Astron. Astrophys. 351, 1066-1074 (1999)

3. Molecular interactions

Carbon dioxide is well studied in chemistry as a solvent to syntethise or separate components in a mixture of chemicals. The complexes formed by this molecule are the subject of numerous theoretical and experimental studies. Molecular interactions are revealed through the behavior of the various bands of a molecule. In particular, infrared spectroscopy is a powerful tool to probe molecular interactions. Experiments in the microwave and radio spectral regions as well as ab initio calculations (Jamróz et al. 1995) and infrared spectroscopy (Kazarian et al. 1996) have shown that CO₂ could act as a Lewis acid if mixed with water, amines or amides. Long before, experiments had inferred the specific interactions of CO₂ with methanol (Hemmaplardh & King 1972), ethanol (Gupta et al. 1973) butan-1-ol and diethylether (Massoudi & King, 1973) among other solvents.

The acidity of the carbon dioxide molecule comes from the carbon atom being bound to two oxygen atoms. It then loses part of its electronic density to the benefit of the oxygen atoms. Molecules possessing at least a lone electron pair can thus interact with this molecule forming an Electron-Donor Acceptor complex (EDA). This interaction influences the strength of the intramolecular bonds. In the CO₂ case, the oxygen atoms are repelled whereas the interaction takes place through the carbon atom, modifying the bond angles. This interaction has strong effects on the infrared spectrum. The degeneracy of the CO₂ bending mode is broken and two distinct modes appear. Ab initio calculations (Jamróz et al. 1995) attribute the higher frequency mode to the out-of-plane vibration (the bent CO₂ defines a plane, see Fig. 2), the other one being the in-plane mode. They also show that the atom giving the electrons lies in the same plane as the deformed molecule. The stabilisation energy of the complex formed is calculated to be of the order of 20 to 40 kJ/mole (Handbook of Physics and Chemistry), representing 10 to 20% of the formation enthalpy of H₂O. It is thus a relatively strong coupling.

Fig. 2a-d. Schematic representation of the possible complex geometries between the CO2 and, from top to bottom, the dimethylether A , acetone B , methanol C and dimethylamine D . The CO2 molecule interacting is deformed and the degeneracy of the bending mode is removed. Atoms are represented as follows: white circle (hydrogen), black filled circle (carbon), grey anthracite (oxygen), light grey (nitrogen). The lobes on oxygen and nitrogen atoms represent the lone electron pairs.

Observable consequences on an infrared spectrum are the splitting of the CO₂ bending mode, and the appearance of a weak new mode. This last one should arise in the 1300-1600 cm^-1 region of the spectrum, corresponding to the symmetric stretching vibration (Shimanouchi 1972) which becomes slightly active when the molecule is bent. The antisymmetric mode is in principle less affected by the interaction as the CO₂ molecule tends to position itself at right angle to the axis defined by the Lewis acid bonds. It will therefore perturb much more the bending than the stretching mode.

© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999
helpdesk.link@springer.de