The structures and stabilities of chainlike (CO2)n (n = 2–6) polycarbonates, where adjacent C atoms are linked by C–O–C bonds, were investigated at the density functional theory (DFT) level (B3PW91/6–311G(2d,p)), including dicarboxylic dianions, [CnO2n+1]2–, and the corresponding acids, [CnO2n+1]H2, and Li salts, [CnO2n+1]Li2. At equilibrium, the most stable systems have Cs, C2, or C2v symmetries. In the gas phase, these dianions are generally metastable with respect to spontaneous ejection of one electron, yet in the presence of counterions they become stabilized, for example, as [CnO2n+1]2–(Li+)2 ion pairs. [CnO2n+1]2– linkages are also stabilized as dicarboxylic acids, [CnO2n+1]H2; we find the latter to have equilibrium conformations of higher symmetry than previously reported in the literature. To the best of our knowledge, none of the [CnO2n+1]X2 (X = Li or H) compounds with n ≥ 2 have been reported in the experimental literature (albeit, the alkyl esters C2O5R2 and C3O7R2 are commercially available). All CO bonds in C2O5X2 to C6O13X2 have single- to double-bond character (≈140–118 pm), indicating that the [CnO2n+1] moieties are held together by strong chemical forces (in contrast to the weakly bound complexes (CO2)n and (CO2)n–, n > 1). Vibrational frequencies were calculated to ensure all conformations were true minima. The IR and Raman intensities show that the high intensity C=O stretching modes (1750 ± 100 cm–1) will help in the spectral characterization of these compounds. Solvation calculations using the polarizable continuum model (PCM) find that C2O52– can be formed via CO32– + CO2 as well as CO3–[Formula: see text], each reaction having ΔG298 < 0 in practically all solvents. This result confirms the experimentally observed large solubility of CO2(g) in molten carbonates, CO3M2 (M = Li, Na, or K). In contrast, starting with n = 2, the reactions [CnO2n+1]2– + CO2 do not proceed spontaneously in any solvent (ΔG298 > 0).

Density functional theory (DFT) calculations were performed on the geometries and energies of CO3 1-/2-(CO2)n clusters with n = 1-5. For small clusters (n = 1 or 2), coupled cluster energies were obtained. Up to three CO2 molecules are bound covalently to the dianion. Only weak electrostatic bonds were found in the monoanions. Calculated binding energies for the monoanions are in reasonable agreement with experimental values. The calculated adiabatic electron detachment energy for the dianion is -0.07 eV at n = 5, indicating that at least six CO2 molecules will have to be added to CO3 2-before the dianionic cluster becomes, in the gas phase, more stable than the monoanionic one. In comparison, for sulfate -carbon dioxide clusters, stabilization occurs at n = 2. Carbonate clusters are compared with sulfate clusters for three solvent molecules: CO2, SO2, and H2O. Carbonate clusters have larger binding energies than sulfate clusters. For a given dianion, binding energies are largest for SO2 and smallest for H2O. However, in all cases, stabilization of the carbonate dianion by clustering is more difficult to achieve than stabilization of the sulfate dianion.Résumé : On a effectué des calculs suivant la théorie de la fonctionnelle de la densité sur les géométries et les énergies d'agrégats CO3 1-/2-(CO2)n dans lesquels n = 1-5. Pour les petits agrégats (n = 1 ou 2), il a été possible d'évaluer les énergies de couplage des agrégats. Le dianion peut se lier d'une façon covalente avec un maximum de trois molécules de CO 2 . On n'a trouvé que de faibles liaisons électrostatiques dans les monoanions. Les énergies de liaisons calculées pour les monoanions sont en accord raisonnable avec les valeurs expérimentales. L'énergie calculée pour le détachement adiabatique d'un électron est de -0,07 eV pour n = 5; ceci indique que, en phase gazeuse, il faut ajouter au moins six molécules de CO 2 à l'anion CO 2 2-avant que l'agrégat dianionique devienne plus stable que l'agrégat monoanionique. Par comparaison, pour les agrégats sulfate -dioxyde de carbone, la stabilisation se produit avec n = 2. On a comparé les agrégats du carbonate avec ceux du sulfate pour trois molécules de solvant : CO 2 , SO 2 et H 2 O. Les énergies de liaison des agrégats du carbonate sont plus élevées que celles des agrégats du sulfate. Pour un dianion donné, les énergies de liaison sont les plus élevées pour le SO 2 et les plus faibles avec H 2 O. Toutefois, dans tous les cas, la stabilisation du dianion carbonate par le biais de la formation d'agrégat se fait plus difficilement que la stabilisation du dianion sulfate.Mots-clés : agrégats carbonate -dioxyde de carbone, monoanions, dianions, théorie de la fonctionnelle de la densité, stabilisation du dianion, énergies de solvatation, énergies de liaison, comparaison avec les agrégats du sulfate.[Traduit par la Rédaction]

Density functional calculations using the B3PW91 functional with the 6-311+G(3df) basis set were performed on the addition of n = 1–4 SO2 molecules to SO3− and SO32−. Geometry optimizations were performed for a large number of possible structures. At n = 4, the dianionic cluster becomes adiabatically more stable than the monoanionic one, with an adiabatic electron detachment energy of 0.30 eV. Monoanionic clusters are characterized by the O–S–O–SO3 moiety having long O–S bonds to SO2 molecules. Dianionic clusters, however, prefer S–S bonding of O3S–O–S(O) with SO2.