Steven G. Mayer, Department of Chemistry, University of Portland, Portland, OR 97203 USA

1. Neat CO2(gas) 10% CO2 30% CO2 30% CO2 90% CO2 90% CO2 Neat CO2(liq) Neat CO2(liq) Investigating the Molecular Interactions Between Solute and Cosolvent Molecules in Supercritical CO2 Steven G. Mayer, Department of Chemistry, University of Portland, Portland, OR 97203 USA We were able to reproduce the Raman spectra obtained by Sum, et al. (J. Phys. Chem. B 1997, 101, 7371-7377) of CO2 incorporated into clathrate hydrates (water cages) and then use the identical conditions to collect infrared absorption spectra of these structures. We found that the enclathrated CO2 exhibited rotational structure similar to gas phase CO2. Calculation of the rotational constant suggests that the CO2 freely rotates inside the water cage. We predicted this phenomenon based on a suggestion by Consani and Pimentel in their paper on enclathrated C2D2 (J. Phys. Chem. 1987, 91, 289-293). Furthermore, we found that the infrared absorption spectra revealed that CO2 exists in several different environments that are highly dependent upon the concentration of CO2 in H2O, the temperature, and the mixing time. Figure 1 shows the ν12ν2 Fermi diad of CO2 (Raman) at 1500 psi and 295K. Notice the broadening and red-shifting of the peaks. Figure 2 shows the asymmetric stretch of CO2 (infrared) at 1500 psi and 295K. Notice that the 30% CO2 spectrum reveals a narrower peak and rotational structure not present in the 90% mixture. Recent work by Ripmeester, et al. (J. Phys. Chem. A2009, 113, 6308-6313) observed similar behavior but our spectra were collected at higher resolution and as a result, we were able to see rotational structure in the asymmetric stretch of CO2. The rotation of CO2 in the clathrate cage was suggested by Ikeda, et al. from their x-ray crystallographic data that they presented in the Journal of Chemical Physics (J. Chem. Phys. 1998, 108 (4), 1352-1359). Figure 1 Figure 2 Absorbance Raman Intensity cm-1 cm-1