Mechanistic Studies Of Some Typical Radical/Molecular Reactions Of Atmospheric Importance And The Single-Molecular Catalysis In The Gas Phase

This thesis focuses on several typical radical-molecular reactions in atmospheric and single molecule catalyzed reactions. The reactions studied in this thesis include the reaction of ROCH2O2(R=H, CH3) with HO2, the reaction of methylidyne radical (CH) with acetaldehyde (CH3CHO), the effects of H2O and HCOOH on the reaction of methylglyoxal with H2O, the effects of H2O on the reaction of NF3with OH radical, the catalytic dehydration and dehydrogenation of ethanol, and the water-mediated dimerization of HNO. Some important results obtained from theoretical calculations are summarized as follows:1. The reaction of HO2with HOCH2O2The detailed mechanism and kinetic of reaction HO2+HOCH2O2have been explored using CCSD(T)/CBS//B3LYP/6-311G(d,p) method. It is revealed that the reaction takes place either on triplet potential energy surface via the hydrogen abstraction routes forming HOCH2OOH and O2or on singlet potential energy surface via the addition-elimination mechanism forming various radical products. On the other hand, the triplet Sn2mechanism and the singlet hydrogen abstraction routes are of no importance. Moreover, the results indicate that the formic acid observed in previous experimental studies is the product of secondary reaction between HO2and the nascent product HOCH2OOH.The rate coefficients have been calculated using transition state theory as a function of temperatures (200-2000K), and the empirical expression of overall rate coefficients is k(T)=8.48×10-24T3.55e2164/t+2.37×10-29T4.70e3954/T cm3molecule-1s-1. The results indicate that the rate coefficient exhibits typitcal non-Arrhenius behavior. At lower temperature (<600K), the reaction is dominated by the triplet abstraction mechanism and the rate constant exhibits negative temperature dependence; at higher temperatures (>600K), the singlet addition-elimination mechanism becomes important and the rate constant shows positive temperature dependence.2. The reaction of HO2with CH3OCH2O2The reaction HO2+CH3OCH2O2has been studied using B3LYP/6-311G(d,p) and CCSD/cc-pVDZ method. In view of the energetic reaction pathways and the structures of the stationary points, the reaction HO2+CH3OCH2O2is similar to the reaction of HO2with HOCH2O2. The results reveal that the dominant product channel on triplet PES is CH3OCH OOH+O2via direct hydrogen abstraction while the triplet Sn2mechanism is of no importance. On the other hand, the main product on the singlet PES is HO2+OH+CH3OCHO via addition-elimination mechanism, which could be viewed as HO2-mediated unimolecular decomposition of CH3OCH2O2.3. The reaction of methylidyne radical (CH) with acetaldehyde (CH3CHO)The mechanism of CH+CH3CHO reaction has been investigated using CBS-QB3//B3LYP/aug-cc-pVTZ method. The entrance channel of CH+CH3CHO reaction is a barrierless process, forming a linear intermediate CH3CHOCH, which could convert to several products. The microcosmic mechanism recommended by present study could not only uncover the products observed by previous experimental studies, but also explain the isotopic effect (CH/CCD) observed in experiment. In view of energy, C2H5+CO is the most exothermic product, additionally, the energy barrier of this product channel is the lowest one. As for hydrogen elimination channel^acrolein(CH2CHCHO) and methylketene(CH3CHCO) are the most preferable co-products. Furthermore, a new product channel C2H3+CH2O has been discovered.4. The effects of H2O and HCOOH on the reaction of methylglyoxal with H2OThe influence of H2O and HCOOH molecules on the gas-phase hydration of methylglyoxal (CH3COCHO) has been explored at the CBS-QB3//B3LYP/6-311G(d,p) level of theory. The results indicate that water-mediated keto-enol isomerization is competitive with the hydration mechanism. However, with single H2O and HCOOH molecule participating in the reaction, the dominant reaction mechanism of methylglyoxal with H2O is hydration rather water-mediated keto-enol ismerization as both H2O and HCOOH molecule could reduce the energy barrier of hydration mechanism significantly. Compared to H2O molecule, the catalytic effect of HCOOH is more efficient.5. The effects of H2O on the reaction of NF3+OHThe reaction of NF3with OH has been studied using BMC-CCSD and coB97XD/aug-cc-pVTZ method. The reaction between NF3and OH takes place mainly through SN2and F-abstraction mechanisms. However, the energy barriers of these two processes are extrmemly high and the products are endothermic, thus it is very difficult to remove NF3by reaction with active OH radical. With additional one H2O molecule taking part in the reaction, the energy barrier of NF3+OH reaction reduces to some extents, however, the reaction is still endothermic. Fortunately, when there are two H2O molecules(H2O-dimer) participating in the reaction NF3+OH, the reaction could take place via a new SN2mechanism, moreover, the energy barrier reduces significantly and the products has been changed to NF2O+(H2O)2-HF, which are highly exothermic(70.5kcal/mol). Thus it can be speculated that (H2O)2induced reaction of NF3+OH is a possible removal pathway of NF3in humid environment.6. The catalytic dehydration and dehydrogenation of ethanolCatalytic dehydration and dehydrogenation reactions of ethanol by various typical small molecules have been investigated systematically at the UMP2(full)/6-31+G(d,p) level. The results reveal the barriers of both dehydration and dehydrogenation can be reduced significantly in the presence of H2O, H2O2, HCOOH, H3PO4, HF, NH3andC2H5OH. The acidic catalysts (e.g., HCOOH, H3PO4, HF) prefers dehydration while the basic catalysts (e.g., NH3) tend to promote dehydrogenation more effectively. The catalytic effect of neutral water-dimer (H2O)2on the dehydrogenation reaction id more efficient than that of dehydration, thus dehydrogenation is the dominant mechanism. The result is consistent with the experimental results of the selective oxidation of ethanol in the supercritical water. The calculated dehydrogenation/dehydration product ratio is in agreement with the experimental value as well. Moreover, the dehydration and dehydrogenation mechanisms of ethanol on zeolite have been revealed using cluster model. The result indicates that the bridging OH group with Bransted acidity and the neighboring oxygen atoms with basicity are the active sites for the catalyzed dehydration of ethanol selectively.7. Water-mediated dimerization of HNOThe influence of H2O molecule on the dimerization of nitroxyl(HNO) has been studied using CBS-QB3//coB97XD/aug-cc-pVTZ method, and the solvent effect has been discussed employing PCM model. It is revealed that water-mediated hydrogen transfer is the dominant mechanism of HNO dimerization, in which H2O-mediated1,2-H transfer of trans-(HNO)2followed by1,3-H transfer of trans-H(O)NNOH-H2O is the most important reaction pathway. Another possible pathway is water mediated two-step1,2-H transfer of cis-(HNO)2.