Theoretical Investigations On The Reactions Mechanisms And Kinetics Of Several Important Radicals Containing S And N Atoms In Atmosphere

The radicals containing S and N atoms (such as CH₂SH, CH₃S, NO₂ and NCO) play significant rloes in the combustion chemistry and atmospheric chemistry. Detailed investigations on the mechanisms and kinetics of those reactions are very important to control atmospheric pollutions. As for the theoretical study, choice of suitable method is one of the keys to calculate mechanisms and kinetics of aim reactions. Many theoretical investigations indicate that decomposition of intermediates in multi-channels reaction plays important roles. Moreover, combinational methods, such as Gn serials, BMC-CCSD and MC-QCISD, prefer to single methods in the single point energy calculation.Using ab initio and density function theory (DFT) chemistry methods, we studied the detailed mechanisms and pathways for serial reactions: CH₃S+NO₂, CH₃S+CO, C₂H₅S+NO₂, CH₂SH+O₂, CH₂SH+NO₂, NCCO+O₂, NCO+CH₃OH and NCO+C₂H₅OH. Moreover, based on the potential energy surface (PES) obtained, rate constants for some important channels are calculated. Thus, useful information and elementary theoretical evidence are provided for further studying these reactions experimentally. The important and valuable results in this thesis are summarized as follows:1. The mechanisms for the reactions of CH₃S+NO₂ and C₂H₅S+NO₂ are investigated on both of the singlet and triplet potential energy surfaces (PESs). (1). The results show that two reactions are more predominant on the singlet PESs, while they are negligible on the triplet PESs. (2). Without any barrier height for the whole process, the main channel of CH₃S+NO₂ reaction starts from the addition of reactants to form the energy-rich intermediate CH₃SONO and then dissociate to CH₃SO+NO. (3). For C₂H₅S+NO₂ reaction, the initial addition of the reactants is barrierless to form energy-rich adduct C₂H₅SONO and C₂H₅SNO₂. The N-O bond in C₂H₅SONO dissociates directly to give out C₂H₅SO+NO, which is the most feasible channel. The heat released in the formation of C₂H₅SNO₂ prompts further reaction. With lower barrier surmounted, CH₃CHS+t-HONO play some roles for the whole reaction. (4). The direct hydrogen abstraction channels are found on the triplet PES. However, with high barriers and unstable products involved, those channels are negligible. Our calculation is in accordance with experimental presumption.2. We calculated the potential energy surface for the reaction of CH₃S with CO, and the rate constants for the feasible channels leading to several products were calculated by TST and multichannel-RRKM theory. The results show that addition/elimination mechanism is dominant, while hydrogen abstraction mechanism is uncompetitive. The major channel is the addition of CO to CH₃S leading to the intermediate CH₃SCO, which decomposes to CH₃ + OCS subsequently. Over the temperature range of 200 to 3000 K, the overall rate constant is positive temperature dependence and pressure independence, and it can be described by the expression as k = 1.10×10^-16T^1.57exp(-3359/T) cm³ molecule^-1 s^-1. At the temperature range between 208 and 295 K, the calculated rate constants are in good agreement with the experimental upper limit data. At T = 1000 and 2000 K, the major product is CH₃ + OCS at lower pressure; while at higher pressure, the stabilization of CH₃SCO is dominant channel. 3. The mechanisms of CH₂SH+O₂ and CH₂SH+NO₂ reactions are investigated fully almost for the first time. The results indicate that both the reactions involve direct hydrogen abstraction and addition/elimination mechanisms, and the latter one is more favorable than the former one. (1). For the CH₂SH+O₂ reaction, the oxidation of CH₂SH to form HSCH₂OO is a barrierless process. The most favorable channel is the rearrangement of the initial adduct HSCH₂OO to form another intermediate H₂C(S)OOH via a five-center transition state, and then the C–O bond fission in H₂C(S)OOH leads to a complex CH₂S···HO₂, which finally gives out to the major product CH₂S + HO₂. (2). For the CH₂SH+NO₂ reaction, the result shows that the title reaction is more favourable on the singlet PES thermodynamically, and it is less competitive on the triplet PES. On the singlet PES, the initial addition of CH₂SH with NO₂ leads to HSCH₂NO₂ without any transition state, followed by a concerted step involving C-N bond fission and the shift of H atom from S to O giving out CH₂S + trans-HONO, which is the major products of the title reaction. With higher barrier height, the minor product is CH₂S + HNO₂, which is formed by a similar concerted step from the initial adduct HSCH₂ONO.