Density functional theory

The structures and stabilities of chainlike [(C[O.sub.2]).sub.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, [[[C.sub.n][O.sub.2n+1]].sup.2-], and the corresponding acids, [[C.sub.n][O.sub.2n+1]][H.sub.2], and Li salts, [[C.sub.n][O.sub.2n+1]][Li.sub.2]. At equilibrium, the most stable systems have [C.sub.s], [C.sub.2], or [C.sub.2v] 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 [[[C.sub.n][O.sub.2n+1]].sup.2-][([Li.sup.+]).sub.2] ion pairs. [[[C.sub.n][O.sub.2n+1]].sup.2-] linkages are also stabilized as dicarboxylic acids, [[C.sub.n][O.sub.2n+1]][H.sub.2]; 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 [[C.sub.n][O.sub.2n+1]][X.sub.2] (X = Li or H) compounds with n [greater than or equal to] 2 have been reported in the experimental literature (albeit, the alkyl esters [C.sub.2][O.sub.5][R.sub.2] and [C.sub.3][O.sub.7][R.sub.2] are commercially available). All CO bonds in [C.sub.2][O.sub.5][X.sub.2] to [C.sub.6][O.sub.13][X.sub.2] have single- to double-bond character (≅ 140-118 pm), indicating that the [[C.sub.n][O.sub.2n+1]] moieties are held together by strong chemical forces (in contrast to the weakly bound complexes ([C[O.sub.2]).sub.n] and [(C[O.sub.2]).sub.n.sup.-], 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.sup.-1]) will help in the spectral characterization of these compounds. Solvation calculations using the polarizable continuum model (PCM) find that [C.sub.2][O.sub.5.sup.2-] can be formed via C[O.sub.3.sup.2-] + C[O.sub.2] as well as C[O.sub.3.sup.-] + C[O.sub.2.sup.-], each reaction having Δ[G.sup.298] < 0 in practically all solvents. This result confirms the experimentally observed large solubility of C[O.sub.2](g) in molten carbonates, C[O.sub.3][M.sub.2] (M = Li, Na, or K). In contrast, starting with n = 2, the reactions [[[C.sub.n][O.sub.2n+1]].sup.2-] + C[O.sub.2] do not proceed spontaneously in any solvent (Δ[G.sup.298] > 0).