| Structures and stabilities of small molecules (ions) have been the hot topic to both experimental and theoretical studies. They are believed to play important roles in various fields such as photochemistry, combustion chemistry and interstellar chemistry, etc. Many of these small molecules (ions) have very short lifetimes under normal conditions. Thus, it has remained a challenge for experimentalist to characterize them spectroscopically or to stabilize them under controlled circumstances. Theoretical and computational chemistry has been a very powerful method to help determine the structures, predict stabilities and possible reaction mechanisms of the small reactive systems. With the computational method, we can not only obtain the possible isomeric forms, chemical bonding and electronic structures, but also to establish a detailed potential energy surface, upon which we can predict the kinetic stabilities of the isomers. Theoretical and computational chemistry can provide fruitful basis for future molecular synthesis and mechanistic study of their reactions. It can also give us useful information for reducing the environmental pollution, exploring the combustion processes, detecting interstellar molecules and understanding various dynamic processes. The C2N+ ion has attracted much interest due to its importance in the interstellar chemistry of dense clouds. On one hand, many C,N-containing neutral species such as C2N2, CH3CN, HC3N, CCN, etc. can effectively produce the C2N+ ion upon electron impact and radiation. On the other hand, a systematic and detailed study on the chemical reactions of C2N+ can be useful to elucidate its depletion mechanism in interstellar and burning processes, and the possibility to synthesize novel interstellar molecules (ions).Using SIFT techniques, Bohme et. al. studied the reactions of C2N+ towards a series of neutral molecules including N2,CO,HCN,C2N2,N2O,H2O,H2S,CH4,NH3, etc. Their experiments concluded that there are two isomeric forms for C2N+. They also confirmed that the reactions of C2N+ can generate many novel compounds containing C-S, C-O and C-N bonding.In contrast to the ample experimental studies, the theoretical part on the C2N+ reactions with neutral species is very rare. Up to now, only Tao et. al. studied the C2N+ reaction with a kind of isovalent molecules H2X (X=O,S) that only contains sigma-bonding. In the present thesis, we apply modern quantum chemical method to thoroughly study the reaction mechanisms of C2N+ with another kind of molecule HCN, which is polar and contains pi-bonding. The detailed reaction potential energy surfaces are presented to discuss the isomerization and dissociation of various isomers. The details are as follows: At the B3LYP/6-311G(d) (geometrical optimization) and CCSD(T)/6-311+G(d,p) (single-point energy calculation) levels, the structures, vibrational frequencies and energetics of a total of 40 C3HN2+ isomers and 73 interconversion or dissociation transition states are obtained. The detailed potential energy surfaces of the reactions of the two C2N+ isomers CCN+ and CNC+ with the polar molecule HCN. For the C2N++HCN ion-molecule reactions,the initial steps are the barrierless electrostatic interaction between the reactants leading to the stabilization of various C3HN2+ isomers. In more detail, the CCN++HCN reaction can lead to three kinds of isomers, i.e., chainlike a2 HCNCCN+, cyclic c1 NC-cCCNH+, and branched chainlike b2 NCC(H)NC+,b1 NCC(H)CN+ and b3 CNC(H)NC+. Amongst, c1 can be viewed as a C(N pi-addition species, while b1-b3 can be viewed as C-H sigma-insertion species. For the CNC++HCN reaction, the main products are chainlike a4 HCNCNC+ and cyclic c6 HCN-cCNC+, both species can coexist. Under conditions with very low pressure, the lower-energy C(N pi-addition species c2 can also be produced,yet it cannot be converted back to a4 once formed. However, when the pressure is very high, a4 and c6 can be effectively stabilized and the formation of c2 is difficult. The formation of C-H sigma-insertio...
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