The Strong Hydrogen Bond in the Formic Acid-Formate Anion System

Hydrogen bond energies and geometric structures of several symmetric and asymmetric formic acid-formate anion dimer complexes have been determined. Ab initio gradient optimization at the self-consistent field (SCF) level in a double-ξ + polarization + diffuse Gaussian function basis set was followed by a series of second-order Moller-Plesset (MP2) single-point calculations for the lowest energy structures. The equilibrium asymmetric dimer with the (anti) formic acid-(syn) formate anion conformation has the highest binding energy on both the SCF and MP2 levels at 28.8 and 33.0 kcal/mol, respectively. Experimentally, a gas-phase ΔH°_D value of 36.8 kcal/mol has been reported for the biformate anion (Meot-Ner, M.; Sieck, L. W. J. Am. Chem. Soc. 1986, 108, 7525). The calculated stability of this anti dimer includes a C-Η···O interaction that is estimated to contribute ~ 2.4 (SCF) or ~ 1.8 (MP2) kcal/mol to the dimer binding energy. The corresponding proton-transferred (syn) formic acid-(anti) formate anion complex is calculated less stable by 5.6 (SCF) or 3.7 (MP2) kcal/mol, even though in the isolated monomer syn-formic acid is the lower energy conformer by 5.8 (SCF) or 5.4 (MP2) kcal/mol in the same basis set. The greater stability of the anti dimer can be attributed both to less interfragment exchange repulsion and to more coulomb attraction compared with the syn dimer conformation. The optimized proton transfer curve as a function of one O−H distance shows a (syn dimer) inflection point and an (anti dimer) minimum, while the MP2 curve shows only a single (anti dimer) minimum. However, the proton transfer energy profiles for a series of fixed R(O···O) distances in the strong Η-bond region each show two minima, with the anti dimer always lower in energy, except at R_min where the minima merge. Two equilibrium symmetric (C_2H) structures were also found higher in energy than the asymmetric anti dimer; an anti-anti conformer by 4.1 (SCF) or ~1.0 (MP2) kcal/mol, and a syn-syn conformation by 7.1 (SCF) or 3.8 (MP2) kcal/mol. Experimentally, both symmetric type conformers and both the asymmetric syn-syn and (syn) formic acid-(anti) formate anion type dimers have been observed in bicarboxylic acid crystal structures. Comparison of bond lengths and angles between calculated and observed structures for the respective conformations shows very good agreement. The existence of all these conformer combinations experimentally and the small energy differences calculated between them theoretically show the sensitivity of the specific conformation of this strong Η-bond system to molecular structure, environment, and level of theoretical treatment.