Theoretical Studies On The Mechanisms Of Several Important Nitrogenous Molecules And Transient Species In Atmosphere Chemistry

Reactions of the small nitrogenous molecules and transient species are theimportant subject in atmosphere chemistry. Especially, in the photochemistryreaction, the lives of those molecules and radicals are usually very short. Itmakes it difficult to detect them in the experiment. At the same time, they playan important role in studing the mechanisms of those reactions. So, it isnecessary to study them in theory. In this paper, the mechanisms of thosereactions were investigated by means of calculation of quantum chemistry athigh levels, which concerned the C₃H₅+NO reaction, the role of neutralmolecule N₃O₂ played in the N₃O₂ -+ h v → NO + N₂O + e-and NO +N2_O= N₂ + NO₂ reactions and the characters of CH₃N on some lowelectronic states. Some conclusions that are made in the present paper may behelpful to further theoretical and experimental studies. The main results aresummarized as follows. 1. The potential energy surfaces (PES) of CH₂=CHC·H₂+NOand ·CH=CHCH₃+NO reactions were investigated at the CCSD(T)/6-311G(d,p)//B3LYP/6-311G(d,p) level. In the CH2=CHC·H2+NO reaction, the mostfeasible attack is the direct radical-radical combination to form intermediateH2CCHCH2NO m1. There are five reaction pathways from m1 to the products.Three of them are feasible in thermodynamics while the other two pathways areunfeasible. The most feasible pathway in them is expressed as H2CCHCH2NOm1 → H2CCHCHNHO m2 → H2CCHCHNOH m3 → P1 H2CCHCN+H2O. Allthe reaction barriers of the five pathways are very large and it is very difficultfor m1 to be converted into other intermediate or products further. This isindicative of a distinct pressure dependence of the rate constants observed in theexperiment.In the ·CH=CHCH3+NO reaction, four pathways were studied. The mostfeasible one in them is expressed as R (·CH=CHCH3+NO)→m1 (trans-CH3CH=CHNO)→m2(cis-CH3CH=CHNO)→m8(CH3-cyc-(CHCHNO))→P1(CH3CHO+HCN). It is analogous to the most feasible pathway ofthe ·CH=CH2+NO reaction. But the former is more significative for it is feasiblein dynamics.2. The roles of neutral molecule N3O2 in the N3O2 -+ h v → NO + N2O + e-and NO + N2O N2 + NO2 reactions were investigated at CCSD(T)/6-311G(d,p) //B3LYP/6-311G(d,p) level. We found the W-shape N3O2 neutralmolecule, which has never been found before, as the intermediate in thephotolysis of the N3O2 anion. However, this neutral molecule N3O2 has a highrelative energy and a low barrier (5.96 kJ/mol) to the dissociation products ofNO + N2O.In the N2O+NO N2+NO2 reaction, all the relative energies of theintermediates and the transition states are very high. But the elementaryreaction in which neutral molecule N3O2 plays the role of transition state ismore feasible than the multicomponent reaction.3. The electronic structure, the C-N bond dissociation reaction and the1,2-H-shift reaction of CH3N on some low-lying states were investigated in thepresent paper. The conclusions are listed as follows.(1) In this paper, the geometries at the CASSCF(12,11)/cc-pVDZ level ofCH3N on 11A', 13A', 11A", 13A", 21A', 21A" and 23A" states were optimizedwith Cs symmetry. And the energies at CASPT2(12,11)/cc-pVQZ level by usingthe CASSCF geometries were calculated. In those states, 11A', 13A' and 13A',23A" are degenerate. They correspond to the 1E and 3E states with C3v symmetry,respectively. The Jahn-Taller effects appear on both degenerative states. And itis more obvious on 3E state than that on 1E state.(2) The C-N bond dissociations of CH3N on 13A",11A',11A",21A',13A'and 23A" states are typical dissociations. Their dissociation energies are301.15,393.86,393.69,304.62,147.92 and 140.47 kJ/mol, respectively. Whenit is on the 21A" state, a low barrier is involved in the C-N bond dissociation ofCH3N. The barrier is around a C-N bond distance of 0.21 nm. The dissociationproducts are all CH3(2B1)+ N(2P) on the 11A', 11A", 21A', 13A', 23A" and 21A"states, in stead of CH3(2B1)+N(4S) on the 13A" state.(3) The 1,2-H-shift processes and products were investigated on 11A', 13A',11A", 13A", 21A', 21A" and 23A" states. We confirmed the reason for thedifferent H-shift products on 11A' and 11A" states predicted by Travers et al.The transition state on 11A' was located and the barrier is very low (15.22kJ/mol). This result can explain the mechanism of short life of CH3N. On theother hand, the crossing in the H-shift reaction of CH3N between the 13A" and11A' states was located, its relative energy is 137.17 kJ/mol, and its spin-orbitcoupling magnitude is 495.188 cm-1. This magnitude is large enough to connect13A" and 11A' states at the crossing. But the large barrier on 13A" between theCH3N and the crossing makes it less competitive compared with that on 11A'.After the comprehensive consideration of the H-shift and the C-N bonddissociation of CH3N, it is concluded that the H-shift reactions on 13A", 11A'and 11A" states are easy to occur, the C-N bond dissociation reactions on 13A',23A" and 21A" states are easy to occur, and both the reactions are hard to occuron 21A' state.