Quantitative Equilibrium Constants between CO2and Lewis Bases from FTIR Spectroscopy

Equilibrium constants measured from the ν2 bending mode of CO2 by FTIR spectroscopy are reported for the electron donor−acceptor interactions of CO2 with three Lewis bases: triethylamine (TEA), pyridine (PYR), and tributyl phosphate (TBP). The average Kc values are 0.046 (CO2−TEA), 0.133 (CO2−PYR), and 1.29 (CO2−TBP) L/mol at 25 °C in the solvent pentane. For the CO2−TBP system, the average enthalpy of association, ΔH°, is −4.7 kcal/mol. Ab initio calculations indicate that steric repulsion of the ethyl groups in TEA cause the binding energy of the CO2−TEA complex to be weaker than that of the CO2−PYR complex by 1.34 kcal/mol, a trend that is in agreement with our spectroscopic data. The lattice fluid hydrogen bonding model was used in conjunction with the spectroscopically determined Kc values to predict bubble points for the CO2−TEA and CO2−TBP systems and CO2 sorption in a hypothetical polymer. These calculations indicate that these relatively weak specific interactions have a measurable effect on phase behavior and can influence sorption of CO2 in polymers.