Theoretical Studies On The Structures And Stability Of Several Silicon And Carbon-related Small Molecules

Silicon, carbon, oxygen, phosphorus and sulfur-related chemistry have obtained considerable attention from various fields. One particular interesting area is the possible role in astrophysical chemistry. Up to now, many silicon-, carbon-, oxygen-, phosphorus- and sulfur-containing molecules and radicals, such as SiC_n,C_nO,C_nS,SiN,SiO,SiS,C_nP and SiCO, have been detected either in dense molecular clouds or circumstellar shells. The discoveries of these molecules and radicals have drawn attention to the rich cosmic chemistry of heteroatomic systems, and possibly led to the inclusion of such molecules into chemical models of the dense molecular clouds. It is well know that extensive experimental and theoretical investigations have been performed on the larger SiC_n, C_nO, C_nP and C_nS which have been expected to be carriers of some interstellar bands. Their direct addition may lead to the formation of several SiC₃X (X=O,S) and [Si, C, P, X] (X=O,P,S) isomers. For example SiC2+CX(X=O,S), CP+SiX(X=O,P,S) and SiP+CX(X=O,P,S), which indicate the small molecules SiC₃X (X=O,S) and [Si, C, P, X] (X=O,P,S) series should be of interstellar interest. On the other hand, silicon, carbon, oxygen, phosphorus and sulfur-related chemistry have been also believed to play a key role in the production of the microelectronic materials. It is well known that binary silicon carbides are used frequently in microelectronic and photoelectronic applications. Oxygen, phosphorus and sulfur are often used as minute dopant to change the character of materials. During SiC_n-doped C_nX (X=O, S) and SiP-doped CO, CS, CP reaction processes, the smaller species SiC₃X (X=O, S) and [Si, C, P, X] (X=O, P, S) may be generated. Detailed theoretical investigations on valence bond structures, stability and spectroscopy of these species may not only be helpful for future identification of new Si, C, O, P and S-containing species in the laboratory and space but also for the elucidation of the reaction mechanisms of the small molecule clusters. The density functional theory and ab initio method are performed to investigate the structures and atbility of several silicon and carbon-related small molecules. The results are as follows:1. At the B3LYP/6-311G(d) level, the potential energy surfaces (PES) are initially explored on the triplet SiC₃O and SiC₃S molecules. The relevant isomers and related transition states are refined at the QCISD/6-311G(d) level. The valence bond structures and possible formation pathways of the stable isomers in the interstellar medium and laboratory are discussed.For the triplet SiC₃O, at the QCISD/6-311G(d) level, the lowest-lying isomer is the linear SiCCCO 1 with the ³âˆ‘electronic state, which possesses great kinetic stability of 59.5 kcal/mol. In addition, two bent chain-like isomers CSiCCO 2 and OSiCCC 5 with considerable kinetic stability. They are predicted to be candidates for future experimental production and astrophysical detection. The valence bond structures of the three stable isomers and possible formation pathway of isomer 1 in the interstellar space are discussed. The predicted structures and spectroscopic properties for the stable isomers are expected to be informative for the identification of SiC₃O and even larger SiC_nO species in the laboratory and interstellar medium.For the SiC₃S, at the QCISD/6-311G(d) level, the lowest-lying isomer is the chain-like SiCCCS ³1 with a great kinetic stability of 54.1 kcal/mol. In addition, cyclic isomers CC-cCSSi ¹9, S-cCCCSi ¹12, S-cCCSiC ¹18, S-cSiCCC 121 and cage-like isomer cage-SiSCCC ¹23 also possess considerable kinetic stability. These six isomers are predicted to be possible candidates for future experimental production and astrophysical detection. The valence bond structures of the stable isomers and possible formation pathways of the SiCCCS 31 in the interstellar medium are discussed.From the potential energy surface of the triplet SiC₃O, the chain-like isomers 1, 2 and 5 are thermodynamically and kinetically stable. No cyclic structure species are kinetically stable. However, the cyclic isomers ¹9, ¹12, ¹18, ¹21 and cage-like isomer ¹23 are stable isomers for the SiC₃S besides chain-like isomer 31, which is different from SiC₃O that has no cyclic isomer that are stable with respect to the separated reactants. Obviously, once the O atom in SiC₃O is substituted by S atom, the cyclic isomer becomes increasingly important and stable, which can be attributed to the fact that the second-row sulfur element has higher preference to formσ-bond and less preference to formÏ€-bond than the first-row oxygen element. Finally, we expect the calculated results will be helpful for the identification of the SiC₃X (X=O, S) species in the interstellar medium and for the production of the laboratory.2. The doublet isomeric potential energy surfaces of the 19-valence-electron [Si, C, P, X] (X=O,S) radicals are initially investigated at the B3LYP/6-311G(d) level. The relevant isomers and related transition states are refined at the QCISD/6-311G(d) level. The valence bond structures and possible formation pathways of some stable isomers in the interstellar medium and laboratory are discussed.For the [Si, C, P, O] radical, at the QCISD/6-311G(d) level, the lowest-lying cyclic isomer O-cCSiP 8 and two bent isomers OSiCP 1 and SiCPO 3 possess considerable kinetic stability. As a result, isomers 1, 3 and 8 are predicted to be possible candidates for future experimental and astrophysical detection. The valence bond structures of three stable isomers are analyzed. Implications of the isomers 1 and 8 in the laboratory and interstellar space are also discussed. The calculated results of structures and spectroscopic properties are expected to be informative for the detection of the stable [Si, C, P, O] isomers in the laboratory and space.For the [Si, C, P, S] radical, at the QCISD/6-311G(d) level, the lowest-lying isomer is the bent SSiCP 1 with considerable isomerization barrier. In addition, the bent isomer SiCSP 5 and the cyclic S-cCSiP 6 also possess considerable isomerization barriers. As a result, isomers 1, 5 and 6 and be regard as kinetically stable isomers and may be detected in the interstellar space. The formation mechanisms of the isomers 1 and 6 in the laboratory and interstellar space are also discussed. It is interesting to compare [Si, C, P, X] (X=O,S) with its isovalent species [Si, C, N, O], which have been experimentally and theoretically studied. On the PES of the [Si, C, N, O], several linear and chain-like isomers are thermodynamically and kinetically stable, cyclic structure species can not be located. On the contrary, besides several chain-like isomers, cyclic isomers become increasingly stable for [Si, C, P, X] (X=O,S) with the P and S atoms substitute the N and O atoms of the [Si, C, N, O], respectively, which may be ascribed to the higher preference of second-row phosphorus and sulfur elements have much trend to formσ-bonding than the first-row nitrogen and oxygen elements. Finally,we expect the spectroscopy characterizations and valence bond structures of the stable [Si, C, P, X] (X=O,S) isomers will not only be helpful for future astrophysical detection and laboratorial production but also for understanding the reaction mechanisms of the CP + SiO and CS + SiP.3. Theoretical calculations at B3LYP/6-311G(d) level are carried out to investigated the singlet potential energy surface (PES) of 18-valence-electron SiCP₂ molecule. The stable isomers and their relevant interconversion transition states are further refined at the QCISD/6-311G(d) level. Three isomers cSiPCP 1, PSiCP 7 and SiCPP 8 possess considerable kinetic stability. Analyzing the valence bond structures of three kinetically stable SiCP₂ isomers, isomer PSiCP 7 contains typical Si≡P and C≡P triple bonds, which is still experimentally unknown may represent a good example for laboratory production of compounds comprising Si≡P or C≡P triple bonds. Finally, the similarities and discrepancies in structure, energy and stability between SiCP₂ and its analogous C₂P₂, Si2P₂, SiCN2 and CSiNP molecules are compared. The changes of 18-valence-electron molecules in structure, energy and stability are indicated.