Asymmetry of the hydrogen bond of hydrogen phthalate anion in solution. A QM/MM study

The geometry of hydrogen bonds and, in particular, the proton location in a hydrogen bond is a fundamental question. In the last years, considerable attention has been devoted to the influence of the environment on the hydrogen bond symmetry. Very recently several experimental results, based on the measurement of the 18O-induced 13C isotope shifts at the ipso carbons of labeled dicarboxylic acids in water and several organic solvents, have shown that the intramolecular hydrogen bonds in those monoanions of dicarboxylic acids are asymmetric in water but also, unexpectedly, in all the studied organic solvents. In this paper, the geometrical features of the intramolecular hydrogen bond in hydrogen phthalate anion solvated by water or by chloroform have been studied theoretically. By means of a continuum representation of solvent. a unique stationary point was found, both in water and chloroform, which corresponds, as in the gas phase, to a potential energy minimum with a symmetric intramolecular hydrogen bond in hydrogen phthalate. To introduce specific solute-solvent interactions, a discrete representation of solvent has also been employed performing QM/MM calculations on the whole systems. The stationary points found in water indicate that the intramolecular proton transfer along the hydrogen bond in hydrogen phthalate is already a double well but with a very small energy barrier. When entropic effects are introduced at 300 K by means of Molecular Dynamics simulations, the most probable configurations generated present an asymmetric hydrogen bond in water. In chloroform the calculated proton transfer energy profile along the hydrogen bond has a single well but, again, only a small percent of the configurations obtained from the dynamics simulations at 300 K can be considered symmetric. The existence of a counterion is another factor that preferentially stabilizes the asymmetric structures. The calculations performed with K+ in chloroform show that the hydrogen bond in hydrogen phthalate anion is clearly asymmetric. The effect of proton delocalization paths, which just involve the motion of the solute keeping the solvent structure frozen, along the hydrogen bond in the structures generated by the classical dynamics simulations has also been taken into account.