Theoretical Investigations On The Reactions Of Several Important Species In Atmosphere Chemistry

The reactions of the isocyanate radical with hydrocarbons and hydrofluorocarbons (HFCs)with active radicals play important roles in various fields, such as combustion chemistry and atmospheric chemistry. 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 reaction mechanisms and the dynamics) is very difficult. Therefore, more and more attentions have been focused on their theoretical researches in recent years.With the quantum chemistry calculation methods, we studied the reactions of the isocyanate radical with hydrocarbons(CH₃,C₂H₅,CH₄ and C₂H₆) and CH₄ with NO₂. The reaction mechanisms are theoretically investigated in detail. As well as we studied the mechanism of hydrofluorocarbons(HFCs)with OH radical. Our calculations provide the elementary theoretical evidence for further experimental research. The following is the main results:1. A detailed computational study has been performed at the QCISD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) level for the NCO with CH₃ reaction by constructing singlet and triplet potential energy surfaces(PESs). The computational results show that the title reaction is more favorable on the singlet PES than on the triplet PES. On the singlet PES, the dominant channel is the barrierless addition of O or N atom to C atom of methyl group to form CH₃NCO (IM1) and CH₃OCN (IM2). On the triplet PES, the favorable channel is the barrierless addition N atom to C atom of methyl group to form an intermediate CH₃NCO (3IM2), which then undergoes N─C scission process to give out CH₃N+CO. And other products channels are minor with high barrier heights.2. The mechanisms of C₂H₅ with NCO was investigated at QCISD(T)/6-311++G(d,p)//B3LYP/6-311+G(d,p) level on both of single and triple PES.. The results indicate that single PES is much lower than that of the triple PES. On the single PES, the initial adduct of the reactant is barrierless and released lots of energy available for further reaction. With the lowest barrier heights and the significant transition states lower than the reactant, the dominant channel is a concerted step of involving H shift and C-O bond scission from the adduct C₂H₅OCN(IM2) to give out the product C₂H₄+HNCO. The secondary product is C₂H₄+HOCN, which is yielded via a similar concerted step form another adduct C₂H₅NCO (IM1). Other products are negligible with high barriers or less stable product. On the triple PES, the most feasible channel is the direct hydrogen abstraction of H in CH₃ group by N atom to form CH₂CH₂+HNCO. However, the product is less stable, and it is not competitive with the dominant channel on the single PES.3. The kinetics of hydrogen abstraction reaction of NCO + CH₄ is studied by ab initio direct dynamics method. The potential energy surface(PES)information is obtained at the MP2/cc-pVDZ level level, and more accurate energies of stationary points are calculated at the G3(MP2)level. By means of the Polyrate 9.1 program ,the rate constants over the temperature range of 400—2000 K are calculated by canonical variational transition state theory(CVT)incorporating small-curvature tunnelling(SCT)contributions proposed by Truhlar et al. The calculated rate constants are found to be in good agreement with the available experimental data.4. The kinetics of hydrogen abstraction reaction of NCO + C₂H6 is studied by ab initio direct dynamics method. The potential energy surface(PES)information is obtained at the MP2/cc-pVDZ level, and more accurate energies of stationary points are calculated at the G3(MP2)level. By means of the Polyrate 9.1 program ,the rate constants over the temperature range of 220—2000 K are calculated by canonical variational transition state theory(CVT) incorporating small-curvature tunnelling(SCT)contributions proposed by Truhlar et al. The calculated rate constants are found to be in accordance with the experimental results.5. The potential energy surface (PES) information of the CH₄+NO₂ reaction is built up at the BMC-CCSD//MPW1K/6-311G(d,p) level. The rate constants are calculated by canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT). The theoretical rate constants are in good agreement with the experimental values. Three feasible channels and the three corresponding transition states, TS1, TS2 and TS3 are identified respectively. Both N and O can attack the carbon atom of CH₄ in the reaction of CH₄ with NCO. The dynamics calculations also exhibit that O abstraction H of CH₄ dominates the title reaction over the temperature range.6. The H-abstraction reaction of CF3CHFCH₂F+OH is investigated by ab initio direct dynamics method. The potential energy surface(PES)information is obtained at the B3LYP/6-311G(d,p) level, and more accurate energies of stationary points are calculated at the level of QCISD(T)/6-31G(d). Four feasible channels and the four corresponding transition states: two channels forα-H abstraction and two channels forβ-H abstraction. The reaction proceeds feasible mainly viaβ-H abstraction with most exothermic.