AN AB INITIO MOLECULAR ORBITAL STUDY ON THE STRUCTURE AND SPECTROSCOPIC PROPERTIES OF MAGNESIUM DICARBIDE

Author Institution: Department of Chemistry, Faculty of Science, Ochanomizu University; National Institute of Materials and Chemical Research, 1-1 Higashi; Hiroshima City University, 3-4-1 Ozukahigashi; Nobeyama Radio Obverservatory, MinamimakiA magnesium-bearing molecule $MgC_{2}$ is one of the candidate to be found in the envelope of a carbon star. Element magnesium may be rich in astrophysical objects because the elemental cosmic abundance of magnesium and silicon is almost the same, while silicon containing molecules such as $SiO, SiS, SiC, SiC_{2}, SiC_{4}, SiH_{4}, SiN$, have been found in interstellar space. Silicon dicarbide $SiC_{2}$ was discovered in IRC+10216. Now, magnesium dicarbide, $MgC_{2}$, is strongly expected to be observed. Since none of the experimental spectroscopic data on $MgC_{2}$ has been reported in any frequency region, ab initio molecular orbital predicition has been requested for its identification. The MR-SDCI+Q calculations with augmented cc-pVQZ basis sets have predicted that the ground state $MgC_{2} (^{1}A_{1})$ has T-shaped structure of $C_{2v}$ symmetry consisting of $Mg^{-}$ cation and $CC^{-}$ moiety with the dipole moment of 7.9 Debye. The CC and MgC distances have been found to be 1.275 and 2.012 \AA in its equilibrium geometry. The MR-SDCI+Q three-dimensional potential energy surface consisting of 497 points were analysed by the 2nd-order perturbation theory, predicting the rotational constants $A_{0}, B_{0}$, and $C_{0}$ to be $51794.0, 11493.9$ and 9378.7 MHz, and the centrifugal distortion constants $\Delta_{J}, \Delta_{JK}, \Delta_{K}, \delta_{J}, \delta_{K}$ to be $0.014, 0.21, -0.023, 0.0027, 0.14 MHz$, respectively. The $\nu_{1} (CC stretching), \nu_{2}(Mg-C_{2} streching)$, and $_{3}$(bending) vibrational frequencies have been estimated to be 1704.2, 594.8 and $455.8 cm^{-1}$, respectively. These results indicate that $MgC_{2}$ molecule is a rigid molecule unlike the analogue $SiC_{2}$, which is known as a molecule with large-amplitude motion. Toward the laser induced fluorescent spectroscopy, the vertical excitation energies for $\bar{A}^{1}A_{1}\leftarrow \tilde{X}{^{1}}A_{1}$ and $\bar{B}{^{1}}B_{2}$ transitions have also been calculated to be 8334 and $13034 cm^{-1}$, respectively, at the CAS-SCF level of theory