Knowledge Bank

Our emission spectra of the C$3$-Xe vdW (van der Waals) complex obtained near the 2$\mathrm{0<mrow\; data-mjx-texclass="ORD">2-</mrow>}$ band of the ~{A}$-$~{X} system of C$3$ showed that a stable linear isomer is present at high C$3$-bending vibrational levels ($vb$ $\ge$ 4) of the ground electronic state. Ab initio calculations at the level of CCSD(T)/cc-pVTZ (effective core potential of the Xe atom) support our spectral assignments. The linear isomer lies about 250 cm $\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ above the T-shaped isomer. From the calculated potential, large amplitude vdW bending motion is expected in the high $vb$ levels; this explains the congested emission spectra observed from vdW bands near the 2$\mathrm{0<mrow\; data-mjx-texclass="ORD">4-</mrow>}$ band of C$3$ . Molecular orbitals at the level of HF-SCF showed that bonding with the 5$p$ orbital of the Xe atom in a linear configuration (along the $a$-axis of C$3$) is stabilized by mixing some high-lying $\pi \ast$ bonding character into one of the C-C bonds. This type of bonding is not possible in the Ar and Kr complexes because in these two complexes 3$p$ or 4$p$ orbitals of the rare gas atoms are used. The isomerization from the T-shaped isomer to the linear isomer probably occurs rapidly in the ~{A} state.