A theoretical study of the effect of a tetraalkylammonium counterion on the hydrogen bond strength in Z-hydrogen maleate

High-level ab initio calculations (B3LYp/6-31+G** and QCISD(T)/6-311+G**) w ere carried out to resolve the disagreement between recent experimental and computational estimates of the relative strength of the intramolecular hyd rogen bond in Z-hydrogen maleate anion with respect to the normal hydrogen bond in maleic acid. The computational estimates for the strength of the in tramolecular hydrogen bond in the gasphase maleate anion are in a range of 14-28 kcal/mol depending on the choice of the reference structure. Computat ional data suggest that the electrostatic influence of a counterion such as a tetraalkylammonium cation can considerably weaken the hydrogen bonding i nteraction (by 1.5-2 times) in the complexed hydrogen maleate anion relativ e 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-hy drogen 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 demonstr ated 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 (th e ban er 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 st rong in condensed media or not depends on how their interactions with their immediate environment affect their strength.