Theoretical Investigations On The Mechanisms For Several Important Haloid Atomic Radical-Molecule Reactions

Reactions of small radicals with neutral molecules play a significant role in diverse environments such as industrial applications, combustion flames, the interstellar medium (ISM), and so on. In this thesis, the reaction mechanisms of radical-molecule reactions, and the stablities of intermediate isomers that relative to industrial application, atmosphere and combustion chemistry, are investigated. We expect that our studies may be helpful for understanding the processes in those chemical reactions. As a result, quantum chemical investigations on the potential energy surfaces of several important atomic radical-molecule reactions have been carried out in this thesis. These reactions include F + propene, F + propyne, F/Cl + acetylene and Cl + HONO. 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 surface (PES) of the F + CH3CH=CH2 reaction is researched at the CCSD(T)/cc-pVTZ//UMP2(FULL)/6-311++G(d,p) level of theory. The atomic radical F? has an unpaired single electron and can have either direct hydrogen abstraction and/or addition-isomerization-elimination mechanism. The most feasible pathway should be the atomic radical F attacking on the C=C double bond in CH3CH=CH2 to form a weakly-bound complex I1 with no barrier, followed by atomic radical F addition to the C=C double bond to form the low-lying intermediate isomer 3 barrierlessly. Starting from intermediate isomer 3, the most competitive reaction pathway is the dissociation of C2-C3 single bond via transition state TS3-P5 leading to the product P5 CH3 + CHF=CH2. It can be expressed as:Path1 RP5: R 2F + CH3CH=CH2→I1→I3→P5 CH3 + CHF=CH2 While in the direct H-atom abstraction reactions, the atomic radical F picking up the b-allylic hydrogen of propene barrierlessly is the most feasible pathway. We show it below: Path12 RP1: R→I9→P1 HF + CH2CHCH2Because the intermediates and transition states involved in the major pathways are all lower than the reactants in energy, this reaction is expected to be rapid. Furthermore, based on the analysis of the kinetics of all channels through which the addition and abstraction reactions proceed, we expect that the competitive power of reaction channels may vary with experimental conditions for this reaction.2. The potential energy surface (PES) of the F + CH3C≡CH reaction is researched at the CCSD(T)/aug-cc-pVDZ//UMP2(FULL)/6-311++G(d,p) level of theory. Two reaction mechanisms including the addition-isomerization-elimination reaction mechanism and the directed hydrogen abstraction reaction mechanism are revealed. For the hydrogen abstraction reactions, i.e. the most probable evolution pathway in this reaction, the HF formation occurs via direct abstraction mechanism dominantly and the H atom picked up by the atomic radical F should come mostly from the methyl group of normal propyne. It can be expressed as: Path9 RP1 R 2F + CH3C≡CH→C1→TSC1/P1→P1 HF + CH2C≡CH On the other hand, for the addition-isomerization-elimination mechanism, the most feasible pathway should be the atomic radical F attacking on the C≡C triple bond in propyne (CH3C≡CH) to form a weakly-bound adduct A1 with no barrier, followed by F addition to the C≡C triple bond to form the low-lying intermediate isomer 5. Subsequently, isomer 5 directly dissociates to P3 H2CCCHF + H via transition state TS5/P3. We show it below: Path1 RP3 R→A1→TSA1/5→5→TS5/P3→P3 H2CCCHF + H The P4 HCCF + CH3 formation channel R→A1→TSA1/1→1→TS1/2→2→TS2/P4→P4 is more feasible than the P5 H3CCCF + H formation channel R→A1→TSA1/5→5→TS5/6→6→TS6/P5→P5 although the feasibility of CH3 formation channel was excluded in the experiment.Based on the analysis of the energetics of all channels through which the additions and abstraction reactions proceed, we expect that the major reaction channel may vary with experimental conditions for the title reaction. Therefore, future experimental studies on this reaction are highly desirable under various pressures and temperatures.3. The potential energy surfaces (PES) of the F/Cl + CH≡CH reaction are researched at the CCSD(T)/aug-cc-pVDZ//CCSD/6-31G(d,p), CCSD(T)/aug-cc-pVDZ//UMP2/6-311++G(d,p) and compound method Gaussian-3 levels of theory. Two reaction mechanisms including the addition-elimination and the hydrogen abstraction reaction mechanisms are considered. In the addition-elimination reactions, the halogen atoms approach C2H2, perpendicular to the C≡C triple bond, forming the pre-reactive complex C1 at the reaction entrance. C1 transforms to intermediate isomer I1 via transition state TSC1/1 with a negative/small barrier for C2H2F/C2H2Cl system, which can proceed by further eliminating H atom endothermally. The two reaction pathways can be summaried as the following:Path 1: R X + CH≡CH→cis-XC(H)CH I1→P1 XC≡CH + H (X=F, Cl) While the hydrogen abstraction reactions also involve C1 for the fluorine atom abstraction of hydrogen, yet the hydrogen abstraction by chlorine atom first forms a collinear hydrogen-bonded complex C2: Path F: R HC≡CH + F→C1 HC≡C(…F)H→TSC2/P2 HC≡C…H…F→P2 C≡CH + HF Path Cl: R HC≡CH + Cl→C2 HC≡CH…Cl→TSC2/P2 HC≡C…H…Cl→P2 C≡CH + HCl According to our results, the presence of pre-reactive complexes indicates that the simple hydrogen abstraction and addition in the halogen atoms reaction with unsaturated hydrocarbon should be more complex. Furthermore, based on the analysis of the kinetics of all channels through which the addition and abstraction reactions proceed, we expect that the actual feasibility of the reaction channels may depend on the reaction conditions in the experiment.4. The potential energy surface (PES) of the Cl + HONO reaction is researched at the CCSD(T)/aug-cc-pVTZ//UMP2(FULL)/6-311++G(d,p) level of theory. Four different mechanisms including hydrogen abstraction, addition-elimination I and II, and addition-isomerization-elimination are revealed. The hydrogen abstraction mechanism involves hydrogen picked-up by atomic radical Cl, via surmounting TSC1-P1 and TSC3-P1 to produce P1 NO2 + HCl. The addition-elimination I mechanism involves atomic radical Cl addition to the hydroxyl-oxygen of HONO, followed by the elimination of NO. The addition-elimination II mechanism occurs by atomic radical Cl kissing the N-atom of HONO to give product P3. The addition-isomerization-elimination mechanism features atomic radical Cl addition to the terminal O-atom of HONO. Based on the analysis of the temperature influence of all channels through which the abstraction and addition reactions proceed, we expect that the actual reaction mechanism of this reaction may depend on the reaction conditions in the experiment.