Hydrogen bonds to carboxylate groups. The question of three-centre interactions

Hydrogen bond geometries as well as their stereochemical preferences have been compared for 255 small molecule carboxylate structures retrieved from the Cambridge Structural Database. A total of 974 independent hydrogen bonds were analysed. Hydrogen atoms cluster in the carboxylate lone-pair directions with no tendency to adopt a central, symmetric position between the two oxygens. The long, secondary H ⋯ O contacts of asymmetric syn hydrogen bonds do not affect either the overall geometries of the interactions or the total syn/anti distribution of accepted H-atoms, presumably due to very unfavourable CO ⋯ H angles. On this basis it is suggested that syn hydrogen bonds to carboxylates should be regarded most frequently as two-centre rather than three-centre (bifurcated) interactions. The statistical data also show that criteria traditionally used to infer three-centre hydrogen bonds are inadequate for carboxylate acceptors.