The Study On The Reaction Mechanism And Kinetics For Two Kinds Of Important Atmospheric Species

Isoprene as a kind of biogenic volatile organic compound is the most abundantnonmethane hydrocarbon released into the atmosphere by deciduous plants, and its majoroxidation products are methacrolein (MACR), methyl vinyl ketone (MVK), and someperoxides, which are the main pollutants resource in lower atmosphere. The reactions ofMACR and MVK with active radical, and the recycle of OH play important roles in thedegradation of isoprene. The NCX(X=O, S) radical, produced by the reaction ofHNCX(X=O, S) with active radicals, plays important role for the reduction of NOxpollutants from fossil fuel combustion emissions. However, most of the above reactionspeoceed barrierlessly, and it is difficult to study the reaction mechanism and kinetics byexperiment, and the related research are very limited. In the present work, we carried outthe theoretical study on the mechanism and kinetics for the following reaction systems:(1)MACR-OH+O2;(2) MACR+Cl;(3) HNCO+CN;(4) HNCS+CN.The geometries optimization of the stationary points and the two-dimensionalpotential surface were carried out by the DFT (M06-2X, B3LYP) theory. Harmonicvibrational frequencies were calculated at the same level, and the intrinsic reactioncoordinate (IRC) calculations were performed to confirm that each transition state iscorrectly linked to the appropriate reactants and products. The energy refinement has beenperformed by means of a multi-level (denoted as HL) energy calculation method. Thetraditional transition state theory (TST), the canonical variational transition state theory(CVT), the microcanonical variational transition state theory (μVT), the CVT coupled withEckart tunneling correction (CVT/Eckart); the CVT coupled with small-curvature tunnelingcorrection (CVT/SCT) were used to calculated the thermal rate constants. The masterequation (ME) simulation was performed to study the temperatue and pressure dependenceof the reaction and the branching ratios of the products. The main results are as follows:(1) The complicated reaction mechanism for MACR-OH+O2reaction was investigatedby using the B3LYP method. O2adds to the central carbon atom of MACR-OHbarrierlessly along the direction of the opposite of–CH3group or–CHO group to form twodifferent imtermediates. The O atom of the peroxy group can abstracts the methyl-H atom, aldehyde-H atom, or the hydroxyl-H atom by the isomerization of the imtermediates, andthen the isomerization intermediates dissociated to different decomposition product. Thechannel of O atom of the peroxy group abstracting the aldehyde-H atom and dissociating toproducts CO, CH3COCH2OH, and OH (p1) has the lower barrier. The kinetics calculationwas carried out by the the canonical variational transition state theory, the microcanonicalvariational transition state theory, and the ME method. The intermediate imM’and theproducts p1are the respective main product at T <260K and T>270K. The rate constantincreases with the increase of the pressure at T <260K and P <50Torr, and the rateconstant is pressure-independent when T>270K.(2) The Energetics and kinetics for the reaction of the methacrolein+Cl wereinvestigated at the HL//M06-2X/6-31+G(d,p) level of theory. The addition of the Cl atomto the terminal carbon atom of the C=C double bond is a barrierless process and the directaldehydic hydrogen abstraction by the Cl atom is predicted having a little barrier. The Clatom in the terminal addition intermediate can further abstract the aldehyde-H, themethyl-H and the methylene-H atoms, while the first two reactions are dominant exitchannels. Combining the CVT and the ME calculation results, the overall rate constant ofthe title reaction at298K and760Torr is predicted to be kOverall,CVT=2.3×1010cm3molecule1s1, and the branching ratios of the terminal addition intermediate of the C=Cdouble bond, the aldehyde-H abstraction products and the methyl-H abstraction productsare about86%,12%and2%, respectively, which is coincident with the reportedexperimental results. The terminal addition intermediate of the C=C double bond and thealdehyde-H abstraction products are the main products at T <300K and T>350K,respectively. At temperatures T <300K, the pressure dependence is obvious when thepressure is below10Torr, and the high pressure limit is about100Torr.(3) A detailed reaction mechanism of the HNCO+CN reaction system has beeninvestigated by considering the attack of CN radical at all possible sites of HNCO based onHL//B3LYP/6-31+G(d,p) and the HL//CCSD/6-31+G(d,p) methods. The reaction isinitiated from three electrostatically attracted complexes Coma, Comb and Comc. Thepathways starting from Comb and Comc are energetically inaccessible due to the higherenergy barriers. The main channel, initiated by C-N addition and followed by N-C bond rupture to generate the product HNCN+CO, has lower energy barrier than that of thehydrogen abstraction channel. The TST/Eckart rate constants calculation results show thatthe tunneling effect is important within the lower temperature range. The two abovereaction channels compete with each other at temperatures around273K and the hydrogenabstraction channel is less competitive at higher temperatures.(4) Based on the HL//B3LYP/6-311+G(2d,p) level of theory, the potertial energysurface for the HNCS+CN raction has been calculated. The MEPs have been determinedby constructing a two-dimensional PES, which indicates that the association of S atom iseasier than the N atom of HNCS with the C atom of CN radical, and the CVT calculationsverify that the rate constants of the former is more than double that of the latter in thetemperature range of200-500K. Master equation calculations show that HNC and NCS,which comes from the association channel of the C atom of CN with the S atom of HNCS,are the dominant products with branching ratio of about0.8, and that HCN and NCS, whichcomes from the association channel of the C atom of CN with the N atom of HNCS,account for about0.2of the final products. The overall rate constant calculated by using themaster equation method is kOverall, ME=5.07×1011cm3molecule1s1at298K and760Torr, and the title reaction has a positive-pressure dependence effect at1-760Torr and298K.