Theoretical Investigations On The Structures And Stability Of Three Types Of Small Molecules Containing Si

Recently, the silicon-containing species have received attention in many fields. One of important reasons is that many silicon-containing species including cyclic and chainlike molecules have been detected in interstellar space, e.g. SiN, SiO, SiS, SiCN, and SiCn(n=1-4). On the other hand, during N and P doped SiC vaporization process, many cyclic species are produced, the theoretical study of the Si, C, N and Si, C, P systems, will be helpful for understanding the initial step of doped SiC vaporization. The structures and stability of three types of small molecules with respective 14, 17 and 18-valence-electron have been studied by quantum chemistry computation using the Gaussian 98 and Molcas5.2 program packages. The dissociation and isomerization potential energy surfaces have been established by locating the isomers and interconversion transition states. The main conclusions are summarized as follows:1. The structures, energetics, and spectroscopies of SiC2P and Si2CN isomeric species are explored at various levels. The effect of various computational methods on the structures, energetics, and spectroscopies have been discussed, for the both thermodynamically and kinetically stable ones, the electronic structures and bonding nature of them are also analyzed.For the SiC2P system, the results show that there are 13 minimum isomers including the chainlike, four-membered ring and three-membered ring structures, which are connected by 19 interconversion transition states. At the final CCSD(T)/6-311+G(2df)//QCISD/6-311G(d)+ZPVE level, the lowest energy isomer is the linear form SiCCP 1 with a structure resonating mainly between (Si=C=C=P(( and (Si=C(-C(P(. The second and third low-lying isomers are two cyclic forms, i.e., four-membered ring species cSiCCP 5 (with Si-C cross-bonding) at just 3.2 kcal/mol and cSiCPC 7 (with C-C cross-bonding) at 10.4 kcal/mol, respectively. All the three isomers 1, 5 and 7 possess considerable kinetic stability either towards isomerization or dissociation, and thus expected to be observable. 5 and 7 may also be of particular interest since no cyclic species containing both Si and P have been known up to now. The results are also expected to be useful for understanding the initial growing step for the P-doped SiC vaporization processes. Finally, the calculated results were compared to those of the analogous molecules C3N, SiC2N and C3P.For Si2CN system, it is shown that fourteen minimum isomers are located connected by 23 interconversion transition states. At the CCSD(T)/6-311+G(2df)//QCISD/6-311G(d)+ZPVE level, the thermodynamically most stable isomer is four-membered ring form cSiSiCN 1 with Si-C cross bonding. Isomer 1 has very strong C-N multiple bonding characters, formally suggestive of a radical adduct between Si2 and CN. Such a high (-electron localization can effectively stabilize isomer 1 to be the ground state. The second low-lying isomer is a linear form SiCNSi 5 (9.8 kcal/mol above 1) with a resonating structure among (Si=C(-N=Si(, ((Si=C=N=Si(, and (Si=C=N–Si(( with the former two bearing more weight. The species 1 and 5 have very high kinetic stability by the barriers at least 25 kcal/mol. So, the isomers 1,5 and 7 of SiC2P and 1, 5 of Si2CN may be detected either in experiment or in interstellar space. The calculated results are compared to the analogous C3N, C3P, SiC2N,and Si2C2. The interstellar implication of SiC2P,Si2CN and the N, P doped SiC vaporization have been discussed also.2. For the four atomic 18-valence-electron Si2NP and SiC2S molecules, the structures, energetics, and spectroscopies of Si2NP and SiC2S isomeric species were explored at ab initio and DFT levels. The singlet and triplet potential energy surfaces for the two molecules have been established, respectively.For SiC2S, total 29 minimum isomers have been located at the B3LYP/6-311G (d) level. At the high CCSD(T)/6-311+G(2df)// QCISD/6-311G (d)+ZPVE level, the energetically lowest isomer is SiCCS 11 which mainly resonates between Si=C=C=S, and Si(C-C(S. In contr...