Geometric H/D isotope effect in a series of organic salts involving short O−H•••O hydrogen bonds between carboxyl and carboxylate groups

A series of organic salts based on the monoanion of 2,3,5,6-tetrafluoroterephthalic acid were synthesized. All the compounds show characteristic hydrogen-bonded chain structures via short O–H⋯O hydrogen bonds between carboxyl and carboxylate groups. Upon deuteration, the overall structures of these compounds do not change. But the short O–H⋯O hydrogen bonds exhibit noticeable elongations (0.003–0.021 A) of the O⋯O distance, known as a positive geometric H/D isotope effect. However, there is no definite relationship between the O⋯O distance and the geometric H/D isotope effect due to complex intermolecular interactions between the cations and anions in the crystal structures. Further investigation reveals that there is a weak negative correlation between the O⋯O distance and the packing effect of the cations which can be described by the plane spacing between two adjacent benzene rings of the acid.