Theoretical Investigations On The Reactions Of Several Important Radicals Systems In Atmosphere Chemistry

The reactions of ketene (CH2CO) and hydrofluorocarbons (HFCs) with active radicalsplay important roles in various fields, such as combustion chemistry and atmosphericchemistry. Due to the short lives of the radicals and the difficulty to obtain the pure species,the experimental research for their structures and reaction features (especially the reactionmechanisms and the dynamics) is very difficult. Therefore, more and more attentions havebeen focused on their theoretical researches in recent years.With the quantum chemistry calculation methods, we studied the reactions of ketene(CH2CO) and hydrofluorocarbons (HFCs) with active radicals or atoms. The reactionmechanisms are theoretically investigated in detail, and rate constants and branching ratios arealso predicted. Our calculations provide the elementary theoretical evidence for furtherexperimental research. The most valuable results in this thesis can be summarized as follows:1. The mechanism for the CH2CO + O(3P) reaction is investigated at theQCISD(T)/6-311+G(3df, 2p)//B3LYP/6-311+G(d, p) level. The computational results showthat the reaction proceeds via two possible mechanisms, i.e., carbonyl carbonaddition-elimination mechanism and olefinic carbon addition-elimination mechanism. Fiveproducts, CH2+CO2,CO + CH2O,CCO + H2O,H + CO + HCO and HCO + HCO, aregenerated. With the lowest energy barrier, the pathway producing CH2 + CO2 dominates thetotal reaction and CH2 + CO2 is the main product. Our results provide the theretical evidencefor the experimental report of DeMore group.2. A detailed theoretical survey on the potential energy surface for the CH2CO + CN reactionis carried out at the QCISD(T)/6-311++G(d, p)//B3LYP/6-311+G(d, p) level. The reactionproceeds through four possible mechanism, i.e. direct hydrogen abstraction, olefinic carbonaddition-elimination, carbonyl carbon addition-elimination and side oxygen addition-elimination.Direct hydrogen abstraction is unfavorable kinetically. With the lowest energy, the olefinic carbonaddition-elimination channel to yield CH2CN+CO is most important among all the channels. Theabove conclusions are in good accordance with experimental results. Furthermore, the channelgenerating CH2NC + CO via olefinic carbon addition-elimination mechanism is considerablycompetitive especially as the temperature increases.3. The reactions of CH2CO with NCX(X=O, S) are theoretically investigated at the levelof QCISD (T)/6-311+G(d, p)//B3LYP/6-311+G (d, p). Our computational results suggest thatboth reactions can proceed via direct hydrogen abstraction mechanism and olefinic carbonaddition-elimination mechanism. The direct abstraction of one of the H atoms in CH2COmolecule by NCX may lead to HCCO + HNCX(PX-1), HCCO + HCNX(PX-2) and HCCO +HXCN(PX-3), respectively. There is some difference between their oefinic carbonaddition-elimination reactions. Both N and O can attack the oefinic carbon atom of CH2CO inthe reaction of CH2CO with NCO. As for CH2CO + NCS reaction, C-and N-attack is bothpossible, while S-dominating attack is not found. With the lowest barrier height, the channlegenerating CH2NCO + CO is considered as a kinetically favourable pathway, which was alsosummarized by Hershberger group. Similarly, CH2NCS + CO is theoretically proved to bemain product for CH2CO + NCS reaction.4. The hydrogen abstraction reaction of CH3CH2F + OH is studied by an ab initio directdynamics method. Three feasible channels and the three corresponding transition states, TS1,TS2a and TS2b are identified respectively. The rate constants over the temperature range of210—3500 K are calculated by canonical variational transition state theory (CVT) with thesmall-curvature tunneling correction (SCT) at the G3//MP2/6-311G(d, p) level. Thetheoretical rate constants and branching ratios are in good agreement with the experimentalvalues. The dynamics calculations also exhibit that α-H abstraction dominates the titlereaction from 210 to 800 K, and the reaction proceeds mainly via β-H abstraction in thetemperature higher than 800 K.5. The H-abstraction reaction of CH3CH2F + Cl is investigated by an ab initio directdynamics method. The potential energy surface (PES) information is obtained at theMP2/6-311G(d, p) level, and more accurate energies of stationary points are calculated at thelevel of QCISD(T)/6-311+G(3df, 2p) and G3(MP2). Both α-H abstraction and β-Habstraction are possible, and three transition states, TS1, TS2a and TS2b are identifiedrespectively. The rate constants over the temperature range of 220—2800 K are calculated bycanonical variational transition state theory (CVT) with the small-curvature tunnelingcorrection (SCT). The theoretical rate constants and branching ratio of k1/k agree well withthe experimental values. The dynamics calculations also exhibit that α-H abstractiondominates the title reaction almost over the whole temperature range.6. The potential energy surface (PES) information of the CH3CHF2 + Cl reaction is builtup at the G3(MP2)//MP2/6-311+G(d, p) level. With the ISPE method, the CVT/SCT rateconstants of the reaction are calculated over the temperature range of 200―2500 K. Thebranching ratios are also decided, as well as the dependence on the temperature. Thecalculated rate constant and branching ratios are both in accordance with the experimentalresults. The calculations also indicate that the rate constants over the temperature range 200―2500 K are fitted by the three-parameter expression: k = (4.62× 10-19)T 2.77 exp(-782.89/T).