Mechanistic And Kinetic Investigations Of The Thermal Decomposition Of Alkyl Radicals

Coal,petroleum and gas,which mainly compose of straight chain alkanes,branched alkanes,cycloalkanes and aromatics,play an important role in our living environment.The detailed reaction mechanisms and kinetic properties of these compounds and their corresponding radicals have been concerned experimentally and theoretically.The thermal decompositions of 1-heptyl,3-Me-l-heptyl and methyl-substiyuted cycloalkyl radicals are performed using the DFT and ab initio quantum chemical methods.These intermediates are derived from the C-H bond seission or abstraction hydrogen atom through a very small number of chemical species(such as H,CH3,C2H5 radicals,etc.).In addition,the reaction mechanisms,thermochemical and kinetic properties of the thermal decomposition of these intermediates are also discussed.The whole dissertation mainly includes the following three sections.1.A detailed theoretical study on the thermal decomposition and isomerization of 1-heptyl radical are performed at the CBS-QB3,BH&HLYP,B3PW91,BLYP,MPW1PW91,and M06-2X levels of theory.The result shows that the geometrical parameter is generally insensitive to the used method and basis set.The energies derived from the CBS-QB3 level are in good agreement with the experimental results.The pyrolysis mechanism of 1-heptyl radical mainly includes the isomerization,the beta site C-C and C-H bonds scission,consisting of 19 reactions.The decomposition reactions are generally endothermic,whereas the majority of isomerization reactions are exothermic and a few reactions are endothermic.The theoretical rate coefficients of conventional transition state theory with Eckart tunneling correction(TST/Eckart)for individual elementary reaction involved in the above reaction system are evaluated over the temperature range of 500-2500 K.Furthermore,the same atom number and reaction order reactions are selected to compare their relative importance at different temperatures.It can be found the isomerization process is more pronounced when the temperature below 1200 K.Product distribution of 1-heptyl radical pyrolysis is predicted based on the steady-state approximation,and the eventual products are ethylene(C2H4),propylene(C3H6),1-butylene(1-C4Hs),1-pentene(1-C5H10),and 1-hexene(1-CaH12).The physical heat sink linearly increases with rising temperature,and it increases about 0.2 MJ·kg-1 in the average per 50 K.The chemical heat sink steeply increases over the temperature range of 873-1023 K,and it increases about 0.7 MJ·kg-1 in the average per 50 K.2.A detailed theoretical investigation on the thermal decomposition and isomerization of 3-Me-l-heptyl radical is carried out at the CBS-QB3 level of theory.The calculation reveals that the detailed reaction mechanisms of 3-Me-l-heptyl radical mainly incorporate 6 reversible intramolecular hydrogen atom transfer and 17 beta site C-C bond scission,consisting of 29 reactions.All investigated decomposition reactions are generally endothermic,while the most of isomerization processes are exothermic.Among the hydrogen atom transfer processes,the 1,3-and 1,2-hydrogen atom migration(R5 and R6,respectively)are prohibited due to their high isomerization barriers,while the 1,6-(R2)and 1,5-hydrogen atom transfer(R3)are kinetically accessible(owing to their low ring strains in the cyclic transition states).The theoretical rate coefficients of conventional transition state theory for individual elementary reaction are evaluated over the temperature range of 500-2500 K.Compared with the 1,5-hydrogen atom shift for the n-heptyl radical,the methyl-substitution increases the rate coefficient by a factor of about 3.0.The decomposition reaction is more advantaged when the temperature above 1200 K.The product distributions are also predicted at different temperatures on the basis of the steady-state approximation.The ultimate and dominant products majorly include ethylene(C2H4),propylene(C3H6),1-butylene(1-C4H8)and 2-hexene(2-C6H12)over the temperature range of 500-2500 K.3.A systematically theoretical study on the thermal decomposition of 2-Me-cyclobutyl,2-Me-cyclopentyl and 2-Me-cyclohexyl radicals is performed using the high-level ab initio CBS-QB3 and CCSD(T)quantum chemical calculations.The calculated results reveal that the detailed reaction mechanism of the thermal decomposition of these cyclic alkyl radicals incorporates the ring opening,vinyl rearrangements(exocyclization),beta-site C-C bond cleavage and CH3-elimination processes.The standard reaction enthalpies(△rH2980)and Gibbs free energies(△rG2980)for each elementary reaction involved in the title reaction system are determined with composite CBS-QB3 method.In the thermal decomposition of 2-Me-cyclohexyl radicals,1,4-vinyl rearrangements reactions are exothermic and spontaneous,while the ring opening,C-C bond scission and CH3-elimination processes are endothermic and nonspontaneous.In the thermal decomposition of 2-Me-cyclopentyl radicals,some ring opening reactions are exothermic and spontaneous,while the rest of ring opening,1,3-vinyl rearrangement,and C-C bond scission reactions are endothermic and nonspontaneous.In the thermal decomposition of 2-Me-cyclobutyl radicals,all ring opening reactions are exothermic and spontaneous,while the 1,2-vinyl rearrangement and C-C bond scission reactions are endothermic and nonspontaneous.Among all investigated elementary reactions,the vinyl rearrangement processes are kinetically accessible and readily proceeds(due to their significantly lower barrier).Compared with the barrier heights for the distinct vinyl rearrangement pathways in these cyclic alkyl radicals,it can be found that the barriers are decreased in the order of 1,3->1,2->1,4-vinyl transfer.The branching ratios are evaluated at different temperatures on the basis of the quasi-steady state approximation(QSSA).The calculated result shows that the 1,2-,1,3-and 1,4-vinyl rearrangement reactions are advantaged at low temperature,while the formations of cycloalkene are favoured at high temperature.