at lower while the hydrogen abstractions are at higher temperatures.The concentration distributions of the products examined show that the stable intermediates IM1 and IM2 have the highest concentrations in about 300-400 K while glycolaldehyde（HOCH2CHO）is the main product in 500-1300 K.At higher temperatures above 1400 K,radical CH2OCH=CH2 is found the main product.The apparent activation energy(Eapp.)-8.9 kJ·mol-1 for 200-700 K is in good agreement with-4.3 kJ·mol-1 from the NIST Chemical Kinetics Database.The apparent activation energy predicted for 1400-2000 K is 39.7 kJ·mol-1.The addition/elimination reaction of EVE and OH was also found possible.The central carbon addition is easier than the terminal one for OH attacking to trans-EVE.These addition processes are exothermic with 139.4 and 115.3 kJ·mol-1.However,the activation energy of OH addition to the terminal carbon is lower than that to the central channel for cis-EVE with heat releasing of 112.3 and 119.1 kJ·mol-1,respectively.Subsequently,the direct decomposition and isomerization channels for intermediates were found with moderate energy barriers.The calculated overall rate constant is in very good agreement with the experimental value.The consumption rate of EVE decreases with increasing temperature in200-800 K and increases in 900-2000 K.The concentration distributions of the products indicate that the addition/elimination products are dominant at lower temperatures while the hydrogen abstractions are increasing with increasing temperature and become dominant above1700 K.