Interaction-Induced Dipole Moments of Carbon Dioxide

The infrared absorption spectrum of carbon dioxide-rich planetary atmospheres, such as Venus or ancient Mars, can be difficult to model due to the presence of collision-induced absorption (CIA) features. These features appear in the 0-250 cm^-1 region and are caused by the interaction-induced (I-I) dipole moments of carbon dioxide dimers. Studying these I-I dipole moments can lead to more accurate atmospheric models, which may then be used to create better estimates of the surface temperature of these planets thus revealing information about their climates either now (for Venus) or in the ancient past (for Mars).

To investigate the I-I dipole moments of CO₂ dimers, a finite-field approach was used to calculate the response properties of a carbon dioxide monomer, along with the energies and dipole moments of a T-shaped CO­₂ dimer as a function of the intermolecular distance. Using the monomer results as fitting parameters, functions modeling these dipole moments were developed and compared to an analytically developed equation by M. S. A. El-Kader and G. Maroulis [1]. The resulting models for the I-I dipole moments presented here are thus based on purely ab initio results and are independent of any experimental variables.

The response properties of the carbon dioxide monomers and the energies and dipole moments of the CO­₂ dimers were calculated using the GAMESS-US ab initio software program and the NWChem ab initio software program. The I-I dipole moment equations were developed using the aug-cc-pVTZ basis set at the RHF, MP2, CCSD, and CCSD(T) levels of theory and the aug-cc-pVQZ and aug-pc-3 basis sets at the RHF and MP2 levels of theory. We have, to the best of our knowledge, reported the first purely ab initio I-I dipole moment model of a CO₂ dimer. The resulting functions had a maximum absolute error below 10^-3 au as shown in Figure 1, indicating that the functions provide an accurate description of the ab initio I-I dipole moments.

References:

1. El-Kader, M. S. A.; Maroulis, G. Chem. Phys. Lett. 670, 95-101 (2017)