Further results agreed that the location for strongest CO2 approach was always at the phosphoryl O atom in all five solvents. The CO2 approach distance at this oxygen atom was always at the shortest distance and the CO2 interaction energy at this location was always largest. These are shown in Figure . CO2 approach distance and interaction energies were only slightly less favorable at the terminal methoxy O atoms of (1) -(3) but were significantly less favorable at the internal etheric O atoms. This is likely influenced by the steric bulk of the PEG arms making approach of the internal O atoms by CO2 more difficult. This also explains why adding etheric groups to TBP fails to improve CO2 absorption: the primary interaction site is always at the phosphoryl O atom, where the interaction is far stronger than at any ether O atom. It has been suggested by Shi, et al. that it might be preferable, for solvents intended to separate CO2 from other sparingly soluble gases like H2, to have low molar volumes, and thus small fractional free volumes (FFV), to minimize the absorption while still maintaining a sufficiently high CO2 solubility through chemical interactions with functional groups. Free volume is related to the density and molar volume of the solvent. Computational results for the FFV for each solvent are shown in Table . T i BP gave the largest FFV, as would be expected due to the branched nature of the isobutyl groups. The FFV of ( ) -( ) steadily decreased as the PEG arms grew longer. These calculated values were in general agreement with the experimental gas absorption data in Table which shows that the molecules with longer PEG arms absorbed less CO2 while branched T i BP absorbed slightly more CO2 than linear TBP. As seen in Figure , a plot of the 25°C CO2 absorption capacity versus solvent FFV showed a linear relationship (r = 0.9945), suggesting that FFV is the dominant factor in CO2 absorption in these solvents.