Theoretical Investigations On The Degradation Mechanisms For Several Peroxide, Carbonaceous And Nitrogenous Radicals In The Atmosphere

The reactions involving free radicals play an important role in many fields such as atmospheric chemistry, combustion chemistry, interstellar chemistry, photochemistry, and biochemistry. The considerable attention has been paid on the mechanisms and kinetics for these reactions during the last decade theoretically and experimentally. In this thesis, quantum chemical and molecular reaction dynamics investigations for a series of important radical-radical reactions have been carried out. The potential energy surfaces including structures, frequencies and energies of intermediate isomers and transition states are explored by hybrid density functional (DFT) and ab intio calculation levels of theory. The rate constants and branching ratios as well as the pressure-and temperature-dependence of various product channels for these multi-well reactions are predicted by variational transition-state theory (VTST) and Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The results obtained in the present study may lay a strong foundation for building important radical-radical reactions models in atmospheric, combustion and interstellar chemistry, and could provide theoretical basis for future experimental study. The main results are summarized as follows:1. Theoretical investigations are performed on mechanism and kinetics of the reactions of halogen peroxy radicals ClOO and FOO with the NO radical. The electronic structure information for both of the singlet and triplet potential energy surfaces (PESs) is obtained at the MP2/6-311+G(2df) level of theory, and the single-point energies are refined by the CCSD(T)/6-311+G(2df) level. The rate constants for various product channels of the two reactions in the pressure range of 1-7600 Torr are predicted. The main results are as follows:(1) For ClOO+NO, on the singlet surface, the addition-elimination mechanism is the most important. First, the N atom of the NO radical can attack the O atom of the ClOO radical to form an energy-riched intermediate IM1 ClOONOtp (21.3 kcal/mol) barrierlessly, then IM1 could isomerizes to IM2 ClOONOcp(22.1 kcal/mol)via a low energy barrier. Both IM1 and IM2 can dissociate to the primary product P1 ClNO+ 1O2 and the secondary product P2 ClO+NO2. On the triplet surface, the direct Cl-abstraction reaction is the most feasible pathway. The Cl-abstraction can take place via a van der Waals complex,3IM1 ONClOO(4.1 kcal/mol), then it fragments readily to give P1'ClNO+3O2 with a small barrier. The kinetic calculations show that at low temperatures, the singlet bimolecular product P1 is the primary product, while at high temperatures, the triplet product P1'becomes the primary one; only at high pressures and low temperatures, the unimolecular products IM1 and IM2 can be found with quite small yields. At experimentally measured temperature 213 K, ClNO is the primary product in the whole pressure range, which is consistent with the previous experiment.(2) The FOO+NO reaction is most likely to take place on the singlet surface. Similarly to ClOO+NO, the N atom of the NO radical can attack the O atom of the FOO radical via the barrierless addition mechanism to form an energy-riched intermediate IM1 FOONOtp (19.8 kcal/mol), then IM1 could isomerize to IM2 FOONOcp( 19.6 kcal/mol)via a low energy barrier. The kinetic calculations show that starting from IM1 arid IM2, at room temperature 298 K and at 1 Torr, P1 FNO+1O2 is the exclusive product with the branching ratio near 1 in consistent with the experimental result. As pressure increases, the yield of IM2 increases and IM2 becomes the primary product when P≧ 38 Torr.2. Theoretical investigations are performed on mechanism and kinetics of the reactions of ethyl radical C2H5 with HOO and NCO radicals.(1) For the C2H5+HO2, the electronic structure information of the potential energy surface (PES) is obtained at the MP2/6-311++G(d,p) level of theory, and the single-point energies are refined by the CCSD(T)/6-311+G(3df,2p) level. The kinetics of the reaction with multiple channels has been studied by using VTST theory and RRKM theory. The calculated results show that the HO2 radical can attack C2H5 via the barrierless addition mechanism to form a energy-riched intermediate IM1 C2H5OOH (68.7 kcal/mol) on the singlet PES. The collisional stabilization intermediate IM1 is the dominant product of the reaction at high pressures and low temperatures, while the bimolecular product P1 C2H5O+OH becomes the primary product at lower pressures or higher temperatures. At experimentally measured temperature 293 K and in the whole pressure range, the reaction yields Pi as major product, and the branching ratio changes from 0.96 at 1×10-4bar to 0.66 at 100 bar. Moreover, the direct H-abstraction product P16 C2H6+3O2 on the triplet PES is the secondary feasible product with the yield of 0.04 at collisionally limit at 293 K.(2) For the C2H5+NCO, the electronic structure information of the PES is obtained at the B3LYP/6-311++G(d,p) level of theory, and the single-point energies are refined by the CCSD(T)/6-311+G(3df,2p) level. The rate constants for various product channels of the two reactions in the temperature range of 200-2000 K are predicted by performing VTST and RRKM calculations. The calculated results show that both the N and O atoms of the NCO radical can attack the C atom of C2H5 via a barrierless addition mechanism to form two energy-riched intermediates IM1 C2H5NCO (89.1 kcal/mol) and IM2 C2H5OCN (64.7 kcal/mol) on the singlet PES. Then they both dissociate to produce bimolecular product P1 C2H4+HOCN and P2 C2H4+HNCO. At high temperatures or low pressures, the reaction channel leading to bimolecular product P2 is dominant, and the channel leading to P1 is the secondary; while at low temperatures and high pressures, the collisionally stabilization of the intermediate plays an important role, and as a result IM2 becomes the primary product.3. The mechanism for the reaction of the cyanogen radical (CN) with the cyanomidyl radical (HNCN) has been investigated theoretically. The electronic structure information of the singlet and triplet potential energy surfaces (PESs) is obtained at the B3LYP/6-311+G(3df,2p) level, and the single-point energies are refined at the CCSD(T)/6-311+G(3df,2p) level as well as by multi-level MCG3-MPWB method. The calculations show that the C atom of CN additions to middle-and end-N atoms of HNCN are two barrierless association processes leading to the energy-rich intermediates IM1 HN(CN)CN and IM2 HNCNCN, respectively, on the singlet PES. The higher barriers of the subsequent isomerization and dissociation channels from IM1 and IM2 indicate that these two intermediates, which have considerably thermodynamic and kinetic stability, are the dominant product at high pressure. While at low pressure, the most favorable product is P2 H+NCNCN, which will be formed from both IM1 and IM2 via direct dissociation processes by the H-N bond rupture, and the secondary feasible product is P4 HCN+(?)NCN, while P5 HCCN+N2 and P6 HCNC+N2 are the least competitive products. On the triplet PES, P14NCNC+HN may be a comparable competitive product at high temperature. In addition, the comparison between the mechanisms of the CN+HNCN and OH+ HNCN reactions is made. The present results will enrich our understanding of the chemistry of the HNCN radical in combustion processes and interstellar space.