Theoretical Investigations On The Structures And Stability Of Important Small Molecules

Recently, the C,N,S atom doped Si-cluster have received experimental and theoretical attention. The reason is their use in the inorganic chemistry, organic chemistry, material chemistry, and furthermore, it is their importance in the interstellar chemistry. In this paper, high level quantum chemistry method has been used to investigate the C,N,S atoms doped Si-cluster, and may be helpful for the further identification in the laboratory or in space. All the calculations were performed with the GAUSSIAN 98/03 program packages. The main results are listed as follows:一,The potential energy surface of the possible interstellar molecule Si2CS is investigated at the B3LYP/6-311G(d) level. Eight singlet isomers are located as energy minima and connected by fifteen transition states. Further calculations are performed using the Quadratic CI calculation including single and double substitutions (QCISD/6-311G(2d)) to get more reliable geometrical parameters, harmonic vibrational frequencies, and infrared intensities of the relevant structures. And at the higher (single point) CCSD(T)/6-311+G(2d)//QCISD/6-311G(2d)+ZPVE level, the global state (at 0.0 kcal/mol) corresponds to a planar four-membered ring form. This is followed by the next most stable chain-like isomer SiCSiS (at 1.7 kcal/mol higher the global state) and its geometries is very sensitive to electronic correlation effects. They represent not only high thermodynamic stabilities, but also high kinetic stabilities. These two isomers may be experimentally or astrophysically observable. Meanwhile, the calculated results are compared with the analogous species C₃O, C₃S, and SiC₂S. 二,New species with molecular formula Si₂NS, not yet observed experimentally, are described theoretically for the first time. Various levels of calculations are applied to obtain the structures, energies, dipole moments, vibrational spectra, rotational constants, and isomerization of Si2NS species. A total of 16 minima which are connected by 21 interconversion transition states on the potential energy surface are located at the DFT/B3LYP/6-311G(d) level of theory. And at the higher (single point) CCSD(T)/6-311+G(2d)//QCISD/6-311G(2d)+ZPVE level of theory, the global state of 1 (at 0.0 kcal/mol) corresponds to a nonlinear SiNSiS with a ²A' electronic state. The next most stable isomer of 2 (only 3.7 kcal/mol higher 1) possesses a rhomboidal-type structure similar to the global state of Si2NO. This is followed by another local minimum of 6 (at 43.8 kcal/mol above 1) which is a nonlinear SiNSSi form with a 2A' electronic state. Note that both 1 and 6 have the important Si≡N triple bonding. Too, the analogue of (NCCS) NSiSiS is unstable due to the higher energy of its own. The bonding natures of the relevant species are also discussed. An analysis of the bonding properties and structural similarities and differences between C₂NO, C₂NS, SiCNO, and Si₂NO is also carried out.