title: A theoretical study of the effect of a tetraalkylammonium counterion on the hydrogen bond strength in Z-hydrogen maleate.
abstract: High-level ab initio calculations (B3LYP/6-31+G and QCISD(T)/6-311+G**) were carried out to resolve the disagreement between recent experimental and computational estimates of the relative strength of the intramolecular hydrogen bond in Z-hydrogen maleate anion with respect to the normal hydrogen bond in maleic acid. The computational estimates for the strength of the intramolecular hydrogen bond in the gas-phase maleate anion are in a range of 14-28 kcal/mol depending on the choice of the reference structure. Computational data suggest that the electrostatic influence of a counterion such as a tetraalkylammonium cation can considerably weaken the hydrogen bonding interaction (by 1.5-2 times) in the complexed hydrogen maleate anion relative to that in the naked anion. The estimated internal H-bonding energies for a series of Z-maleate/R4N+ salts (R = CH3, C2H5, CH3CH2CH2CH2) range from 8 to 13 kcal/mol. The calculated energy differences between the E- and Z-hydrogen maleates complexed to Me4N+, Et4N+, and Bu4N+ cation are 4.9 (B3LYP/6-31+G(d,p)) and 5.7 and 5.8 kcal/mol (B3LYP/6-31G(d)). It is also demonstrated that the sodium cation exerts a similar electrostatic influence on the hydrogen bond strength in bifluoride anion (FHF-). The present study shows that while low-barrier short hydrogen bonds can exist in the gas phase (the barrier for the hydrogen transfer in maleate anion is only 0.2 kcal/mol at the QCISD(T)/6-311+G//QCISD/6-31+G level), whether they can also be strong in condensed media or not depends on how their interactions with their immediate environment affect their strength.
authors.affiliation: Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, USA. rbach@Udel.edu
authors.forename: R D
authors.initials: RD
authors.lastname: Bach
authors.forename: O
authors.initials: O
authors.lastname: Dmitrenko
authors.forename: M N
authors.initials: MN
authors.lastname: Glukhovtsev
mesh_terms: D004791:Enzyme Inhibitors / Q000737:chemistry; D005650:Fumarates / Q000737:chemistry; D006860:Hydrogen Bonding; D008298:Maleates / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D004791:Enzyme Inhibitors; D005650:Fumarates; D008298:Maleates; D000644:Quaternary Ammonium Compounds; C032005:fumaric acid; C030272:maleic acid