Rationalizing the strength of hydrogen-bonded complexes. Ab initio HF and DFT studies

A comparative study of the relative stabilities of 17 multiply hydrogen-bonded complexes has been carried out using ab initio Hartree-Fock and density functional methods at the HF/6-311(d,p) and B3LYP/6-311-(d,p) levels, respectively. Predicted hydrogen-bond geometries, relative stabilities, solvent and structural effects, and electrostatic potential contours are discussed in conjunction with experimental data. The B3LYP method, which secures a better agreement of the optimized geometries with the available X-ray data, has also been applied to calculate the gas-phase free energies and enthalpies. The computations reveal that the frequently used incremental approach, which takes into consideration the primary and secondary electrostatic interactions, can often be deceptive in interpreting the stabilities of the multiply hydrogen-bonded dimers. The explanation that reduced entropy enhances the stability of dimers involving intramolecular hydrogen bonds in their monomeric parts compared to similar structures lacking such bonds has also been found to be misleading. A comparison of the calculated results with available experimental stabilities measured in CHCl3 solutions shows that water present in the solvent may cause dramatic changes in relative stabilities. Electrostatic potential contours calculated at the B3LYP/6-311(d,p) level provide a useful qualitative explanation of the stability differences in the investigated complexes.