High accuracy studies on the ground state and transition state of SiC2

The long controversial isomers of the interstellar molecule SiC2, i.e. the cyclic C-2v isomer A, the linear C-infinity v isomer B, together with the newly discussed D-infinity h isomer C have been studied to reveal the nature of the stable structure. The spin-restricted MP2, QCISD, CCSD(T), the Gaussian-2 method G2MP2, the density-functional theory (DFT) method BLYP, and the hybrid Hartree-Fock-DFT methods Becke3LYP and BhandhLYP were employed with both 6-31G* and 6-311G* basis sets. The DFT methods fail to reproduce some experimental conclusions except for BhandhLYP. Based on the accurate calculation of MP2, QCISD, CCSD(T) and G2MP2, the experimentally detected isomer A is indeed more energetically favorable than the linear C-infinity v isomer B. But their energy difference decreases with the increase of the correlation level and the size of the basis set. The predicted assignment of the electronic absorption spectrum of A is B-2 <-- A(1), which is identical to the experimental outcome. The predicted vibrational spectra of A are also in excellent agreement with experimental data. It has been revealed that the linear isomer is not a true stationary point by the QCISD and CCSD(T) methods. It was suggested that the triangular form be best described by the half-to-half hybrid bonding of the ionic and covalent characters between Si and C-2 fragments with about 0.56 e of the charge transfer at the 6-311G* level. After considering the bond-length of the C-2 fragment, it is closer to the C=C in cyclopropene rather than the triple carbon bond suggested previously. The electronic energy of isomer C is too high to be stable compared with the other two states, which was interpreted by means of the orbital correlation diagram. Investigation on the transition state TS for the isomerization between A and B reveals that isomer B will automatically transform to the more stable C-2v form, which shows that B is dynamically unstable. As a conclusion, the C-2v isomer is not only thermodynamically but also dynamically stable, which explains the fact that B has not been experimentally detected. (C) 1998 Elsevier Science B.V. All rights reserved.