Theoretical Studies On The Mechanisms And Dynamics Property Of Several Important Reactions

The study and determination of reaction rate constants has always beenone of the main research fields in chemistry. It is one of the most activesubjects to predict the rate constants theoretically. In this thesis, ab initio anddensity functional theory combined with the direct dynamics methods havebeen used to study the following chemical reactions:(1) Reactions CH3OCl + Cl and CH3OCl + OH. CH3OCl is alsoexpected to undergo homogeneous gas-phase photochemical reactions withCl atom and OH radical. The rate constants of the two reactions areimportant to estimate whether CH3OCl may take part in O3 destroyingcycles, or it may act as a relatively stable sink for chlorine leading to lowerO3 depletion rates. It is especially important to understand the incinerationmechanism and rate constants for important elementary reactions.(2) Reactions C2H5Cl/C2D5Cl + Cl. Incineration is a cleanest methodfor disposal of halogenated hydrocarbons, which are potential sources ofchlorine in the stratosphere, where they can destroy ozone. The currentstudied reactions play an important role in the incineration of halogenatedhydrocarbons. To incinerate halogenated hydrocarbons in a more efficientand less hazardous way, it is necessary to further investigate all aspects ofthis kind of combustion process.(3) Reactions HBr + C2Hn+2Cl3-n (n=1,2). The reactions of chlorinatedalkyl radicals with HBr have been extensively studied during the pastdecades. This is due to their importance for fundamental chemical kineticsand thermodynamics. The measured rate constants can be used to determinethe heats of formation for these radicals. The experimental rate constants ofthe reactions were only reported at 300―677 K and 457―787 K. Thus, it isdesirable to augment available rate constants theoretically, especially in thehigh temperature region.The above-mentioned reactions play an important role in atmosphereand combustion chemistry. So the studies on reaction mechanism and therate constants of these reactions are important for environmental andstratospheric ozone protection. There have many experimental investigationsconcerning rate constants for these reactions, but the theoretical studies arelittle. The main object is to provide more accurate results of the reactionpath and the temperature dependence of rate constants and to explore thereaction mechanism of these reactions. The theoretical results may provideuseful information for further experimental studies.By means of the Gaussian 98 program, at the lower level of theory(MP2 and BH&HLYP), the geometries and frequencies of the stationarypoints (reactant, transition state, complex and product) are calculated. Theminimum energy path (MEP) is calculated at the same level by intrinsicreaction coordinate (IRC) theory. Furthermore, selected points along theMEP, the force constant matrices as well as the harmonic vibrationalfrequencies are obtained. In order to obtain more accurate information ofenergy, the energies of the selected points on the MEP are refined at thehigher level (G3(MP2), G3, a-QCISD(T), MC-QCISD and G2M)). ByPOLYRATE 8.4.1 program within involve the conventional transition statetheory (TST) and canonical variational transition state theory withsmall-curvature tunneling correction (CVT/SCT) is applied to obtain thereaction rate constants. The main results can be summarized as follows:1. For the reactions CH3OCl + Cl → CH3O + Cl2 (3a) and CH3OCl +Cl → CH2OCl + HCl (3b), the potential energy surface information isobtained at the MP2/6-311G(d, p) and MP2/cc-pVTZ levels, andhigher-level energies for the stationary points and a few extra points alongthe minimum energy path are calculated at the G3(MP2)//MP2/6-311G(d, p),G3//MP2/6-311G(d, p) and MC-QCISD// MP2/cc-pVTZ levels of theory.The rate constants of the reaction channels for (3.1) and (3.2) calculated bycanonical variational transition-state theory with the small-curvaturetunneling (CVT/SCT) correction are well consistent with the availableexperimental values in the measured temperature range. For reactionCH3OCl + Cl, the two reaction channels are almost competitive at lowtemperature, with a slight predominance via Cl-abstraction, whileH-abstraction leading to the formation of CH2OCl + HCl becomes majorreaction pathway with the temperature increasing. The variational effect isimportant in the whole temperature range, while the SCT correction only hassome contribution at lower temperature. The rate constants within220―2200 K of the two reactions are fitted by the three-parameterexpressions: k1 = 3.09×10-15T1.15 exp(783.9/T) , k2 = 1.5×10-16T1.71exp(719.3/T) and k = 3.79×10-16T1.62 exp(813.3/T) cm3molecule-1s-1.2. For the reactions CH3OCl + OH → CH3O + HOCl (4.1) andCH3OCl + OH → CH2OCl + H2O (4.2), the potential energy surface (PES)information is built up at the MP2/6-311(d, p), a-QCISD//MP2, G3//MP2,and G3(MP2)//MP2 levels. The reaction proceeds dominantly viaH-abstraction especially from out-of-plane hydrogen leading to theformation of CH2OCl + H2O over the whole temperature range, whileCl-abstraction is much less competitive. The energy profile of the reaction isrefined with the ISPE method at the a-QCISD(T)//MP2 level. The CVT/SCTrate constants of reaction CH3OCl + OH are in good agreement with theexperimental values within 250―341 K. For reactions (4.1) and (4.2), thevariational effect has a minor contribution in the whole temperature range,while the SCT correction plays an important role in the lower temperatureregion. The rate constants over the temperature range 220―2000 K arefitted by the three-parameter expression: k1 = 1.87×10-21T2.67 exp(534.6/T),k2a =6.12×10-23T3.89 exp(415.1/T), k2b=5.06×10-20T2.72exp(108.2/T), k2=3.45×10-20T2.78 exp(126.0/T), and k =2.38×10-20T2.83 exp(184.7/T) cm3molecule-1s-1.3. The potential energy surface (PES) information of theC2H5Cl/C2D5Cl + Cl reaction is obtained at the MP2/6-311(d, p) level, andhigher-level energy corrections at the stationary points and the extra pointsalong the reaction path are performed at the G3 level. The CVT/SCT rateconstants of reactions are in good agreement with the experimental values.The SCT correction plays an important role for two reactions over the wholetemperature region, and the variational effect is only important for α-H/Dabstraction. The rate constants within 220―2200 K of the two reactions arefitted by the three-parameter expressions: k1a = 8.36×10-17 T1.7exp(580.3/T),k1b = 3.12×10-20T2.8 exp(253.2/T), k1 = 2.55×10-19 T2.6exp(792.8/T), k2a =1.76×10-17T1.9 exp(218.4/T), k2b = 4.86×10-21 T3.1exp(102.9/T), and k2 =5.54×10-20T2.8 exp(403.0/T) cm3 molecule-1s-1.4. For the reactions CH3CHCl + HBr → CH3CH2Cl + Br (6.1) andCH3CCl2 + HBr → CH3CHCl2 + Br (6.2), the potential energy surfaceinformation is obtained at the BH&HLYP/6-311G(d, p) level, andhigher-level energy corrections for the stationary points and a few extrapoints along the minimum energy path are carried out by the G2M theory.The rate constants of the reactions (6.1) and (6.2) calculated by theimproved canonical variational transition-state theory with thesmall-curvature tunneling (ICVT/SCT) correction are well consistent withthe available experimental values in the corresponding measuredtemperature regions. For reactions (6.1) and (6.2), the variational effect andSCT correction are all important in the lower temperature region. The rateconstants of the two reactions within 220―2200 K are fitted by thethree-parameter expressions: k1 = 2.71 × 10-19 T1.90 exp(1524 /T) and k2 =5.02 × 10-19 T1.89 exp(121 /T) cm3 molecule-1s-1.