Ab Initio Investigation Of The Reaction Mechanism For Interstellar Small Species

Chemical processes involving C2 has been of great significance in the study of both high temperature chemistry and interstellar chemistry. Investigation of the C2 related reactions have invoked great inclination due to their important role in exploring high temperature chemistry of hydrocarbons. C2 has been involved in soot formation in hot flames, in diamond synthesis from gas pyrolysis, and in the formation and decay of the newly discovered C60 fullerenes. In astrophysical science C2 has been detected hi the carbon stars, comets, and difruse and translucent clouds. Investigation of chemistry involving C2 has been of astrophysical interests in the studies of a variety of interstellar objects. In this thesis, we explore the detailed reaction mechanism of C2 with H2O, H2S in theoretical perspective.The reaction of C2 with water and the possible subsequent reactions have been investigated using ab initio methods. Species involved in the reactions on both singlet and triplet potential energy surfaces were optimized at both M011er-Plesset correlation energy correction truncated at second-order (MP2) and Quadratic Configuration Interaction including single and double substitutions (QCISD) levels in conjunction with 6-311++G** basis set. Single point calculations were performed using coupled cluster theory including single, double, and triple substitutions (CCSD(T)) in conjunction with 6-311++G(d,pd) basis setwith the geometry optimized at MP2/6-311++G** level. On the basis of computational energetic results at CCSD(T)/6-311-H-G(d,pd)//MP2/6-311-H-G** plus zero point energy correction level of theory the addition reaction channel on the singlet surface dominates over the H-abstraction channel on both singlet and triplet surfaces. The most feasible reaction following the formation of addition intermediate (CCOH2) is the intra-molecular hydrogen migration leading to hydroxyethyne (HCCOH). Rearrangement of HCCOH can lead to several other isomer intermediates. Elimination of atomic and molecular hydrogen from HCCOH, CCOH2, and other isomer intermediates is also surveyed. The implication of our computational results regarding the reaction of C2 with H2O in interstellar space and combustion process is discussedThe reaction mechanism of C2 + H2S has been investigated using ab initio method. On the basis of calculations using CCSD(T) in conjunction with 6-311-H-G(d,pd) basis set with the geometry optimized at MP2/6-311++G** level, the H-abstraction reaction on the triplet energy potential surface is an exothermic process with formation of a precomplex as an intermediate, which further dissociate to yield HS(2Σ+) + CCH(2Σ+) with an energy barrier of 4.9 kcal mol-1. The addition of H2S(! AI) to C2(?+g) leads to a bound intermediate HaSCC(1A') (3,3-dihydrodicarbonsulfide), which can further isomerize into HSCCH(1A) (thiohydroxyacetylene) in an one-step hydrogen migration process.