The Study Of Rate Constants On The Unimolecular Reactions HCO₂, HOCO And CH₂XCHFO(X=H,F) Using The RRKM Theory

CO, CO2 and HFC (hydrochlorofluorocarbon) may have great impact on the atmosphere and global climate. HCO2, HOCO and CH2XCHFO(X=H, F) are important intermediates in the reactions of CO+HO→H+CO2, H+CO2→CO+ HO and dissociation of HFCs oxidation, respectively. The study of the unimolecular reactions HCO2, HOCO and CH2XCHFO(X=H, F) can improve the understanding of the total reactions of CO, CO2 and HFC in atmosphere. This work may have good meaning for the atmospheric environment.In the three unimolecular reactions, we use Gaussian 03 to optimize the geometries of the reactants and transition states to get the vibrational frequencies. The single-point-energy of each geometry is optimized with CCSD(T) or QCISD(T) method. The barrier height of each reaction is obtained from zero-point-energy correction. Then the total number of states, the density of state and the rate constants are calculated with L.Yao and S.H.Lin (YL) method according to RRKM theory.In the first reaction, the dissociation of HCO2, HCO2→H+CO2 is discussed. The geometries and single-point-energy of HCO2 and H...CO2 are optimized at MP2/6-311++G** and CCSD (T)/aug-cc-pVTZ level, respectively. The potential energy surface (PES) is presented through zero-point-energy correction. The rate constants calculated with YL method show that harmonic rate constants are higher than anharmonic values.In the second part, HOCO is the intermediate of the reaction H+CO2→CO+ HO. The geometries and single-point-energy of reactions and transition states are optimized at MP2/6-311++G** and CCSD (T)/aug-cc-pVTZ levels, respectively. Then PES is presented through zero-point-energy correction. The PES indicates two channels and three unimolecular reactions, among which one unimolecular reaction predominates the total reaction. The lifetime corresponding to the rate constants of this unimolecular reaction calculated with YL method are close to the lifetime of HOCO in experimental research.In the third part, six reactions of CH2XCHFO(X=H, F) are studied. From the optimized geometries, the six reactions can be determined, respectively, as C-C bond scission, C-H bond scission, C-F bond scission, the elimination of HF and two isomerization reactions. The rate constants of these unimolecular reactions are calculated with YL method. The results show that the higher the barrier height is, the lower the rate constant is.Both in microcanonical and canonical cases, in the three parts above, the discrepancies are obvious between the harmonic and anharmonic rate constants calculated with YL method, so that the anharmonic effect of these unimolecular reactions can not be neglected.