Theoretical Studies On The Hydrogen Abstraction Reaction Of Esters

As one important class of volatile organic compounds(VOCs), esters have been widely used in industry, particularly as a solvent and in the manufacture of perfumes and flavoring. They can be produced naturally through vegetation and be emitted directly into the atmosphere, and the degradation of some oxygenated compounds is also a source of esters. Hydrofluoroethers (HFEs) have been developed as the third generation alternative of chlorofluorocarbons (CFCs) that is used in many industrial applications, the studies of the OH or Cl reactions with fluorinated ethers have attracted much attention. However, Esters can be produced as the major oxidation products of the HFEs, only a limited number of experimental studies focused on the fate of esters. To ascertain the environmental impact of esters released into the troposhere, theoretical investigations of the reactions related to esters are very desirable. The estimation of rate constants for a specific chemical reaction is one of the most active subjects in theoretical chemistry. In this thesis, ab initio or density functional theory direct dynamics methods have been used to investigate the reaction mechanism and calculate the rate constants. Herein, our goal is to perform an ab initio direct dynamics calculation to shed light on the mechanism of this reaction. The potential energy surface (PES) of the reaction is obtained directly by density functional theory (DFT) and ab initio calculations. Then, the rate constants are evaluated by the variational transition-state theory (VTST). The following hydrogen abstracted reactions are studied:1. Reaction CF3CH2OCHO+Cl→products. A dual-level direct dynamics method is employed to investigate this reaction. For the title reaction, there exist two kinds of hydrogen abstraction channels, that is, H-abstraction from the -CHO and–CH2- groups, as follows: CF₃CH₂OCHO+Cl→CF3CH2OCO+HCl→CF₃CHOCHO+HClIn addition, there maybe exist other possible reaction pathway, such as chlorine addition to CF₃CH₂OCHO to form intermediate CF₃CH₂OClCHO followed by further dissociation, CF₃CH₂OCHO+Cl→CF₃CH₂OClCHO→CF₃CH₂OClCO+H→ClCHO+CF3CH2O→ClOOCH+CF3CH2Theoretical calculations suggest that the reaction proceeds exclusively via two hydrogen abstraction channels, while the Cl additon-climination channel is unfavorable. The potential energy surface is explored by low-level calculations at the B3LYP/6-311G(d,p) level combined with high-level single-point energy calculations at the BMC-CCSD level. Furthermore, the rate constants of the two H-abstracion channels are evaluated by means of canonical variational transition-state theory (CVT) with the small-curvature tunneling (SCT) correction over a temperature range of 200-1000 K. The calculated CVT/SCT rate constants are in reasonable agreement with the experimental values. The present results indicate that the formyl-H-abstraction channel is the major reaction pathway at low temperatures, while as the temperature increases, methylene-H-abstraction channel becomes more important and competes with the former. The Arrhenius expression is fitted to be k = 0.17×10-20 T 3.13 exp(-23.9/T) cm3 molecule-1s-1 (200-1000 K). The present study may assist in further laboratory work.2. Reaction CH3C(O)OCH3+Cl→products. The hydrogen-abstraction reaction mechanism and the kinetic of the Cl radical with methyl acetate CH3C(O)OCH3 is investigated theoretically. For the title reaction, there exist three kinds of hydrogen abstraction channels, that is, H-abstraction from–(O)CCH3 and–OCH3 groups(out of plane hydrogen and in-plane hydrogen), as follows: CH₃COOCH₃+Cl→CH₂C(O)OCH₃+HCl →CH₃C(O)OCH₂+HCl the present results indicate that the out of plane hydrogen abstraction from the methoxy end is the dominant channel, so we mainly calculate its rate constant.The direct dynamics method is employed in the calculation of the rate constant. The optimized geometries and frequencies and the gradients of the stationary points are calculated at BB1K/6-31+G(d, p) and B3LYP/6-311G (d, p) levels. The energetic information of potential energy surface is abtained at BB1K/6-31+G(d, p) level. The BB1K method is specifically designed for kinetic calculations and is know to produce reliable thermodynamic and kinetic parameters for radical reactions including reactions with Cl atoms. The present results indicate that the out of plane hydrogen abstraction from the -OCH3 group (methoxy end) is the dominant channel, while the in-plane hydrogen abstraction from it and the hydrogen abstracton from the–(O)CCH3 group may be neglected for the title reaction.The theoretical rate constant for reaction is calculated by canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT) at the BB1K/6-31+G(d, p) level. The calculated CVT/SCT rate constants are found to be in good agreement with the available experimental values in the corresponding measured temperature regions. The three-parameter expression for this reaction within 200-700 K k = 0.37×10-15 T 1.47exp (173.9/T) cm3molecule-1 s-1.