Hydrogen-bond symmetry in zwitterionic phthalate anions: Symmetry breaking by solvation

The cationic nitrogen of zwitterion 1 is located symmetrically with respect to its intramolecular OHO hydrogen bond. Incorporation of one O-18 allows investigation of the H-bond symmetry by the NMR method of isotopic perturbation. In both CD3OD and CD2Cl2 equilibrium isotope shifts are detected at the carboxyl and ipso carbons. Therefore, 1 exists as a pair of interconverting tautomers, not as a single symmetric structure with its hydrogen centered between the two oxygens. The H-bond is instantaneously asymmetric, and there is an equilibrium between solvatomers (isomers or stereoisomers that differ in solvation). The broader implications of this result regarding the role of the local environment ("solvation") in breaking symmetry are discussed.