| The combustion process of hydrocarbon fuels in engine is very complex.How to improve the combustion efficiency of hydrocarbon fuels and reduce the environmental pollution in combustion process has become the focus of the design for engine.The study on the combustion reaction mechanism of hydrocarbon fuels can open a new way to solve the problems.In order to fully understand the combustion reaction mechanism of hydrocarbon fuels,it is necessary to provide detailed reaction lists as well as accurate thermo-kinetic parameters and transport parameters.In addition,a large number of radicals are produced in the combustion process of hydrocarbon fuels,and the reactions involved in these radicals cannot be measured by experimental methods.Thus,these reactions of radicals are studied by theoretical calculation in this thesis.Alkenes are important intermediates produced in the combustion process of hydrocarbon fuels,which plays a key role in the study of reaction mechanism for alkanes and aromatics.However,compared with the studies of reaction mechanisms for alkanes,the study on reaction mechanisms for alkenes is not perfect,especially the reaction mechanisms for alkenes at low-temperature.Meanwhile,the calculation of accurate thermal and kinetic parameters for large molecular systemes of alkenes is also a key problem that puzzles the accuracy of the reaction mechanism for alkenes.In this thesis,the calculation of kinetic parameters for important reactions of alkenes reaction mechanisms at low-temperature is studied,including the high-pressure-limit rate constants and pressure-dependent rate constants.The simple rate rules method is used to obtain the high-pressure-limit rate rules and pressure-dependent rate rules for reaction classes of alkenes combustion mechanisms.Meanwhile,the accurate thermal and kinetic parameters for large molecular systems at low-level are obtained by using the isodesmic reaction method.The results of this thesis can provide accurate kinetic parameters for the construction of reaction mechanisms for alkenes.This thesis consists of six chapters.The cycloaddition,intramolecular H-shift and concerted elimination reactions of alkenyl-peroxy radicals are three kinds of important reaction classes of alkenes at low-temperature.However,there are few reports on the pressure-dependent rate constants for these reaction classes.In the third chapter of this thesis,the cycloaddition,intramolecular H-shift and concerted elimination reactions of alkenyl-peroxy radicals are selected as the research object.The cycloaddition reaction classes are divided into(1,n)-endo-cycloaddition and(1,n)-exo-cycloaddition(n=5,6,7)reaction subclasses according to the position of the single electron in the products and the distance between radicals and addition carbon atoms in the reactants.The intramolecular H-shift reaction classes are divided into(1,n)H-as,(1,n)H-s,(1,n)H-vt and(1,n)H-vi(n=4,5,6)reaction subclasses according to the size of the transition states and the types of carbon atoms connected to the hydrogen atoms.The concerted elimination reactions are divided into three reaction subclasses according to the different types of C-H bonds that are broken in reactant molecules.In the third chapter,the B3LYP/6-31G(2df,p)method is used for geometry optimization and frequency analysis.The energy barriers are calculated by the Gaissian-4(G4)method.The traditional transition state theory(TST)and Rice-Ramsberger-Kassel-Marcus/Master Equation(RRKM/ME)method are used to calculate the high-pressure-limit rate constants and pressure-dependent rate constants,respectively.The high-pressure-limit rate rules and pressure-dependent rate rules are calculated by the simple rate rules method.The results in this chapter show that there is great uncertainty in using the same rate constants for the same reaction class.It is necessary to further divide the reaction class into different reaction subclasses according to the characteristics of the reaction center,and then use the simple rate rules method to obtain the rate rules of each subclass,so as to reduce the uncertainty of the rate constants.In addition,the pressure-dependent rate constants of these three reaction classes increase with the increase of temperature and pressure.The intramolecular H-shift reaction of hydroperoxy-alkenyl-peroxy radicals is an important pressure-dependent reaction class in the reaction mechanism of alkenes at low-temperature.Howerer,there is no report on the pressure-dependent rate constants and rate rules for this reaction class.At the same time,the rate constants of intramolecular H-shift reactions for hydroperoxy-alkenyl-peroxy radicals and hydroperoxy-alkyl-peroxy radicals are different,but there is lack of quantitative comparative study.In the fourth chapter of this thesis,the intramolecular H-shift reaction class is divided into different reaction subclasses according to the types of carbon atoms and the size of the transition states.The M05-2X/6-311G(d,p)method is used for the geometry optimization and frequency analysis.The CBS-QB3 method combined with the traditional transition state theory is used to calculate the high-pressure-limit rate constants.The Rice-Ramsberger-Kassel-Marcus/Master Equation(RRKM/ME)theory is used to calculate the pressure-dependent rate constants at the range of 0.01atm～100atm.Meanwhile,the high-pressure-limit rate rules and pressure-dependent rate rules are also given according to the simple rate rules method.The results of the fourth chapter show that the pressure-dependent rate constant increases with the increase of temperature and pressure.In addition,the rate constants of intramolecular H-shift reaction for hydroperoxy-alkenyl-peroxy radicals and hydroperoxy-alkyl-peroxy radicals show different temperature dependence.In general,the high-level ab initio method can be used to get accurate results for small molecular systems,but it can not be used for large molecular systems.The isodesmic reaction method can obtain accurate thermodynamic parameters for large molecular systems at a low-level.In the fifth chapter of this thesis,the isodesmic reaction method is applied to calculate the kinetic parameters for large molecular systems.The concerted elimination reactions of hydroperoxy-alkenyl-peroxy radicals,which lack accurate kinetic parameters for large molecular systems,are studied theoretically.In the fifth chapter,the energy barriers of target reactions under the B3 LYP method is modified by the isodesmic reaction method,and compared with high-level G4 method,the results show that the deviation range of the energy barriers calculated by the B3LYP/6-31+G(d,p)method and G4 method is 0.14～4.04kcal/mol without correction,while the deviation range is reduced to-0.66～0.29kcal/mol after correction by the isodesmic reaction method,which is within the chemical precision(1～2kcal/mol).Therefore,the accurate kinetic parameters for large molecular systems can be obtained by modifying the low-level calculation values with the isodesmic reaction method.In this thesis,the cycloaddition,intramolecular H-shift and concerted elimination reaction of alkenyl-peroxy radicals and intramolecular H-shift and concerted elimination reaction of hydroperoxy-alkenyl-peroxy radical are studied theoretically,which not only provided accurate kinetic parameters for the construction of database and combustion reaction mechanism,but also solved the difficulty of accurate kinetic parameters calculation for large molecular systems.In addition,the sixth chapter also briefly introduces the correlation between alkenes combustion reaction mechanisms and biofuel combustion reaction mechanisms,and explains the necessity of studying the influence of pressure on the thermodynamic parameters. |
