| Reactions of radicals and ions play a significant role in diverse environments such as combustion flames, the interstellar medium (ISM), and planetary atmospheres. In this thesis, quantum chemical investigation on the potential energy surfaces of a series of important radicals and ions withσ-bonded orπ-bonded molecules as well as radical-radical reactions have been carried. 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. The results obtained in the present thesis may be helpful for further theoretical and experimental studies of these kinds of reactions. The main results are summarized as follows:1. A detailed theoretical investigation on the potential energy surfaces (PESs) at the CCSD(T)/6-311+G(2df,p)//B3LYP/6-311g(d,p)+ZPVE computational levels is reported for the reactions of CCN with a series ofσ-bonded molecules XHn andπ-bonded molecules C2H2. Firstly, the cyanomethylidyne (CCN) and the other reactant approach each other forming a complex 1. Subsequently, the carbenoid insertion is confirmed as the most favored entrance channel to form isomer 2. For the previous experimental study of CCN with CnH2n+2, a hydrogen-abstraction mechanism was suggested, which is not in agreement with our result. Fo(rX,n)=(C,4), (N,3), (O,2), (P,3), (S,2), the main reaction pathways are as follow: R CCN+ HnX→HnX…CCN 1→H(n-1)XC(H)CN 2→P1 Hn-2XC(H)CN+H R CCN+ HnX→HnX…CCN 1→Hn-1XC(H)CN 2→Hn-2XC(H)2CN 3→P1 Hn-2XC(H)CN+H So, only the product P1 Hn-2XC(H)CN+H should be almost exclusively observed. In addition, the product C2H3CN has been detected in interstellar space and the isomers H3P-CCN and H2S-CCN are considered to be similar to the ylides in nature, being"ylide-like radicals". For(X,n)=(F,1), (Cl,1), the main reaction routs can be expressed below: R CCN+ HnX→HnX…CCN 1→XC(H)CN 2(?)XC(H)NC 6+(M or hv) R CCN+ HnX→HnX…CCN 1→XC(H)CN 2(?)XC(H)NC 6→R The isomers XC(H)CN 2 and XC(H)NC 6 can be stabilized by collisions or radiation. Without such stabilization, 2 and 6 may well dissociate back to the reactant R CCN+HX. For the reaction of CCN with C2H2, the main reaction channels can be indicated: Path 1: R CCN+C2H2→HCC(H)CCN 1→c-C(H)C(H)C-CN 2→P2 c-C(H)CC-CN+H Path 2: R CCN+C2H2→HCC(H)CCN 1→c-C(H)C(H)C-CN 2→CC(H)C(H)CN 5→HCCC(H)CN 4→P4 l-3HCCCCN+H Path 3: R CCN+C2H2→HCC(H)CCN 1→c-C(H)C(H)C-CN 2→H2CC(C)CN19→H2CCCCN 3→P4 l-3HCCCCN+H Path 4: R CCN+C2H2→HCC(H)CCN 1→c-C(H)C(H)C-CN 2→H2CC(C)CN 19→H2CCCCN 3→P3 CCCCN+H2 Formation of cyclic HC4N may be more competitive than that of linear HC4N due to the lower-energy and simpler pathway of P2 c-C(H)CC-CN+H. Formation of CCCCN radical is the least competitive though P3 CCCCN+H2 is almost isoenergetic to P2 c-C(H)CC-CN+H. The studied CCN reactions could be of combustion and astrophysical interest and could provide efficient routes to form novel cyanogen-containing molecules in interstellar space.2. A potential energy surface involving the main structures of the C2F+H2O reaction calculated at the CCSD(T)/6-311+G(2d,2p)//B3LYP/6-311G(d,p)+ZPVE level is carried out. The main results can be written: R C2F+H2O→H2O…CCF 1→P5 HCCF+OH R C2F+H2O→HCC(OH)F 7→H2CC(F)O 6→H2FCCO 5→P1 CH2F+CO R C2F+H2O→HOCC(H)F 2→HCC(OH)F 7→H2CC(F)O 6→H2FCCO 5→P1 CH2F+CO R C2F+H2O→HOCC(H)F 2→HFCC(H)O 4→H2FCCO 5→P1 CH2F+CO The most kinetically competitive channel is the quasi-direct hydrogen-abstraction route forming P5 HCCF+OH. The overall H-abstraction barriers (4.5, 4.7 and 4.2 kcal/mol) for the C2F+H2O reaction are comparable to the corresponding values (5.5, 3.7 and 5.7 kcal/mol) for the analogous C2H+H2O reaction. The much less product is P1 CH2F+CO via the addition-elimination process. Furthermore, addition of a second H2O can catalyze the reaction with the H-abstraction barrier significantly reduced to a marginally zero value (0.5 kcal/mol). This is also indicative of the potential relevance of the title reactions in the low-temperature atmospheric chemistry.3. Four chloride-related radical–radical reactions, i.e., CH3+CH3-nCln (n=1,2,3) and CH3+CCl2, are theoretically studied for the first time by means of the Gaussian-3//B3LYP potential energy surface survey combined with the master equation study over a wide range of temperatures and pressures. The main reaction routs can be depicted: Path 1: nR CH3+CH3-nCln→H3C-C(H)3-nCln na→nP1 H2C=C(H)3-nCln-1+HCl Path 2: nR CH3+CH3-nCln→H3C-C(H)3-nCln na→[H3C-C(H)3-nCln-1…Cl n (only for n=2, 3) ]→nP2 CH3-C(H)3-nCln-1+Cl Path 3: 4R CH3+CCl2→H3C-CCl2 4a→4P1 H2C=CCl2+H. Path 4: 4R CH3+CCl2→H3C-CCl2 4a→H2C-C(H)Cl2 4b→[H2C(Cl)-C(H)Cl 4c]→4P2H2C=C(H)Cl+Cl Path 5: 4R CH3+CCl2→H3C-CCl2 4a→4P3 H2C=CCl+HCl Our calculated results show that the three CH3+CH3-nCln reactions can barrierlessly generate the former two kinetically allowed products P1 H2C=C(H)3-nCln-1+HCl and P2 CH3CH3-nCln-1+Cl with the very high predominance of P1 over P2. For the CH3 reaction with the biradical CCl2, which inevitably takes place during the CH3+CCl3 reaction and yet has never been studied experimentally or theoretically, H2C=CCl2+H and H2C=C(H)Cl+Cl are predicted to be the respective major and minor products. The results are compared with the recent laser photolysis/photoionization mass spectroscopy study on the CH3+CH3-nCln (n=1,2,3) reactions. The predicted rate constants and product branching ratios of the CH3+CCl2 reaction await future experimental verification. 4. A detailed mechanistic study on the singlet potential energy surfaces at the CCSD(T)/6-311+G(2df,p)//B3LYP/6-311G(d,p) computational levels was reported for the reactions of SiCN+/SiNC+ with a series ofσ-bonded molecules HX (X=H, CH3, F, NH2). In contrast to the carbene-featured analogous CCN+/CNC++H2X (X=O,S) reactions, the title reaction SiCN+/SiNC++H2O are not associated with any competitive silylene-insertion characters. The main reaction channels are shown: Path R(X): SiCN++HX→HX…SiCN+ 1→HX…SiNC+ 4→XSiNCH+ 6→P1 SiX++HCN Path R'(X): SiNC++HX→HX…SiNC+ 4→XSiNCH+ 6→P1 SiX++HCN The initial gas-phase condensation between SiCN+/SiNC+ and HX (except the non-ionic H2) effectively forms the adduct HX…SiCN+/HX…SiNC+. The stability of the adduct increases with the electron donating ability of X. Interestingly, the same major product P1 SiX++HCN for both reactions SiCN+ and SiNC+ can be obtained via the process of–CN(?)NC interconversion. The product P2 SiX++HNC is minor one. Even at low temperatures, reactions with the electron donors NH3, H2O and HF can also proceed rapidly via no barrier. This suggests that such reactions may be useful in the synthesis of novel Si-X bonded species. However, the reactions of completely saturated CH4 and H2 produce fragments only at high temperatures.
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