Knowledge Bank

A magnesium-bearing molecule $MgC2$ 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 $SiO,SiS,SiC,SiC2,SiC4,SiH4,SiN$, have been found in interstellar space. Silicon dicarbide $SiC2$ was discovered in IRC+10216. Now, magnesium dicarbide, $MgC2$, is strongly expected to be observed. Since none of the experimental spectroscopic data on $MgC2$ 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 $MgC2(1A1)$ has T-shaped structure of $C2v$ 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 $A0,B0$, and $C0$ 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 $0.014,0.21,-0.023,0.0027,0.14MHz$, respectively. The $\nu 1(CCstretching),\nu 2(Mg-C2streching)$, and $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$(bending) vibrational frequencies have been estimated to be 1704.2, 594.8 and $455.8cm-1$, respectively. These results indicate that $MgC2$ molecule is a rigid molecule unlike the analogue $SiC2$, which is known as a molecule with large-amplitude motion. Toward the laser induced fluorescent spectroscopy, the vertical excitation energies for $A¯1A1\leftarrow X~\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A1$ and $B¯\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2$ transitions have also been calculated to be 8334 and $13034cm-1$, respectively, at the CAS-SCF level of theory.