Theoretical Investigations On The Oxidation Of Carbon-chain Radicals L-CnH In Space

The carbon-chain radicals CnH(n=1-8) have been detected in the interstellar medium,and show their important roles in the organic chemistry, combustion chemistry andinterstellar chemistry. Some of them have been studied on their chemical reactivity andgeometries. The very recent observation of molecular oxygen in interstellar space appealsfor the great need of mechanistic understanding of the oxidation processes of variousinterstellar species. In this thesis, we carried out detailed quantum chemical investigationson the potential energy surfaces (PES) for a set of reactions i.e. l-CnH+O2(n=3-6). Toquantitatively evaluate the kinetic competition of different products in the reactions, wehave also performed the master equation rate constant calculations. Our results will enrichthe understanding of the chemistry of l-CnH radicals.The main results are summarized as follows:1. We report for the first time the oxidation mechanism of the chainlike l-C3H bymolecular oxygen, which is known as one of the interesting carbon-chain hydrocarbonseries CnH detected in space. This reaction is also relevant to the combustionprocesses where various carbon hydrides are involved. A detailed potential energysurface (PES) is constructed at theCCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(d,p)+ZPVE level including variousfragmentation channels. Three types of fragmentation channels are identified as theC-transfer product P2(CO2+C2H), the C,O-exchange product P1(CO+HC2O) and theO-transfer product P6(3O+HC3O). The initially entered unstable dioxygen isomer1aHCCCOO (-26.6) would either undergo the direct O-extrusion to give P6(the intrinsicbarrier7.5kcal/mol) or take a1,2-O-shift (0.8kcal/mol barrier) to give a stableisomer5HCCC(O)O (-139.2) that can either dissociate to P1or to P2. The intrinsicbarrier from5to P1and P2is29.1and23.6kcal/mol, respectively. Clearly, theentrance thermicity26.6kcal/mol of1a can sufficiently initiate the subsequent formation of all the three products. To quantitatively evaluate the kinetic competitionof the three products, we performed the master equation rate constant calculations. Itwas shown that at298K, the most favorable product is P2(64.8%) followed by P6(23.6%) and P1(11.6%). Interestingly, at elevated temperatures, the ratio of P6increases with the decrease of P2, whereas that of P1is little changed. Notably, thethermodynamically most stable product P1is kinetically the least favorable, indicativeof the importance of considering the kinetics. The dominant formation of P2(CO2+C2H) shows that the important carbyne radical l-C3H can be effectivelydegraded by O2via the chain-shortening step. The reactivity of the cyclic c-C3Hradical towards O2is also discussed. The results are expected to enrich ourunderstanding of the chemistry of the simplest C3-radical in both combustion andinterstellar processes.2. Reactions of the carbon-chain radicals are of great importance in the combustionand astrophysical processes. The kinetics of the butadiynyl radical, C4H, has receivedrecent attention. While there has been sufficient knowledge concerning the oxidationof the ethynyl radical, C2H, oxidation of the higher even-numbered members C2nH(n>1) is hardly known. In this work, to enrich the C4H-chemistry, we report the firststudy of the oxidation mechanism of C4H. At theCCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(d,p)+ZPVE level, the potential energysurface survey is presented covering various product channels P1(CO+HC3O),P2(C3H+CO2), P3(HCO+C3O), P4(HC4O+3O) and P5(OH+C4O) accompanied by themaster equation rate constant calculations. Despite the similarity in the potentialenergy surface, the kinetics of C4H+3O2differs dramatically from that of theanalogous C2H+3O2reaction. For the C4H+3O2reaction, the O-abstraction productP4(HC4O+3O) is almost the exclusive product, whereas the lowest C,O-exchangeproduct P1(CO+HC3O) and other products have little importance. By contrast, theC2H+3O2reaction favors the C,O-exchange product HCO+CO. Being overallbarrierless and mainly associated with the molecularâ†'atomic oxygen conversion, theC4H+3O2reaction should play an important role in the soot formation and interstellarchemistry where C4H is involved.3. To further help understand the reaction properties of the long carbon chainradicals, we report the first theoretical survey of the l-C5H+O2and l-C6H+O2 reactions at the CCSD(T)/cc-pVTZ//B3LYP/6-311++G(d,p)+ZPVE level. The carbontransfer was found to be the main pathway in the l-C5H+O2reaction, giving the mainproduct P2(CO2+C4H). Yet, the case is a little different for l-C6H+O2. The energybarrier between the C,O-exchange (resulting in P1(CO+HC5O)) and the oxygentransfer (resulting in P3(3O+HC6O)) is only0.1kcal/mol, indicative of theirco-predominance. The present results were compared with the shorter l-CnH radicalswith O2. It’s easy to conclude: when the l-C2n+1H radicals react with oxygen, the mainpathway should be the C-trance process, with one main product l-C2nH+CO2, but thatis a little complex for the reactions of l-C2nH with O2.4. Silicon-methylidyne (SiCH) radical is considered to be important both as anastrophysical molecular and as the prototypical organosilicon compound. Severaltheoretical studies on various aspects of SiCH structure and spectroscopy have beenreported. The studies on the ground-state structure and the possibility of isomerizationin early years showing that SiCH(2Π) is the global minimum, with CSiH ca.50kcal/mol higher in energy. An investigation of the laser-induced fluorescence (LIF)spectra of SiCH and SiCD showed that the radical has a1.693SiC double bond inground state and a1.612triple bond in the excited state. We gave the firstdiscussion on the reaction of SiCH+O2using density functional theory. At theCCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(d,p)+ZPVE level, we found28isomersin the [SiCHO2] system. Based on these isomers we constructed the PES of the SiCH+O2reaction, and found the most feasible pathway is a multi-step isomerisation of1SiC(H)OO to5SiOC(H)O and the dissociation of5to P1(HCO+SiO). A minorthermodynamically pathway is Râ†'1â†'2â†'3â†'6â†'8â†'P2(HSi+CO2). It’snoticeable that the two pieces of P1are both important interstellar molecules. So ourresults may be a key evidence to confirm the existence of SiCH radical in the outerspace for the future.