Theoretical Investigations On The Mechanisms For Reactions Of NO₂ With Several Important Radicals

As major atmospheric pollutants released by combustion process, nitrogenoxides have attracted attention for experimental and theoretical chemists. Thereactions of nitrogen dioxide with some important radicals are one ofimportant topics in planetary atmospheric chemistry as well as in combustionchemistry. In this thesis, the theoretical investigations on the potential energysurfaces of some important reactions of radical with nitrogen dioxide arecarried out. Important information of potential energy surfaces such asstructures and energies of intermediate isomers and transition states, possiblereaction channels, reaction mechanisms and major products are obtained. Theresults obtained in the present thesis may be helpful for further theoretical andexperimental studies of this kind of reactions. The main results aresummarized as follows:1. The potential energy surfaces (PES) of CHX+NO2 (X=F, Cl) reactionsare firstly studied. After considering the potential energy surface of CH2+NO2reaction obtained by other authors, the main reaction pathways of CHX+NO2(X=H, F, Cl) reactions are analyzed and expressed as follows:Path 1 CHX+NO2→HXCNO2→HXCO+NOPath 2 CHX+NO2→HXCNO2→XCN(O)OH →XCNO+OHPath 3 CHX+NO2→HXCNO2→HXCNO+3OPath 4 CHX+NO2→HXCNO2→XCN(O)OH →X(OH)CNO→XCOH+NO 或HX+OCNO→HX+CO+NOFor the CH2+NO2 reaction, the most feasible pathway is Path 1 with theproducts H2CO+NO, Path 2 is much less competitive, Path 3 is the leastfavorable. For the CHF+NO2 reaction, the most competitive pathway is Path 1with the products HFCO+NO, Path 2 forming HFCO+NO and Path 4 givingHF+OCNO or HF+CO+NO are the second and third feasible pathway,respectively. For the CHCl+NO2 reaction, the dominant decompositionpathway is Path 1 to give product HClCO+NO, while Path 3 leading toHClCNO+3O and Path 4 forming ClCOH+NO may have less contribution tothe reaction.2. The theoretical calculations on the potential energy surfaces of thereactions of CH2Y, CHCl2, CCl3 with NO2 (Y=H, F, Cl, OH) are performed.The main results are summarized as follows:(1) These reactions proceed mostly through singlet pathways and less gothrough triplet pathways. They are all initiated by the carbon-to-nitrogenattack barrierlessly leading to adduct H2YCNO2(Y=H, F, Cl, OH) orHnCl3-nCNO2 (n = 0-1).(2) For CH2Y+NO2(Y=H, F, Cl) reactions,the following reaction pathways areobtained.Path 1: CH2Y + NO2 → H2YCNO2 → H2YCONO → CH2YO + NO (for Y =H, F) or CH2O + YNO (for Y = F, Cl)Path 2: CH2Y + NO2 → H2YCNO2 → trans-H2YCONO → CHYO + HNOPath 3: CH2Y + NO2 → H2YCNO2 → trans-H2YCONO → CHYO + HONPath 4: CH2F + NO2 → H2FCNO2 → trans-H2FCONO → cis-H2FCONO →trans-HFCONOH → CHFO + HONFor the CH3 + NO2 reaction, CH3O + NO (via Path 1) and CH2O + HNO (viaPath 2) are the most favorable products with a comparable yield, and CH2O +HON via Path 3 is the least competitive product. For the CH2Cl + NO2reaction, the most feasible product is CH2O + ClNO (via Path 1) and thesecond feasible product is CHClO + HNO (via Path 2). For the CH2F + NO2reaction, four products can be formed, i.e., CH2O+FNO and CH2FO+NO (viaPath 1), CHFO + HNO (via Path 2), and CHFO + HON (via Path 3 and Path4). The order of their feasibility should be CH2O+FNO ≈ CH2FO+NO ≈CHFO + HNO > CHFO + HON. In addition, the major product CH2O + ClNOin CH2Cl + NO2 reaction can further dissociate to give species CH2O + Cl +NO, while in CH2F + NO2 reaction, the similar process CH2O + FNO →CH2O + F + NO is thermodynamically prohibited.(3) For reactions of CHCl2, CCl3 with NO2, after considering the potentialenergy surface of CH2Cl+NO2 reaction, the important pathways for theCHnCl3-n+NO2 (n = 0-2) reactions can be indicated as:Path 1 CHnCl3-n + NO2 → HnCl3-nCNO2 → HnCl3-nCONO→ CHnCl2-nO +ClNO (n = 0-2)Path 2 CHnCl3-n + NO2 → HnCl3-nCNO2 → HnCl3-nCONO→ CHnCl2-n + HNO(n = 0-1)Path 3 CHCl2 + NO2 → HCl2CNO2 → HCl2CONO → CHClO + ClONPath 4 CHCl2 + NO2 → HCl2CNO2 → HCl2CONO →CCl2O + HONBy comparison, it is easily found that the features of potential energy surfaceson the three CHnCl3-n + NO2 (n = 0-2) reactions are almost similar. For thethree reactions, isomer b HnCl3-nCONO (n = 0-2) can undergo 1,3-chlorinemigration from C to N atom associated with N-O1 weak bond cleavageleading to species CHnCl2-nO (n = 0-2) + ClNO as the most favorable product.It should be pointed out that CCl2O + ClNO is the only primary product thatcan be obtained for the CCl3 + NO2 reaction. The fragment ClNO in the mostfavorable product can further dissociate to form Cl atom and NO. The primaryproduct CHnCl2-n (n = 0-1) + HNO can be obtained for both reactions CH2Cl +NO2 and CHCl2 + NO2, and it has less contribution to both reactions. Allreactions involve the same middle-N association process CHnCl3-n + NO2 →HnCl3-nCNO2 (n = 0-2) followed by isomerization to HnCl3-nCONO.For the reactions of CH2Cl, CHCl2, CCl3 with NO2, with the increase ofchlorine substitution, the barrier heights of rate-determining process(HnCl3-nCNO2 → HnCl3-nCONO (n = 0-2)) increase. As a result, the oppositereactivity trend is obtained, i.e., k(CH2Cl +NO2) > k(CHCl2 +NO2) > k(CCl3+NO2).(4) For CH2OH+NO2 reaction, the four feasible pathways are shown:Path 1: HOCH2NO2 1 → P1 CH2O + HONO-transPath 2: HOCH2NO2 1 → HOCH2ONO-trans 2a → P2 HOCHO + HNOPath 3: HOCH2NO2 1 → HOCH2ONO-trans 2a → P3 CH2O + HNO2Path 4: HOCH2NO2 1 → HOCH2ONO-trans 2a → HOCH2ONO-cis 2b → P4CH2O + HONO-cisThe most competitive pathway is Path 1, Paths 2, 3, and 4 may competeintensively each other, but they all can't compete with Path 1.3. The potential energy surface (PES) of the CN+NO2 reaction isinvestigated. Our calculations indicate that the title reaction proceeds mostlythrough singlet pathways, less go through triplet pathways. This reaction ismost likely initiated by the carbon-to-nitrogen approach to form adduct a(NCNO2) with no barrier. By analysis, there are two competitive pathways:Path 1: R → a → b1 (b2) → P1 (NCO + NO)Path 2: R → a → b1 → b2 → c1 (c2) → P2 (CNO + NO)Path 1 is the major channel and Path 2 is the minor channel.4. A detailed doublet potential energy surface of the CCO + NO2 reactionsystem is characterized. The most feasible pathway is the barrierlessassociation of CCO with NO2 forming a (OCCNO2) followed by oxygen shiftto b (O2CCNO), which will dissociate to product P1 (CO2 + CNO). While thesuccessive isomerization and fission processes of b to products P2 (OCNO +CO) and P3 (c-OCC-O + NO), both of which can further decompose to P4(2CO + NO), are much less competitive.5. The detailed mechanistic study on the reaction HCCO+NO2 isperformed. This reaction proceeds most likely through the singlet HCCONO2potential energy surface (PES) initiated by the carbon (neighboring to Hatom)-to-nitrogen attack leading to adduct OCCHNO2 (1) with no barrier.Subsequently, there are three possible pathways as follows:Path 1: OCCHNO2 1 → O-cCCHN(O)O 2 → P1 HCNO+CO2Path 2: OCCHNO2 1 → OCCNOHO (3a, 3b, 3c) → OCOHCNO (4a, 4b) → P1HCNO+CO2Path 3: OCCHNO2 1 → OCCNOHO 3a → O-cCCN(OH)O (5a, 5b) → P2HONC+CO2Their feasible order is Path 1 > Path 2 ≈ Path 3.For the reactions CHX (X=F, Cl), CH2Y (Y=H, F, Cl, OH), CN, CCO,HCCO with NO2, Since the isomer and transition state involved in the mostfavorable pathway are all lower than the reactants R in energy, they areexpected to be fast, as is confirmed by experiment. So these reactions may beof significance in atmospheric and combustion chemistry. While for thereactions CHCl2, CCl3 with NO2, because rate-determining transition states infeasible pathways all lie above the reactants, the two reactions may beimportant in high-temperature processes.