Density functional theory (DFT) calculations on the structures and stabilities of [C_nO_2n+1]^2– and [C_nO_2n+1]X₂ polycarbonates containing chainlike (CO₂)_n units (n = 2–6; X = H or Li)

Abstract

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).

Theoretical studies on clusters of carbonate with carbon dioxide, CO31–/2–(CO2)n, forn= 1–5 — Comparison of carbonate clusters with sulfate clusters

“…The central C-O bond distances are different in the C s case (1.40 and 1.44 Å), but identical (1.42 Å) in the C 2 case. The most stable structures for each n have C 2 symmetry, in agreement with the structures found by Bruna et al,18 except for the C 1 structure IVa(-2), which is 0.02 eV lower than the C 2 structure IVb(-2).…”

“…The lowest energies were obtained for C s structures. In a recent paper by Bruna et al, 18 structures of the dianionic clusters CO 3 2-(CO 2 ) n (n = 1-6) with C 2 symmetry were investigated by density functional theory (DFT) methods.…”

Carbon dioxide transport in molten calcium carbonate occurs through an oxo-Grotthuss mechanism via a pyrocarbonate anion

“…and larger oligomers C n O 2− 2n+1 are structurally stable with their energetics strongly dependent on the counterion [13]. Polymeric phases of CO 2 have been also predicted by DFT [14].…”

Sr[C2O5] is an Inorganic Pyrocarbonate Salt with [C2O5]2– Complex Anions

“…Since their discovery by Bohem and Mehta in 1938, pyrocarbonate esters ( R 1 –[C 2 O 5 ]– R 2 ) have found extensive use in organic syntheses and biochemistry. − In contrast, pyrocarbonate salts with [C 2 O 5 ] 2– complex anions have not been reported yet. The formation of [C 2 O 5 ] 2– -groups in melts, aqueous carbonate solutions, or in the gas-phase has been proposed earlier. − These pyrocarbonate groups were suggested to be formed by the reaction of CO 2 with the [CO 3 ] 2– -anion. However, the evidence for the existence of isolated [C 2 O 5 ] 2– -groups is circumstantial, as their presence was either inferred from two weak Raman peaks in the spectrum of an eutectic Li 2 [CO 3 ]–Na 2 [CO 3 ] melt or from computer simulations without experimental benchmarks.…”