Theoretical Study of the Low-Barrier Hydrogen Bond in the Hydrogen Maleate Anion in the Gas Phase. Comparison with Normal Hydrogen Bonds

Abstract

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 Møller−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 Å) 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.

Temperature-dependent polymorphism of N-(4-fluorophenyl)-1,5-dimethyl-1H-imidazole-4-carboxamide 3-oxide: experimental and theoretical studies on intermolecular interactions in the crystal state

“…The relatively long N-H bond and a large value of N-H…O bond angle in (I) allow one to classify this intramolecular hydrogen bond as a strong one. Moreover, the short N…O distance suggests that it could also be considered as a low-barrier hydrogen bond [35,36] similar to intramolecular hydrogen bonds observed for benzopyrane derivatives [37][38][39]. To elucidate this observation, low-temperature X-ray measurements were undertaken.…”

Co-crystal/salt crystal structure disorder of trichloroacetic acid–N-methylurea complex with double system of homo- and heteronuclear O–H···O/N–H···O hydrogen bonds: X-ray investigation, ab initio and DFT studies

“…Many researches are devoted to very strong hydrogen bonds, because of their important role as a transition state in biochemical reactions and enzyme catalysis [6][7][8][9][10][11][12]. Their unusual stabilization energy makes them to be distinguished from ordinary hydrogen bonds.…”

A neutron scattering study of strong-symmetric hydrogen bonds in potassium and cesium hydrogen bistrifluoroacetates: Determination of the crystal structures and of the single-well potentials for protons

“…39 On the other hand, calculations of the isolated maleate ion lead to a LBHB at variance with the crystal structure. 38,40,41 We are not aware of any theoretical modeling of very large proton displacements ͑ϳ2 Å͒ for KHM in the crystal field.…”