<input ... ></input><input ... >A macrocyclic amidourea recently synthesized by Brooks et al., when dissolved in DMSO along with tetrabutyl ammonium fluoride, removes CO₂ from the atmosphere to form a complex in which a CO₃ 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-ζ 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 CO₃⁻² complex (and its HCO₃⁻ 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 CO₃⁻² and HCO₃⁻ complexes can be distinguished by ¹³C NMR. The CO₃⁻² complex also has a distinctive H−N stretch, perturbed by the H-bonding to the CO₃ group. We also calculate the CO₃⁻² 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 CO₂. 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 CO₂.