OBSERVATION OF A LINEAR ISOMER OF THE C${_3}$-Xe VAN DER WAALS COMPLEX

Abstract

Our emission spectra of the C${_3}$-Xe vdW (van der Waals) complex obtained near the 2$^{2-}_0$ band of the \~{A}$-$\~{X} system of C${_3}$ showed that a stable linear isomer is present at high C${_3}$-bending vibrational levels (${v_b}$ $\geq$ 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 $^{-1}$ above the T-shaped isomer. From the calculated potential, large amplitude vdW bending motion is expected in the high $v_b$ levels; this explains the congested emission spectra observed from vdW bands near the 2$^{4-}_0$ 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 $\pi^*$ 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.