Theoretical Studies On The Gas Phase Reaction Mechanisms For Several Important Small Molecules

Gas phase reactions of small molecules play a crucial role in atmosphericchemistry, interstellar chemistry, astrochemistry and combustion chemistry. In thisthesis, the potential energy surfaces of the ion-molecule and radical-radical reactionsof relevance to Titan atmosphere, Interstellar space, Earth’s atmosphere have beenexpolored using quantum chemical methods. Important information such asgeometries and energies of the reactant, isomers, transition states and products areobtained. Possible reaction channels as well as reaction mechanism are also beenprovided. The results obtained in this thesis are compared with previous experimentalfindings and may shed some light on future experimental investigations of these kindsof reactions. The main results are summaried as follows:1. Titan is known as the second Earth, because its thick atmosphere is muchsimilar to the primitive Earth’s. Benzene is a very important species in the atmosphereof Titan. It is the basic unit for the polycyclic aromatic hydrocarbons （PAHs） whichare considered to be responsible for the formation of organic haze layers in Titan’satmosphere. In the upper atmosphere of Titan, benzene primarily comes from thefollowing steps: C₄H₂⁺+C₂H₄→C₆H₅⁺+H, C₆H₅⁺+H₂→C₆H₇⁺, C₆H₇⁺+e^-→C₆H₆+H. The complex doublet potential energy surface for the ion-molecule reactionof C₄H₂⁺with C₂H₄are investigated at the B3LYP/6-311G（d,p） andCCSD（T）/6-311+G（2df） levels. The terminal C atom attacking on C₂H₄molecule mayhave two possible patterns:（i） attacking the C═C bond of C₂H₄to form thethree-membered cyclic intermediate1（cCH₂CH₂CH-C₃H）（-37.1）;（ii） attacking onecarbon atom of C₂H₄, the linear intermediate13（H₂CCH₂CHCCCH）（-15.6） can be formed. With the large heat released from the initial step, the three-membered cyclicintermediate1and the chainlike intermediate13can undergo further evolutionleading to six products, that is, P₁（C₆H₆⁺），P₂（C₆H₅⁺α+H）, P₃（C₆H₅⁺β+H）, P₄（C₄H₄⁺+C₂H₂）, P₅（C₆H₄⁺+H₂）, and P₆（C₃H₃⁺+C₃H₃） with the relative freeenergies of-115.4,-35.3,-10.1,-13.3,-23.2, and9.1kcal/mol, respectively. In thefinal product distributions, P₂and P4may be the most favorable product withconsiderably large yields. P₃may be the second feasible product. The yields ofproduct P5should be even much smaller. There is almost no yield of products P₁andP₆. Our theoretical prediction on the products relative abundance is in good agreementwith the experimental study performed by Vincent et al. We aslo have investgated themechanisms of the rest of the sequence reaction C₆H₅⁺+H₂→C₆H₇⁺and C₆H₇⁺+e^-→C₆H₆⁺H at the same theortical level. The attack of molecule H2on the C6H5+αleads to complex C₆H₇⁺without any encounter barrier. We calculate the pointwisepotential energy curve to confirm that C₆H₅⁺α+H₂→C₆H₇⁺is a barrierless process.Complex C6H7can dissociate to benzene and H atom via an H atom eliminated. Thebarrier height of the step C₆H₇→C₆H₆⁺H is25.3kcal/mol. It is expected that thesetheoretical results will be useful for understanding the formation mechanism ofbenzene in the atmosphere of Titan.2. Carbon cation and ethylene are very important species in interstellar space.The reaction C⁺+C₂H₄is often the first step in the synthesis of more complex neutraland ionic long carbon chains. A detailed theoretical study for the poorly understoodion-molecule reaction of C⁺with C₂H₄is explored at theCCSD（T）/6-311++G（3df,2pd）//DFT/B3LYP/6-311G（d,p）+ΔZPVE, CCSD（T）/6-311++G（3df,2pd）//QCISD/6-311G（d,p）+ΔZPVE, and G3B3levels of theory. On thedoublet potential energy, there are three initial attack patterns, that are,（i） the carboncation attacks the C=C bond of C₂H₄to form the chain-like intermediate6CH₂CCH₂（-168.7）,（ii） and （iii） C⁺attacks one of the C-H of ethane to form chain-likeintermediate14（-146.6） and6（-145.5）. The large heat released from the initial step can drive1,6, and14take subsequent isomerization or dissociation reactions leadingto seven products, that are, P₁（c-C₃H₃⁺+H）, P₂（c-C₃H₂⁺+H₂）, P₃（l-HC₃H⁺+H₂）,P₄（l-C₃H₂⁺+H₂）, P₅（l-C₃H⁺+H₂+H）, P₆（CH+C₂H₃⁺）, and P₇（C+C₂H₄⁺）.Comparing every optimal channel of forming possible products, we found the finalproduct distributions. At low-temperature interstellar environment, P₆and P₇are themost feasible products. P₁and P₂are the second and third competitive products. Thereis almost no yield of products P₃, P₄,and P₅. Because the isomers and transition statesinvolved in the most feasible pathways all lie below the reactant, the title reaction isexpected to be fast. Our theoretical results are consistent with the experimentalinvestigations by Sonnenfroh et al, and supplement two undetected products. We hopethe results may provide some useful information for understanding the generationmechanism of complex carbon chain-containing compounds in cosmos.3. ClO radical is one of the most important chemical constituents in the Earth’satmosphere. Scientists have confirmed that the loss of stratospheric ozone moleculesis closely related to the presence of ClO. CN radical is another important chemicalsubstance in the Earth’s atmosphere. Due to its large electronegativity, the CN groupis often regarded as a pseudohalogen. The ClO and CN double radical reaction isinvestigated at the CCSD（T）/6-311++G（2df）//B3LYP/6-311G（d）+ΔZPVE level. Onthe singlet PES, the attack of ClO radical on CN radical may have five kinds ofentrance channels. We obtain four kinds of products, i.e., P₁（ClNCO）, P₂（CO+ClN）,P₃（ClNC+O）, and P₄（NO+CCl）. At room-temperature or low-temperatureconditions, P₁is the most favorable products, and there is almost no yield of productsP₂, P₃and P₄. At high-temperature condition, P₁and P₄are the most feasible products,P₃is the second competitive products, and there is almost no yield of products P₃. Webelieve that the coalescent of ClO and CN in the earth’s can inactivate the ClO radicaland reduce the ozone destruction in the atmosphere.4. Recently, the small clusters containing elements of Si, P and S have receivedexperimental and theoretical attention. The structures, energetics, spectroscopies and stabilities of the doublet Si2PS radical are explored at the density functional theoryand ab initio levels. Fifteen minimum including the chainlike, three-membered ring,four-membered ring and cagelike structures are located, connected by24interconversion transition states. Four isomers S-cSiPSi4（12.1）, P-cSiSiS5（27.6）,PSiSiS10（0.0）, and cagePSSiSi15（3.5） are predicted to possess very high kineticstability. All the four isomers are very promising candidates for future laboratory andastrophysical detection.To aid the future identification of the Si2PS isomers either inthe laboratory or in interstellar space, the calculated vibrational frequencies, dipolemoments, and rotational constants for the relevant isomers are also presented. The setof dissociation fragments SiS （³Î£ï¼‰ and PSi （²Î ï¼‰ can associate to form the isomer1and2directly with no barrier. Moreover, the isomers1and2may isomerize to the stableisomers4and5. The similarities and discrepancies among Si₂PS, C₂PS and SiCPS arefound. The possible formation strategies and the implications of the stable isomers inthe laboratory and space are also discussed in detail. The theoretical results in thispaper are expected to be useful for future identification of the Si₂PS radical either inlaboratory or in interstellar space.