Catching CO2 in a bowl

Increased concentrations of CO(2) in the atmosphere contribute to global climate change. Improved methods are needed for removing CO(2) from the flue gas of power plants and/or directly from the atmosphere. A macrocyclic amidourea recently synthesized by Brooks et al., when dissolved in DMSO along with tetrabutyl ammonium fluoride, removes CO(2) from the atmosphere to form a complex in which a CO(3) group is held by a number of O-H-N bonds within the bowl-shaped cavity of the macrocycle. We have calculated the structure, stability, and vibrational spectra of this complex, using density functional techniques and polarized double-zeta basis sets. Both basis set superposition errors and polarizable continuum effects on the complex geometry and stability have been evaluated. The calculated structure is in good agreement with experiment. We predict that this CO(3)(-2) complex (and its HCO(3)(-) analogue) have larger formation constants by several orders of magnitude than the analogue complex of the amidourea macrocycle with Cl(-) (particularly in DMSO solution compared to aqueous solution). Our calculations also indicate that the CO(3)(-2) and HCO(3)(-) complexes can be distinguished by (13)C NMR. The CO(3)(-2) complex also has a distinctive H-N stretch, perturbed by the H-bonding to the CO(3) group. We also calculate the CO(3)(-2) complex to absorb within the visible region, unlike the free macrocycle or typical metal carbonates. Macrocycles of this type may provide a useful route to the absorption of atmospheric CO(2). Our calculations also indicate that changing the solvent from DMSO to water and/or heating the complex will be an efficient way to decompose it to release CO(2).