Quantum Chemistry Study On The Reaction Mechenism Of Ketene With H And CN Free Radicals

Reactions of ketene (CH2CO) with some small molecular free radicals play an important role in combustion and atmospheric chemistry. The experimental and theoretical studies to these reactions have attracted many interests of chemists these years. Using ab initio and DFT methods, the reactions about ketene with H and CN free radicals have been investigated in this thesis, as following:1. The reaction of ketene with atomic hydrogen may occur through four different channels, that is: CH2CO+H →TS1→IM2→TS2→CH3+CO (1) →TS6→IM1→TS3→IM2→TS2→CH3+CO (2) →TS4→HCCO＋H2 (3) → TS5 → IM3 →TS7 → IM4 → TS8 → C2H2 + OH (4)All the stationary points on the reaction paths are optimized by using ab initio QCISD calculations at 6-311G (d,p) basis set level. The single-point energy corrections of every stationary point on the minimum energy path (MEP) are performed at the level of CCSD(T) and G2 theory. All the transition states are verified through vibrational analysis. The computed intrinsic reaction coordinate (IRC) confirms that the transition states represent an energy maximum on passing from reactants to products.The channel (1) and the channel (2) are both the stepwise reactions which can produce CH3+CO ultimately, but the process of their forming the intermediate (IM2) are different. This situation results from that the atomic hydrogen attacks the different position of the ketene molecule. The enthalpy change of the reactions is –160.35 kJ?mol-1. The rate constants of the reactions are calculated based on traditional transition state theory (TST) and the results suggest the channel (1) is the major channel on the condition of low temperature, which are in good agreement with experimental results. The channel (3) is a typical hydrogen abstraction reaction. The rupture of C-H bond and the formation of H-H bond are concerted. The calculated potential barrier is 58.71 kJ?mol-1, which is higher than the potential barrier of channel (1), so the hydrogen abstraction reaction is a minor channel .The last channel is a three-step reaction. It forms the ultimate products C2H2+OH via three transition states and two intermediates. The calculated results suggest that it is very difficult to occur because the forward and reverse potential barrier of the rate controlling step, IM3 →TS7 → IM4, are 221.80 and 197.78 kJ?mol-1 respectively. 2. There are two channels of reaction CH2CO+CN, that is: CH2CO+CN →IM1→TS1→CH2CN+CO (5) →TS2→HCN+HCCO (6)The calculation results at B3LYP/6-311G (d,p) suggest that the channel (5) is a typical addition-elimination reaction. The CN free radical reacts with ketene to form the intermediate (IM1) firstly. This reaction is found to be a barrierless process, and then IM1 can undergo further decomposition process to produce ultimate products. The enthalpy change of the channel is -258.76 kJ?mol-1. The calculated potential barrier of decomposition step is 30.37 kJ?mol-1 by CCSD(T) single-point energy correction, which is less than that of the channel (6). So we consider that the addition-elimination reaction is the major channel of the reaction CN free radical with ketene. The channel (6) is a hydrogen abstraction reaction. It is a minor channel of reaction for its potential barrier (86.49 kJ?mol-1) is higher than the channel (5).