Theoretical study of the low-barrier hydrogen bond in the hydrogen maleate anion in the gas phase. Comparison with normal hydrogen bonds

Relative strengths of normal and low-barrier hydrogen bonds (LBHBs) in the gas phase were analyzed by means of quantum-mechanical and thermodynamic calculations on the mesaconic/citraconic and several maleic/fumaric cis/trans isomerization equilibria. All geometries were fully optimized with correlation effects included via second-order Moller-Plesset perturbation theory. The cis isomer of the maleic monoanion (also known as hydrogen maleate) is greatly stabilized in the gas phase owing to the formation of an intramolecular low-barrier hydrogen bond more than 20 kcal/mol stronger, in free energy terms, than the corresponding normal intramolecular hydrogen bond in maleic diacid. The very short internuclear distance (2.41 Angstrom) obtained at the MP2 level between the hydrogen donor and the hydrogen acceptor in hydrogen maleate, as well as the high value of the NMR chemical shift for the participating proton, are two other characteristics experimentally attributed to the formation of an LBHB. The transition state structure for proton exchange in the maleic monoanion is symmetrical. In this structure, the interactions of the central hydrogen atom with the acceptor and the donor atoms are classified as covalent by Bader's theory of molecular structure. In any case, our calculations indicate that the zero-point energy for maleate monoanion is above the energy barrier for proton transfer. This fact allows free motion of the hydrogen atom lying on the ground vibrational state in accordance with the single symmetrical minimum experimentally predicted in nonpolar solvents.