Theoretical Studies On The Structure And Stability Of Silicon, Germanium-related Cyanides

Silicon-containing chemistry has attracted continuing interest from various fields. One particular interest is their possible role in astrophysical chemistry. The concentration of Si in interstellar space is only slightly lower than those of C, N and O. However, due to the limitation of spectroscopic data, the number of detected Si-containing molecules is far fewer than those of C, N, O-containing species. Up to now, more than ten Si-containing species have been detected in space. It is well known that the C, N-containing molecules occupy a special realm in the interstellar chemistry. A number of such species naked or hydrogenated are found in space such as HC2n+1N (n=0-5), HCCN, H₂CCN, CH₃CN, etc., with some of them containing the isomeric forms such as HNC, HNCCC, HCCNC and CH₃NC. For the analogous Si,C,N-contaning species, only the simplest SiCN and SiNC radicals have been detected in space. Promisingly, existence of the new molecules [H,Si,C,N] (three isomeric forms HSiCN, HSiNC, HCNSi) and [H₃,Si,C,N] (three isomeric forms H₃CNSi, H₃SiCN and H₃SiNC) has been recently confirmed in laboratory, and has been recently confirmed by a combined IR spectra and theoretical calculations. The germanium chemistry on the earth already has a long history. A number of germanium-containing species including the free radicals, positive ions and anions have been studied due to their important role in organometallic chemistry and material sciences. However, the germanium chemistry in space has not received attention until very recently. In year 2002, the Ge III 1088.46 was detected and measured in the planetary nebulae (PNe) for the first time. The results indicated that Ge is synthesized in the initial steps of the s-process and therefore can be self-enriched in the planetary nebulae (PNe). We bear in mind that the astrophysical chemistry of its smaller congener, silicon, has received increasing interest. As was stated: "it is likely that with better spectroscopic data from the laboratory, many more (silicon-bearing molecules) may be found."Similarly, we can optimistically expect that the detection of Ge-containing molecules is just a matter of time though up to now, no such gas-phase species are known in space. Although silicon, germanium and carbon have iso-valence electrons, their chemical properties are rather different from each other. Silicon is usually reluctant to form multiple-bonding whereas carbon prefers. We have been interested in the structures, stabilities and bonding properties of a series of [Hx,Si,C,N] and [Hx,Ge,C,N] species. Despite the iso-valence, the [Hx,Si,C,N] and [Hx,Ge,C,N] isomers should have distinct features from the analogous [Hx,C,C,N] ones. In fact, the mono-hydrogenated [H,Si,C,N] and [H,Ge,N,C] isomers each have singlet ground states, whereas HCCN and HCNC have triplet states. It is of great interest to investigate the structuresand stabilities of the Si≡C, Si≡N, Ge≡C å’ŒGe≡N triply bonded, silylene,germylene or carbene-involving isomers ranging from chainlike to cyclic forms. Due to the predomination of H, C, N and O in space, the molecules containing the combination of Si, Ge with these elements, e.g. [Hx,Si,C,N],[Hx,Ge,C,N] are very promising. In dense interstellar space, the environmental temperature is very low and the molecules with good kinetic stability can exist once generated. Unfortunately, the detailed potential energy surface information of even the simplest hydrogenated Si, C, N-molecule, [H,Si,C,N] has not been reported yet. On the other hand, hydrogenated Si, Ge , C, N-containing species [Hx,Si,C,N], [Hx,Ge,C,N] also play important roles in the coating of microelectronic and photo-electronic materials. Knowledge of the structural, stability and bonding properties of [Hx,Si,C,N] [Hx,Ge,C,N] are very useful for understanding the chemical vaporization processes of SiCN, GeCN films, and for gaining insight of the atomic or radical interaction with substrates, i.e., SiHm+HnCN (m+n=x), GeHm+HnCN (m+n=x). The main conclusions are summarized as follows: 1. At the Gaussian-3//B3LYP/6-31G(d) leve, both the singlet and triplet potential energy surface study are performed on the [H,Si,C,N]. Six kinetically stable isomers cNHCSi 18, HNCSi 15, HSiCN 31,HSiNC 32,HCNSi 33 and HNCSi35 are predicted to be potential candidates for futurelaboratory and astrophysical detection. The isomer cNHCSi 18 are particularly promising. The results reported here are expected to stimulate future characterization of the first mono-hydrogenated SiCN molecule. 2. At the Gaussian-3//B3LYP/6-31G(d) leve, a detailed potential energy surface study is performed on the completely unknown [H2,Si,C,N] radical at various high levels. Eight kinetically stable isomers H2SiCN 1, H2SiNC 2, H2CNSi 3, H2CSiN 4, H2NCSi 5, HCNSiH 7, and CHSiNH 10a, 10b are predicted to be potential candidates for future laboratory and astrophysical detection. The former three-lowing isomers 1, 2 and 3 are particularly promising. The results reported here are expected to stimulate future characterization of the first even-hydrogenated SiCN molecule. Some interesting features such as the silylene, Si≡C triple bonding, cumulenic sp-hybridized Si-bonding, intra-and inter-molecular donor-acceptor bonding are found. 3. The detailed Gaussian-3//B3LYP/6-31G(d) calculations are performed on the potential energy surface of [H3,Si,C,N]. Eleven new kinetically stable isomers SiCNH3 5, H2CNSiH 8, H2CSiNH 9, H2NCSiH 10, H2NCHSi 11, H2N-cCHSi 12, H2NSiCH 13, cCH2SiNH 22, cNH2SiCH 23, cSiH2NCH 25 and cSiHCHNH 26 are predicted with the least conversion barrier of 18-37 kcal/mol. Three new kinetically meta-stable isomers H2SiCNH 15, H2SiNCH 17 and cSiH2CNH 24 are predicted with lower conversion barriers of 11-13 kcal/mol. Up to now, very few such species are experimentally known. The...