Theoretical Investigations On The Reaction Mechanism For Several Important Nitrogenous Radicals

Reactions of nitrogenous radical or molecule play a significant role in diverse environments such as combustion process, environmental chemistry, biochemistry, atmospheric chemistry, organic chemistry, and interstellar environment. In this thesis, detailed quantum chemical investigations on the potential energy surfaces of a series of important nitrogenous radical and molecule as well as radical-radical or radical-molecule reactions have been carried. Important information of potential energy surfaces such as structures and energies of reactants, products, intermediate isomers, and transition states, possible reaction channels, reaction mechanisms and major products are obtained. For deeper understanding of the reaction mechanism, we carried out master equation rate constant calculations and obtained information of rate constants and product in wide range of temperature and pressure for several reaction. The results obtained in the present thesis may be helpful for further theoretical and experimental studies of these kinds of reactions and detection of interstellar molecules in space. The main results are summarized as follows:1. The NCO + C2H4 reaction is simple and prototype for reaction of the NCO radical with unsaturated hydrocarbons, and is considered to be important in fuel-rich combustion. In this paper, we for the first time performed detailed theoretical investigations for its reaction mechanism based on Gaussian-3//B3LYP scheme covering various entrance and decomposition channels. The most favorable channel is: firstly the NCO and C2H4 approach each other, forming a weakly-bound complex L1 OCN…C2H4, followed by formation of isomer L2 OCNCH2CH2 via a small barrier of 1.3 kcal/mol. Transition states of any decomposable or isomeric channels for L2 in energy are much higher than reactants, which indicate that adduct L2 has stabilization effect in this NCO + C2H4 reaction. The direct H-abstraction channel leading to P1 HNCO + C2H3, might have an important contribution to the eventual products in high temperature. These results can well explain available kinetic experiment. Moreover, reaction mechanism for the title reaction is significantly different from the NCO + C2H2 reaction which proceeds on most favorably to generate the products HCN + HCCO and OCCHCN + H via a four-membered ring intermediate.2. The HCNO+CN reaction is one potentially important process during the NO-reburning process for the reduction of NOx pollutants from fossil-fuel combustion emissions. To compare with the recent experimental study, we performed the first theoretical potential energy surface investigation on the mechanism of HCNO+CN at the G3B3 and CCSD(T)/aug-cc-pVTZ levels based on the B3LYP/6-311++G(d,p) structures, covering various entrance, isomerization and decomposition channels. The results indicate that the most favorable channel is to barrierlessly form the entrance isomer L1c NCCHNO followed by successive ring-closure and concerted CC and NO bond-rupture to generate the product P1 HCN+NCO. However, the formation of P4 3HCCN+NO predicted as the only major product in the recent experiment, is kinetically much less competitive. This conclusion is further supported by the master equation rate constant calculation. Future experimental reinvestigations are strongly desired to test the newly predicted mechanism for the CN+HCNO reaction. Implications of the present results are discussed.3. The self-recombination of the methylene amidogen radical (H2CN) is known to be fast and should play an important role in determining the concentration of H2CN radicals in both combustion and astrophysical processes. The rate constants of H2CN+H2CN have been determined by previous experiments, whereas its detailed evolution process and product distribution are still unclear. In this work, by means of quantum chemical and master equation calculations, we for the first time theoretically explored the potential energy surface and kinetics of the H2CN+H2CN reaction. At the CCSD(T)/6-311++G(2df,p), CCSD(T)/aug-cc-pVTZ and Gaussian-3 single-point levels based on the B3LYP/6-31++G(d,p) structures, the dominant channel was found to be (R) H2CN+H2CN H2CNNCH2 (L1) r-CH2NNCH2 (r1) N2+C2H4 (P1) with a zero overall barrier. The calculated rate constants are in agreement with available experiments. Of particular interest, since the formed product involves molecular nitrogen, the H2CN+H2CN reaction might have important contribution to the nitrogen-recycling in a number of conflagrant and astrophysical processes.4. The reaction of the methylene amidogen radical (H2CN) with hydroxyl (OH) is potentially important in a number of chemical processes. In this paper, we performed the first theoretical potential energy surface investigation on the mechanism of H2CN+OH at the CCSD(T)/6-311++G(2df,p), G3B3, CCSD(T)/aug-cc-pVTZ and CCSD(T)/aug-cc-pVQZ single-point levels using the B3LYP/6-31++G(d,p), BH&HLYP/6-31++G(d,p), and QCISD/6-311++G(d,p) optimized geometrie,covering various entrance, isomerization, and decomposition channels. Two reaction channels are feasible in thermodynamics and kinetics: 1) the quasi hydrogen-abstraction of H2CN by OH to form product HCN+H2O via a weakly-bound complex NC(H)H···OH, and 2) the addition-elimination to form HCN+H2O via a stable intermediate CH2NOH. According to the master equation rate constant calculations in the wide ranges of temperature (50-1100 K) and pressure (120-1300 Torr) range, when the temperature is below 400 K, the effective stabilization takes place, making CH2NOH as the dominant product. Once the temperature reaches 500 K, the formation of the product HCN+H2O by the quasi-direct H-abstraction process becomes favorable. The calculated rate constants are consistent with available experiments. Moreover, under the experimental conditions (298 K, 120 and 200 Torr), the H2CN+OH reaction favors the effective condensation forming H2CNOH, whereas the previously suggested hydrogen-abstraction mechanism prevails only after 500 K. The implications of the present study in combustion and astrophysical processes are discussed.