On the charge delocalisation in partially deprotonated polycarboxylic acid anions and zwitterions forming (-)O-H•••O(-) interactions in the solid state

A database analysis of the structural effects of inter-anion hydrogen bonding interactions between mono-deprotonated polycarboxylic acid and zwitterionic molecules has been carried out. It has been shown that the deprotonated -COO groups, irrespective of the charged or neutral nature of the species, are strongly polarised in the solid state, behaving as -COO(-) groups. In contrast, the protonated groups, whether in anions or zwitterions, behave exactly as in neutral molecules. The same behaviour is shown by the smallest dicarboxylic acid anion, the hydrogen oxalate anion. Charge localisation onto the deprotonated carboxylate groups is seen as a means of exploiting the stabilising contribution of dipole-dipole interactions in conjunction with charge-assisted O-H . . .O interactions. The few cases of symmetric or nearly symmetric O-H . . .O interactions are discussed.