Solvent effects on hydrogen bonds - A theoretical study

Hydrogen-bonded interactions in the acetic acid dimer and in complexes formed by acetic acid with acetaldehyde, acetamide, ammonia, methanol, and phenol and in corresponding complexes between the acetate anion and the same ligands as before were studied in the gas phase and in solution by means of quantum chemical DFT/BLYP calculations. Three solvents (heptane, DMSO, and water) of largely varying polarity were chosen. The polarized continuum model was used for the description of the solvent. Optimized geometries, reaction energies, and Gibbs free energies of complex formation were computed. In the neutral complexes an opening of the weaker of the two hydrogen bonds formed in the complex is observed with increasing polarity of the solvent. This opening is interpreted by the creation of optimal conditions for separate solvation of the subsystems of the hydrogen bond in competition with the geometrical requirements for the formation of this bond. Even though almost all reaction energies are found to be negative, only the strongly bound complexes, acetic acid dimer, and acetic acid-acetamide are stable according to Gibbs free energy results. The main factors for this finding are the entropy loss on the formation of the bimolecular complex and the changes of the free energy of solvation. Solvation effects are interpreted in terms of dipole moments, solvent-accessible surfaces, and cavity volumes of the separate molecules and of the complexes.