Interanionic O(-)-H center dot center dot center dot O(-) interactions: A solid-state and computational study of the ring and chain motifs

The O(-)-H . . .O(-) interaction formed by the anions HCO3-, HC2O4-, HC4O4- and HC5O5- (HA(-)), obtained upon monodeprotonation of the corresponding c arbonic, oxalic, squaric and croconic acids (H(2)A), has been investigated theoretically and experimentally. The ring (RING) and chain (CHAIN) hydroge n bond motifs established between these anions have been analysed in terms of geometry and energy and their occurrence in crystalline salts investigat ed by searching the Cambridge Structural Database (CSD) and the Inorganic C hemistry Structural Database (ICSD). It has been shown that hydrogen carbon ates form RINGs, with the notable exception of NaHCO3, while only CHAINs ar e known for hydrogen oxalates. Hydrogen squarates and hydrogen croconates c an form both RINGs and CHAINs. The structures of Rb+ and Cs+ hydrogen croco nates, which present the two alternative motifs, have been discussed togeth er with that of the hydrated salt NaHC5O5 . H2O. The relationship between R ING and CHAIN has been examined in the light of ab initio calculations. A r igorous quantum chemical study of the nature of the interanionic O(-)-H . . .O(-) interaction in both vacuum and condensed phase has shown that the in teraction energy is dominated by the electrostatic component which becomes attractive at short O . . .O distances (less than 2.5 Angstrom) if the net ionic charge on the anion is delocalised away from the -OH group. It has be en demonstrated that the RING motif is slightly metastable with respect to dissociation in the gas phase, but becomes stable in the crystal owing to t he influence of the Madelung field. However, the CHAIN motif is unstable bo th in the gas phase and in the crystal. It is argued that interanionic O(-) -H . . . O(-) interactions ought to be regarded as stabilising bonding inte ractions rather than proper intermolecular hydrogen bonds because the RING and CHAIN aggregates are not energetically stable on an absolute scale of b onding energy (i.e., in the absence of counterions). The presence of very s hort non-hydrogen-bridged O . . .O contacts resulting from charge compressi on of polyatomic anions bridged by alkali cations is also discussed.