Theoretical Investigation On NCCX (X=O, S) Systems And Their Protonation Reactions

The chemistry nature of carbon, nitrogen, oxygen, and sulfur-containing species has attracted continuing interest from various fields. One particular interest is their possible role in astrophysical chemistry. Up to now, some carbon-, nitrogen-, oxygen-, or sulfur-containing molecules have been detected in interstellar space. Therefore, a detailed theoretical study concerning carbon-, nitrogen-, oxygen-, or sulfur-containing molecules may be helpful for future identification of these species either in laboratory or in space and also for elucidation of the vaporization processes and the formation mechanism of materials chemistry. Furthermore, the protonation processes of some important neutral species are especially important in ionic environments or interstellar space. Many protonated species such as HCO+, HOC+, H3O+, N2H+, HCS+, H3+, HCNH+, and HC3NH+ have been identified in space.In this thesis, we investigated the structures, stability and bonding nature of [N,C,C,S] and [N,C,C,O] systems and their protonation system, [H,N,C,C,S]+ and [H,N,C,C,O]+, at the B3LYP/6-311++G(d,p) and CCSD(T)/6-311++G(2df,2p) (single-point) levels by quantum chemisty computation. Furthermore, the protonation reaction processes and products of [N,C,C,S] and [N,C,C,O] isomers were also discussed in detail.On the doublet suface of [N,C,C,S] system, the lowest-lying isomer was found to be NCCS with bent structure. The computed results indicate that for [N,C,C,S] system only three isomers, nonlinear chain NCCS, CNCS, and linear CCNS are kinetically stable. The three isomers should be experimentally observable provided that accuracy experimental conditions are available. Furthermore, the electronic structures and bonding nature of NCCS, CNCS, and CCNS isomers were analyzed. Also, the heats of formation of the three important isomers were determined at higher levels. Possible formation strategies of the relevant isomers in laboratory and space were discussed in detail.For the doublet [H,N,C,C,S]+ system, the lowest-lying isomer was found to be HNCCS+. Ten iomsers were calculated to possess considerable kinetic stability and may be obserbed in the laboratory and interstellar space among the loctaed twenty-four iomers. The protonation reaction processes and products of the stable [N,C,C,S] isomers were discussed in detail, and the calculation results indicated that the [N,C,C,S] isomers may be significantly stabilized upon protonation. Furthermore, the analyzed results of quartet state [N,C,C,S] and [H,N,C,C,S]+ systems indicat that the stability of quartet isomers are low, and the quartet state distribution has only very minor contribution to the isomer distribution.For the protonation system of [N,C,C,O], [H,N,C,C,O]+, eighteen isomers were located as energy minima, and the lowest-lying isomer is HNCCO+. Eight isomers were calculated to possess considerable kinetic stability among the located eighteen iomers. Therefore, they could be obserbed in the laboratory and in interstellar space. The protonation reaction processes and products of the stable [N,C,C,O] isomers were discussed in detail. The calculation results indicated that the [N,C,C,O] isomers may be significantly stabilized upon protonation. This result is in agreement with the [N,C,C,S] system isomers.On the singlet suface of the [H,N,C,C,S] system, the lowest-lying isomer was found to be SC(H)CN. Among the loctaed seventeen iomers, ten isomers possess reasonable kinetic stability and should be obsvervable in laboratory or in interstellar space. The possible covalent structures of the singlet [H,N,C,C,S] isomers with considerable stability are briefly discussed, and the adiabatic ionization and vertical ionization energies of the relevant isomers were determined at higher levels. Furthermore, thirteen isomers were located as energy minima on the triplet [H,N,C,C,S] surface. However, the analyzed results indicat that the stability of triplet isomers are lower, and the triplet state distribution has very minor contribution to the overall isomer distribution.