Computational Study of the Ro-Vibrational Spectrum of CO-CO2

Abstract

An accurate ab initio ground-state intermolecular potential energy surface (PES) was determined for the CO-CO 2 Van der Waals dimer. The Lanczos algorithm was used to compute ro-vibrational energies on this PES. For both the C-in and the O-in T-shaped isomers, the fundamental transition frequencies agree well with previous experimental results. We confirm that the in-plane states previously observed are geared states. In addition, we have computed and assigned many other vibrational states. The rotational constants we determine from J = 1 energy levels agree well with their experimental counterparts. Planar and out-ofplane cuts of some of the wavefunctions we compute are quite different, indicating strong coupling between the bend and torsional modes. Because the stable isomers are T-shaped, vibration along the out-of-plane coordinates is very floppy. In CO-CO 2 , when the molecule is out-of-plane, interconversion of the isomers is possible, but the barrier height is higher than the in-plane geared barrier height. I. INTRODUCTIONCO-CO 2 is a Van der Waals dimer composed of the two monomers CO and CO 2 , both of which are of astrophysical interest. Its infra-red and microwave spectra have been recorded and its structure determined. 1-3 CO-CO 2 has two stable isomers. Both are T-shaped with the CO 2 monomer at the top of the T and the CO monomer the stem of the T. The lower-energy isomer has the C of CO close to the C of CO 2 . The higher-energy isomer has the O of CO close to the C of CO 2 . We shall refer to these isomers as C-in and O-in. Both isomers are shown on the right side of Figure 1. The C-in isomer was first studied by Legon and Suckley 1 and later by others. 4,5 The existence of the O-in isomer was predicted by ab initio calculations 6 and later confirmed. 2 For both isomers, two inter-monomer transition frequencies have been observed. One is for an in-plane state and the other for an out-of-plane state. 2,3 Harmonic frequencies have also been computed with ab initio methods. 5-7 Although the experimental frequencies for the lower energy C-in isomer are rather close to the ab initio harmonic frequencies, the experimental O-in frequencies are not close to the harmonic values. In the C-in case, the agreement is good enough that the experimental in-plane vibration was assigned to the "CO rock/geared bend".In this paper, we report a new four-dimensional (4-D) ab initio potential energy surface (PES) that is a function of the intermolecular coordinates of CO-CO 2 , and energy levels computed on it. The PES is built using points computed at the CCSD(T)-F12b/VTZ-F12 level. The only approximation in the energy-level calculation is the separation of the high frequency intra-monomer coordinates from the low frequency inter-monomer coordinates. Energy levels and wavefunctions are computed with the Lanczos method and a large spherical-harmonic type basis. It has been demonstrated that such calculations are accurate for other Van der Waals dimers. 8-11 Probability Density (PD) and wavefunction cut plots are used to...