COMPETING INTRAMOLECULAR AND INTERMOLECULAR HYDROGEN-BONDS FOR ORGANIC SOLUTES IN AQUEOUS-SOLUTION

For molecules stabilized by intramolecular hydrogen bond(s) in the gas phase, there is a competition in aqueous solution between the intramolecular hydrogen bonds and those which may form to the water solvent. Whether the intramolecular hydrogen bond is maintained or disrupted for the most stable solute structure in solution depends on the chemical nature of the molecule. 1,2-Ethanediol, 2-hydroxybenzoic acids and the neutral and protonated conformers and tautomers of 2-(4(5)-imidazolyl)-ethylamine (histamine) are discussed from this respect in this paper. The structures of monohydrated polar molecules obtained in gas-phase optimization are compared with the solution structure around the solute in water for the 4-hydroxybenzoic acid and imidazole. Geometries have been optimized at the ab initio HF and MP2 levels using the 6-31G* basis set. The corresponding energies were obtained for several systems in single point calculations using the 6-311++G** basis set. Solution simulations and relative hydration free energies were obtained using the Monte Carlo method and the statistical perturbation theory utilized in the BOSS program.