Theoretical Study On The Mechanism Of Reaction Between MBO And O₃ Under Some Chemical Modelling Conditions

In recent years,haze weather becomes the big problem in some area due to the serious pollution of the atmosphere.The appearance of haze will not only reduce the visibility in the atmosphere,but also impair human health directly.Research shows that haze weather is closely related to Volatile Organic Compounds（VOCs）in the atmosphere,and,as a kind of component of VOCs,Biogenic Volatile Organic Compounds（BVOCs）results in the appearance of Second Organic Aerosol（SOA）,which is found to be an important component of fine particulate matter(PM_2.5).2-methyl-3-butene-2-ol（MBO,（CH₃）₂C（OH）CHCH₂）,which is mainly generated by vegetation,is a typical BVOCs.This compound tends to reacts with OH,O₃ and NO₃in the atmosphere to generate some gas-phase organic compounds and free radicals.The presence of these compounds has a significant effect on the atmospheric chemical process and will directly lead to air pollution.In this paper,we focus on the reaction between MBO and O₃.Up to now,there are some speculation of the reaction mechanism on this reaction based on the experimental results.As we know,in such experimental research,NOx is always introduced to eliminate the OH radical which is produced in the reaction,this causes the reaction mechanism become very complex.So the speculation of the reaction mechanism becomes unreliable.It is necessary to do detailed theoretical research on the mechanism of the reaction to confirm or reveal the reaction details.Additionally,experiments find that different environmental conditions,such as humidity,acidity or lighting,have considerable influence on the atmospheric chemical process.Therefore,constructing model system to simulate the atmospheric humidity and acidity is very helpful to understand the real chemical process involved in atmosphere.In Chapter 1,the composition of atmospheric VOCs and its influence on atmospheric environment are summarized.In particular,we review the experimental research on the reaction between MBO and O₃,theoretical research on some atmospheric chemical reactions in gas phase,and experimental and theoretical research on the influence of different chemical environment on atmospheric reactions.In Chapter 2,the theoretical methods used in this thesis are briefly introduced,including the Artificial Bee Colony Algorithm and ABCluster software package,the basic knowledge of density functional theory,the transition state theory,the calculation method and the basis sets,and the Gaussian software package for quantum chemistry calculation.In Chapter 3,the structure and relative stability of the complexes formed by O₃、MBO、（H₂O）_n（n=1,6,12）are studied.The global searching of stable structure for（H₂O）₆、（H₂O）₁₂以及O₃?（H₂O）₆、MBO?（H₂O）₆、O₃?（H₂O）₁₂、MBO?（H₂O）₁₂ is carried out by using ABCluster combined with Gaussian 09 software.Geometry optimization is performed at M06-2X/6-31++G（d,p）level of theory,except for MBO?（H₂O）₁₂,which is optimized at B3LYP/6-31G*level.The most stable structure is chosen by comparison the relative energy and frequency analysis for different structure.As the result,we conclude that the complex of MBO·O₃ is easy to form due to its high binding energy for the O₃、MBO、H₂O system.In the humid condition,MBO tend to bind more water molecules,and the analysis of the binding energy and frontier molecular orbital indication that the binding of water molecules on MBO can influence its chemical reactivity.In Chapter 4,the mechanism of the reaction between MBO and O₃ is studied in detail with quantum chemistry calculations.The reactants,intermediates,transition states and reaction products are calculated at M06-2X/6-31++G（d,p）level with Gaussian 09 software package.Intrinsic Reaction Coordinate（IRC）calculations are carried out at same level of theory to confirm the reaction path.Results show that the complex of MBO·O₃ is formed as the first step for the reaction.The energy released in this process（that is the binding energy）is 310.9 k J/mol,which is enough to overcome the reaction barrier for any of the subsequent steps.The formed complex of MBO·O₃ can be decomposed through two reaction channels:the products are multi-carbon Criegee intermediate（CH₃）₂C（OH）CHOO,HCHO,and Criegee intermediate H₂CO₂,2-hydroxy-2-methyl-propanal（HMPr）respectively.Among the products,HCHO and HMPr are VOCs,which play an important role in the formation of atmospheric aerosols and air pollution.The Criegee intermediates produced can either decompose or react with other atmospheric components.In Chapter 5,we simulate different atmospheric chemical environment by introduce H₂O、NH₃、H₃O⁺to the reaction system of MBO and O₃,and focus on the two reaction channels of decomposition of MBO·O₃ complex.All the calculations are carried out at M06-2X/6-31++G（d,p）level,and the stationary structure and transient state structure are confirmed by analysis of calculated vibrational frequencies at the same level,IRC calculations are carried out at same level of theory to confirm the reaction path.By analyzing the results,we found that the introduce of H₂O and NH₃ into reaction system of MBO and O₃ increases the binding energy of the corresponding complexs,and lower the reaction barrier.This effect can be understood as catalysis,that is,H₂O and NH3 act as catalysts in the reaction process.As for the introduce H₃O⁺into the reaction,the proton transferring is come up during the reaction,which change the products from（CH₃）₂C（OH）CHOO⁺and H₂COO to[（CH₃）₂C（OH）CHOOH]⁺and[H₂COOH]⁺.Quantitative analysis of the binding energy and reaction barrier in H₃O⁺condition shows that the reaction mechanism and reaction velocity change apparently.In the Appendix,we present the global searching of stable structure of clusters formed by some atmospheric species and water molecules.All the calculation are carried out by using ABCluster combined with Gaussian 09.By analyzing the results,we conclude the most stable structure of 14 clusters,including（H₂O）₆、（H₂O）₁₂、O₂?（H₂O）₆、O₂?（H₂O）₁₂、Cl?（H₂O）₆、Cl?（H₂O）₁₂、O₃?（H₂O）₆、O₃?（H₂O）₁₂、BO32?（H₂O）₆、BO32?（H₂O）₁₂、MBO332?（H₂O）₆、MBO332?（H₂O）₁₂、MBO232?（H₂O）₆ and MBO232?（H₂O）₁₂,in which,BO32 is 3-butene-2-ol,MBO332 is 3-methyl-3-butene-2-ol,and MBO232 is 2-methyl-3-butene-2-ol.In summary,what we conclude in this thesis:1.In the reaction of MBO and O₃,binding complex is formed as first step,follow by two decomposed reaction channel,which produce（CH₃）₂C（OH）CHOO/HCHO and（CH₃）₂C（OH）CHOO/H₂CO₂ respectively.The reaction mechanisms of（CH₃）₂C（OH）CHOO and H₂CO₂ are also presented.All the calculated results support the speculation of the reaction mechanism deduced by experimental results in literature.2.It is quite different for the binding energy as H₂O、NH₃、H₃O⁺introduced in to MBO·O₃.This means that the reaction energy provided by forming complexes is different for introduction of H₂O、NH₃、H₃O⁺.The sequence of this energy is H₃O⁺>NH₃>H₂O.3.The reaction products change as introducing different species into reaction of MBO and O₃.For reaction of MBO and O₃,as well as H₂O or NH₃is introduced,the products are（CH₃）₂C（OH）CHOO,HCHO,（CH₃）₂C（OH）CHO（HMPr）and H₂COO.For reaction of MBO and O₃ with H₃O⁺,the products are[（CH₃）₂C（OH）CHOOH]⁺、HCHO、（CH₃）₂C（OH）CHO（HMPr）、[H₂COOH]⁺and H₂O.4.The reaction barrier changes under different condition.The addition of H₂O、NH₃lower the barrier of the two reaction channels,which act as catalyst.The addition of H₃O⁺lower the reaction barrier apparently,and relative height of the two channels changes,which lead to the change of its reaction mechanism and reaction velocity.5.The introduction of H₂O、NH₃、H₃O⁺into MBO与O₃ will change their performance in atmospheric process.