Theoretical Investigations On The Structures And Stability Of Several Important Small Molecules

Recently, the Si, N, P, and O atom doped C-cluster have received experimentaland theoretical attention. The reason is their use in the inorganic chemistry, organicchemistry, material chemistry, and furthermore, it is their importance in theinterstellar chemistry. In this paper, high level quantum chemistry method has beenused to investigate the Si, N, P, or O atoms doped C-cluster, and may be helpful forthe further identification in the laboratory or in space. All the calculations wereperformed with the GAUSSIAN 98 and MOLCAS program packages. The mainresults are listed as follows:1. CCSD(T)//DFT/B3LYP method were used to study the PESs of SiC₃N andSiC₃P molecules. Both of SiC₃N and SiC₃P are members of SiC₃X (X = N or P)species. For the relevant isomers, the calculations on the structures, frequeneies, andenergies are carried out at the QCISD and CCSD levels. The CASPT2//CASSCFmethod is used to test the relevant species's multi-configurational effects.On the PES of the SiC₃N species, linear SiCCCN 1 is the most stable isomerwith the 2П electronic state. It can be viewed as a resonance structures between·|Si-C≡C-C≡N| and |Si=C=C·-C≡N|, with the former bears somewhat more weightthan the latter. The other isomers such as linear SiCCNC 2, SiNCCC 3, and SiCNCC7, the bent structures such as CCSiCN 4 and CCSiNC 5, the three-membered ringsuch as CN-cCCSi 11 and CN-cSiCC 12, all represent high thermodynamic andkinetic stabilities. The highest relative energies of the eight isomers mentionedabove are about 45kcal/mol, and the least interconversion barriers are more than 10kcal/mol. So they can exit in the low temperature conditions, such as interstellarspecs.On the PES of the SiC3P species, the global-minimum structure is linearSiCCCP 1 with 2П electronic state. From the results of the bond lengths, thedominant electronic configuration and the NBO analysis, it can be viewed as aresonance structures among ?|Si?C≡C?C≡P|, |Si=C=C??C≡P| and |Si=C=C=C=P|?.At the same time, three-membered ring isomers CC-cCPSi 10 represent large kineticand thermodynamic stabilities. It is different from the SiC3N species, not only linearstructure and three-membered ring isomers have high stabilities on the SiC3P's PES,but also four-membered ring isomers P-cCCCSi 14, P-cSiCCC 16, five-memberedring isomer cCPCCSi 18 and even cage-like isomer cage-PSiCCC 25 also representconsiderable stabilities. Although the relative energies of some relevant isomers areabout 80kcal/mol, their interconversion barriers are about 10kcal/mol. The highbarriers make them stable in the special condition once formed and hard to transformor decompose into other structures. The appearance of the stable multi-memberedring and cage-like structures on the SiC3P's PES may be due to the second-row Patom which has much less trend to form Ï€-bond than the corresponding first-row Natom.The high thermodynamics and kinetic stabilities make the relevant SiC3N andSiC3P isomers detectable in space or in the laboratory. The discovery of may cyclicspecies containing Si atoms in space, such as cSiC2,cSiC3 and cSi2C2, and thedetection of SiCN isomer stimulate the study of the new Si-, C-, N-, or P-containingSiCnX (X = N or P) species. On the other hand, the theoretical study of the Si, C, Nand Si, C, P systems, may be helpful to understand the initial step of the growingmechanism during the N-or P-doped SiC vaporization process. We hope that ourstudy can be helpful to distinguish the species in the future investigation.2. In order to study the PES of NC3P and C3P2, which are members of NCnY (X,Y = N, P) species, the DFT/B3LYP/6-311G(d) method was used. High levelQCISD/6-311G(d) was taken to get more exact structures, harmonic vibrationalfrequencies and dipole moments. To get reliable energies, the CCSD/6-311G(d)single-point energy calculations are performed based on the B3LYP/6-311G(d) andthe QCISD/6-311G(d) geometries. To test the relevant species' multi-configurationaleffects, the CASPT2//CASSCF method has also been used in this paper.For the triplet PES of NC3P, the ground state NCCCP 31 has a linear structureand 3Σ-electronic states. It may be best described as a resonant structure among|N≡C?C≡C??P|?, |N≡C??C=C=P|? and ?|N=C=C=C=P|?. Linear isomers CNCCP 32,CCCNP 33 and CCNCP 35 also have high thermodynamic and kinetic stabilities.Among the relevant isomers, the isomerization barriers are more than 15kcal/mol,and the relative energies are less than 65.0kcal/mol. On the triplet NC3P's PES, noisomes which containing cyclic or cage-like structures have higher stabilities andcan be found in interstellar space or in the laboratory.Among the isomers of C3P2 species, PCCCP 31 is the only triplet isomer whichhas high stability. Its symmetry is D∞h and the electronic is 3Σg-. The bond lengthsand the spin densities suggest a resonant structure among |P≡C?C≡C??P|?,?|P=C=C=C=P|? and |P≡C??C??C≡P|. The singlet isomers with high stabilities arebent PCPCC 12, three-membered ring PC-cCCP 16, CC-cCPP 17, five-memberedring cPCPCC 113, cPCCCP 126, and cage-like cage-PPCCC 116, cage-PCPCC 118.Among all the relevant isomers, the isomerization barriers are more than17kcal/molhigh and the relative energies are less than 60 kcal/mol.NCCCP 31 and PCCCP 31 are the ground states of the NC3P and C3P2 species,respectively. And they have the highest stabilities of all the energies. We compared itwith the well known isomer NCCCN, which can be viewed as a resonant structurebetween |N≡C??C??C≡N| and ?|N=C=C=C=N|?. Whem the P atom replace the Natom, the weights of conjugative triple bonding are getting decrease and that ofcumulene structure are increasing. It may be due to the second-row P atom whichhas much less trend to form Ï€-bond than the corresponding first-row N atom. Itshould be considered that for the XCnY species, the DFT/B3LYP/6-311G(d) andQCISD/6-311G(d) are reasonable for the structures and the harmonic vibrationalfrequencies. To get more exact energies, the multi-configurational CASPT2//CASSCF method is necessary. The relevant isomers mentioned above withconsiderable thermodynamics and kinetic stabilities may be detected in space or inthe laboratory. The calculated results may be helpful to the future generation anddetection.3. All the structures of the SiC2O species are researched at the DFT/B3LYP/6-311G(d) level, and total of eleven isomers are located. The geometrical parameters,harmonic vibrational frequencies and infrared intensities were obtained. The single-point energies were followed by the CCSD(T)/6-311G(2df) method with theinclusion of DFT/B3LYP/6-311G(d) zero-point vibrational energies. Furthermore,the structures and frequencies are refined at the QCISD/6-311G(2d) level and theenergies at the CCSD(T)/6-311G(2df)//QCISD/6-311G(2d)+ZPVE level. Finally,the CASSCF(10,12)/cc-pVTZ structural optimizations were performed to investigatethe multi-configurational effect of the relevant species followed by CASPT2(10,12)/cc-pVTZ single-point energy calculations.Among the eleven minima obtained at the DFT/D3LYP/6-311G(d) level, linearisomer SiCCO 1 is the global minimum. It is C∞v symmetry and 1Σ+ electronic state.Compared with the analogous SiCNN and SiCCS species, SiCCO can be viewed asSi=C=C=O and Si≡C?C≡O resonate structures and the former bears somewhat moreweight than the latter. The three-membered ring species O-cSiCC 5 is anotherisomer with high stability. Its relative energy is 39.4kcal/mol. Both of the isomersare stable towards thermodynamic and kinetic stabilities. In 2005, the linear SiCCOwas generated for the first time and probed by Fourier transform microwavespectroscopy. The experimental spectroscopic constants 2624.2202 MHZ is close tothe result of our QCISD calculation 2.609745 GHZ.Since the fragments such as SiC, CO, C2O, and SiC2 were known to exist ininterstellar space, they maybe reaction with each other to form the isomers SiCCOand O-SiCC. It is reasonable for us to believe that these two isomers could be foundin interstellar space. We hope that the calculated results may be helpful to the futureexperimental and interstellar detection.