Theoretical Study On Structures And Stability Of The Interstellar Molecule NC₃O

Recently, the N,O atom doped C-cluster have received experimental and theoretical attention. The reason is their use in the inorganic chemistry, organic chemistry, and material chemistry. Furthermore, it is their importance in the interstellar chemistry. It is well known that with the rapid development of spectrometric detective techniques, more than 120 species have so far been identified in interstellar and circumstellar dust clouds. Among them, many nitrogen- or oxygen-containing molecules have been detected, such as CnO (n=1, 2, 3, 5), CN, NO, CnN (n=1, 3, 5) and HCN. Moreover, extensive experimental and theoretical investigations have been performed on the larger CnO and CnN clusters. The mixed NCnO species may present a bridge between the CnN and CnO clusters and should be of astronomical interest.Understanding the properties of the structure, bonding and stability of NCnO clusters may be helpful for future identification of new N, C and O-containing species either in the laboratory or in interstellar space and also for the elucidation of the formation mechanism of O-doped CnN clusters The NC₃O species that we study in this paper is belong to the doped clusters. Although these isomers have not been detected in interstellar space,some fragments such as CO,C2O,C3,NO and CN were known to exist in interstellar space which means the molecules that contain C,N,O atoms may exist in interstellar space. It is reasonable for us to speculate that many NC₃O isomers may be generated. We hope that the calculated results may be helpful to the future experimental and interstellar detection.On the other hand, it should be mentioned that the analogous PC₃O and NC3S clusters have obtained theoretical studies. The studies showed that besides the stable quasi-linear structure PCCCO and linear structure NCCCS,there are chainlike and ring structures with considerable thermodynamics and kinetic stabilities. Considered that NC₃O and PC₃O belong to the XC₃O(X = N or P) species, in the same way, NC₃O and NC3S belong to the NC3Y (Y=O or S) species, they should have some analogous characteristics. Up to now, no ring species containing N atom have been found in interstellar space. Therefore it is necessary to take study on the PES of the NC₃O species to make sure whether there are isomers with the high stabilities that can be detected in the laboratory or in space. Up to now, no ring species containing N atom have been found in interstellar space.In this paper, high level quantum chemistry method has been used to investigate the NC₃O cluster and may be helpful for the further identification in the laboratory or in space. All the calculations were performed with the GAUSSIAN 98 packages. The main results are listed as follows:All the structures of the NC₃O species are researched at the DFT/B3LYP/6-311G(d) level, and total of 32 minimum 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(3df,2pd) method with the inclusion of DFT/B3LYP/6-311 G(d) zero-point vibrational energies. Furthermore, the structures and frequentcies are refined at the QCISD/6-311G(2d,p) level and the energies at the CCSD(T)/6-311G(3df,2pd)//QCISD/6-311G(2d,p)+ZPVE level.Among the 32 minima obtained at the DFT/D3LYP/6-311G(d) level, linear isomer NCCCO 1 is the global minimum. It is C∞v symmetry and 2Пelectronic state. We compared it with the analogous radical PCCCO and NCCCS, which can be described as a resonant structure among the follow three forms: (1) ?|X=C=C=C=Y| (2) |X≡C?C?=C=Y| (3) |X≡C?C≡C?Y|? (X=N,P;Y=O,S) The relative weight order of the three resonant forms of ground-state NCCCO is (2)>(1)>(3) and that order is (1)>(2)>(3) for PCCCO. That means when the P atom replaces the N atom, the weights of conjugative ttriple bonding are getting increased. For NCCCS the order is (3)>(2)>(1), the weights of conjugative triple bonding are getting increased. The difference can be explained by the fact that C=O double bonding are much stronger than the C=S bonding which strengthen the stability of molecule with the cumulene structure. Chainlike isomers CNCCO 2,CCNCO 3,CCOCN 4 and branched-chain isomer CC(O)NC 23 also represent considerable stabilities. Although the relative energies of some relevant isomers are about 80kcal/mol, their interconversion barriers are about 24kcal/mol. The high barriers make them stable in the special condition once formed and hard to transform or decompose into other structures. On the NC₃O's PES, no isomers which containing cyclic or cage-like structures have higher stabilities and can be found in interstellar space or in the laboratory ,but on the PC₃O's PES there are cyclic and cage-like structures with considerable thermodynamics and kinetic stabilities. It may be due to the first-row N atom which has much more trend to formÏ€-bond than the corresponding second-row P atom. The relevant isomers mentioned above with considerable thermodynamics and kinetic stabilities may be detected in space or in the laboratory. We hope that the calculated results may be helpful to the future experimental and interstellar detection.