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Theoretical Studies on the Reaction Mechanism and Rate Constants of Atmospheric Pollutants Including Fluorine or Bromine

Author: ZhaoJianXiang
Tutor: WangLi
School: Henan University
Course: Physical and chemical
Keywords: Rate constants Reaction Mechanism Dual-level Direct DynamicsMethod Small-Curvature Tunneling Correction
CLC: O643.12
Type: Master's thesis
Year: 2013
Downloads: 22
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Abstract


The study and determination of reaction rate constants has not only been one ofthe main research fields in experiments, but also be paid lots of attention bytheoretical study.The following reactions are investigated by dual-level direct dynamics method:CH3CHBr+HBr→productsCH3CBr2+HBr→productsCH2CH3-nXn+HBr （X=Cl, Br and n=1,2）→productsCHF2CF2OCH3+OH→productsCH2FCF2OCH3+OH→products1. The hydrogen abstraction reactions of CH3CHBr+HBr→CH3CH2Br+Br （R1）and CH3CBr2+HBr→CH3CHBr2+Br （R2） are investigated by dual-level directdynamics method. The equilibrium geometries of the reactants, transition states（TSs）,products, and complexes involved in the reactions are computed at theMPW1K/6-311+G（d,p） level. The single point calculations for the stationary pointsare performed at the modified GAUSSIAN-2method （G2M（RCC5）） based on theMPW1K optimized geometries, respectively. Starting from the saddle-pointgeometries, and going downhill to both the asymptotic reactant and product channels,we construct the minimum-energy path （MEP） by intrinsic reaction coordinate theoryin mass-weighted Cartesian coordinates at the MPW1K level. By means of thePolyrate9.7program, the rate constants are calculated using the improved canonicalvariational transition state theory （ICVT） with the small-curvature tunnelingcorrection （SCT）. The rate constants calculated using the ICVT/SCT method at theG2M（RCC5）//MPW1K/6-311+G（d,p） level are observed to agree well with theavailable experimental values for reaction R1. The rate constants of R2are smallerthan those of R1, which indicates that the bromine substitution decreases thereactivity of ethyl radical. Finally, the rate constants are fitted by two models, that is,three-parameter and four-parameter expressions over a wide temperature range of200-2000K. Finally, the enthalpies of formation for CH3CHBr2and CH3CBr2speciesare evaluated at the G2M（RCC5）//MPW1K/6-311+G（d,p） level by isodesmicreactions.2. The reactions of HBr with CH2CH2Cl, CH2CHCl2, CH2CH2Br, and CH2CHBr2 radicals are calculated by dual-level direct dynamics method. The geometricparameters of the reactants, transition states, complexes, and products are optimized atthe MPW1K/6-311+G（d,p） level. In order to obtain more accurate energies and barrierheights, single-point calculations are carried out by the modified Gaussian-2（G2M）method with the optimized geometries. The dual-level potential profile along thereaction path is further refined with the interpolated single-point energies （ISPE）method. The information on the potential energy surface is used to evaluate the rateconstants by means of the POLYRATE9.7program. The rate constants are calculatedusing the VTST proposed by Truhlar and coworkers. The specific level of VTST thatwe used is CVT theory or CVT with the SCT method. For all reactions, thevariational effect is slightly larger in the low-temperature range, whereas the SCTeffect plays an important role at lower temperatures for R2and R4. We have providedreasonable theoretical values for the rate constants, which will be especially useful inthe case of the reactions R2-R4, for which there are no well-determined experimentalrate constants. Our calculation shows that chlorine or bromine substitution decreasesthe reactivity of ethyl radicals.3. For the molecule CHF2CF2OCH3, two stable conformers, SC11 and SC12with Csand C1symmetry, are formed by the rotation of-CHF2group. Due to the Cs symmetryof SC11structure, two hydrogen atoms in the-CH3group are equivalent and the thirdone is different from them. For the reaction CHF2CF2OCH3(SC11)+OH, hydrogenatom can be abstracted from-CH3（R1a1） and-CHF2（R1b1） groups to form threereaction channels:（1） H atom located at-CHF2group;（2） H atom located at-CH3group and lied in the plane defined by the C-C-O-C;（3） H atom located at-CH3groupand lied out of C-C-O-C plane, which two H atoms （H2and H3depicted in Figure1）are the same because of the Cs symmetry of CHF2CF2OCH3molecule.Four hydrogen atoms for the SC12structure with C1symmetry are not equal, sofour distinct hydrogen abstraction channels are situated according to the differentpositions of these four H atoms. However, only reaction channels related withabstraction of two H atoms out of the C-C-O-C plane are identified because of theC-H bond rotation. In summary, three different reaction channels, i.e., one from-CHF2site and two from-CH3site, are identified.Dynamics calculations are carried out by using the variational transition statetheory with interpolated single-point energies method （VTST-ISPE） at theBMC-CCSD//BMK/6-311+G（d,p） level. Two conformers with similar stability are confirmed for each reactant. H-abstraction from-CH3group is the major reactionchannel at lower temperatures, which is consistent with the C-H bond dissociationenergy. The agreement between calculated and experimental rate constants is seen tobe remarkably good. Finally, the total rate constants are fitted by four-parameterexpression over a wide temperature range of200-2000K.4. There are two conformers for CH2FCF2OCH3, SC11（Cs symmetry） and SC12（C1symmetry）. Three H-abstraction channels can be expected for the reactionCH2FCF2OCH3(SC11)（Cs）+OH: one is H-abstraction from-CH2F site because twoH atoms are identical; two other channels are H-abstraction from-CH3group. Actually,only two reaction channels exist since the transition states associated withH-abstraction from-CH3group are the same. For the reaction CH2FCF2OCH3（SC12）（C1） with OH radical, four channels are situated. Two channels are H-abstraction from-CH2F group according to two different hydrogen positions. Three channels should bedetected for H-abstraction from-CH3group. However, only two reaction channelsrelated with abstraction of two H atoms out of the C-C-O-C plane are confirmed. Thehydrogen abstraction from-CH3group are more favored. H-abstraction from-CH3group is the major reaction channel at lower temperatures, which is consistent withthe C-H bond dissociation energy. In addition, hydrogen abstraction from SC11ispredominant in the low-temperature range, while conformer SC12plays a moresignificant role with the temperature increasing. The contributions of both conformersare almost the same in higher temperatures.

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CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Chemical kinetics,catalysis > Chemical kinetics > The mechanism and kinetics of chemical reactions
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