Theoretical Investigations On The Mechanisms Of Several Important Radical-Molecule Reactions In The Environment

Reactions of small radicals with neutral molecules play a significant role in diverse environments such as combustion flames, the interstellar medium (ISM), and planetary atmospheres such as Saturn's moon Titan. As a result, quantum chemical investigations on the potential energy surfaces of several important radical-molecule reactions have been carried out in this thesis. Those reactions include C3H+H2O, HCO/HOC+C2H2, C2H+H2CO and HCS+O₂. Important information of potential energy surfaces such as structures and energies of intermediate isomers and transition states, possible reaction channels, reaction mechanisms and major products are obtained from the theoretical investigations. Some conclusions that are made in the present thesis may be helpful for further theoretical and experimental studies of this kind of reactions. The main results are summarized as follows: 1. The potential energy surfaces (PES) of the C₃H+H₂O reaction are researched at the CCSD(T)/6-311G(2d,p)//B3LYP/6-311G(d,p) level. The carbenoid insertion of C₃H to the H-O bond of H2O leads to the very low-lying isomers HCCCHOH 1a (-69.9), 1b (-67.9). We show two most feasible pathways from isomer 1: Path 1: HCCCHOH (1a,1b)→P5 HCCCHO+H Path 2: HCCCHOH (1a,1b)→H2CCCHO (2a,2b)→H2CCHCO (3a,3b)→P1 C2H3+CO The rate-determining transition states for Path 1 and Path 2 are TS1a/P5 (-26.1) and TS1a/2b (-26.6), respectively. So both TSs can be considered as isoenergetic and both channels can be highly competitive. Moreover, we firstly suggest that the most viable mechanism of the propynal HCCCHO(P5) formation in space is through the radical-molecule C3H+H2O reaction . 2. At the level of CCSD(T)//B3LYP adding the high class calculation-G3//B3LYP and G3//MP2, the potential energy surfaces (PES) of the HCO/HOC+C2H2 reaction are investigated detailed. The two former low-lying reaction pathways for the HCO+H2CO reaction can be summarized as the following: Path 1: R1 HCO+C2H2→HCCHCHO 4→H2CCHCO 1→P1 C2H3+CO Path 2: R1 HCO+C2H2→OCH…HCCH Com1→P1 C2H3+CO And the most feasible channel is Path 1 with 10 kcal/mol barriers. In contrast, the HOC-reaction can take a barrierless H-donation process leading to P1 C2H3+CO. 3. At the level of CCSD(T)//B3LYP adding the high class calculation-CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311G(d,p)+ZPVE, CCSD(T)/6-311+G(3df,2p)//MP2/6-311G(d,p)+ZPVE , CCSD(T)/6-311+G(3df,2p)//QCISD/6-311G(d,p)+ZPVE and G3//MP2, the potential energy surfaces (PES) of the C2H+H2CO reaction are investigated detailed. The results indicate the gas-phase reaction R C2H+H2CO (0.0)...