Low-Barrier Hydrogen Bonding in Aqueous and Aprotic Solutions of Dicarboxylic Acids: Spectroscopic Characterization

We present additional spectroscopic evidence for the formation of low-barrier hydrogen bonds (LBHBs) within vicinal and geminal dicarboxylic acid monoanions. Hydrogencis-cyclohexane 1,2-dicarboxylate, displays low-field1H NMR signals in aprotic solvents at 19.3 to 19.7 ppm, depending on the solvent. The LBHB in hydrogen 2,2-dimethylmalonate is further characterized by the observation of a positive value for the deuterium isotope effect on the chemical shift, Δ[δH− δD], of 0.8 ± 0.3 ppm. Low-field1H NMR signals are not observed for hydrogentrans-cyclohexane 1,2-dicarboxylate, hydrogen succinate, or hydrogen malonate under the same conditions. These compounds lack internal structural constraints forcing close contact between the caboxylic acids, although close contacts are possible, confirming that compression of carboxylic acid groups facilitates LBHB formation. Internally strained dicarboxylic acid monoanions also display low field1H NMR signals in aqueous solutions (90/10, acetone-d6/H2O) at low temperatures, at which proton exchange is slowed. The low field signals at −50°C are centered at 20.2 ppm for hydrogen maleate, 19.0 ppm for hydrogen 2,2-dimethylmalonate, and at 19.2 ppm for hydrogencis-cyclohexane 1,2-dicarboxylate.