The study of Cooperativity of Hydrogen Bonding of Acetone with Water

Hydrogen bonds are one of the most important types of intermolecular interactions. They play a huge role in many physical-chemical and biological processes. Water and aqueous solutions deserve special attention in terms of intermolecular interactions. Water can be imagined as a mixture of various associates coexisting at equilibrium with each other. Despite the presence of different models describing the hydrogen bonded complexes in bulk water, the authors concur to present water as highly associated liquid with a three-dimensional hydrogen bonded net. Water can influence the structure, acid-base properties and reaction ability of dissolved molecules. Specific interactions with water (solvent) have the biggest impact on the physical-chemical properties of solutes. The peculiarities of water are associated with the phenomenon of cooperativity, which leads to the increase in the energy of a many-particle complex compared with binary complexes. Cooperativity determines stability of many biologically important supramolecular systems. Dissolution of organic molecules in water is also accompanied by the cooperative effects. Therefore, the studies aimed to quantify the cooperativity of hydrogen bonds of solutes in water are of particular interest. The complexes with water molecules are investigated in gas phase and inert solvent including quantum-chemical calculations. The energy of a hydrogen bond in water clusters is highly increased compared with the energy of water dimer. Experimental investigation of hydrogen bonding of molecules in bulk water is a difficult task. In the current work, the investigation of hydrogen bonding of acetone with water is carried out by IR-spectroscopy and DFT calculations. First, the stretching vibration frequencies of the C=O group referred to the free acetone molecule and to the complex with the water molecule in various aprotic solvents are obtained. Special attention is focused on the purification of substances. An experimental methodology for the qualitative determination of the cooperative effect in many-particle complexes of a solute molecule with water is proposed. It is based on the linear dependence of the stretching vibration frequencies of acetone C=O group in various aprotic solvents upon the solvent parameter S-VW derived from the solution thermochemistry of alkanes. It is shown that binding of an acetone molecule with water clusters cooperatively strengthens compared with binary complexes. The shift accompanying the cooperative effect is determined as a deviation of the vibration frequency in bulk water from the linear dependence of the C=O group stretching vibration frequencies of acetone in equimolecular complex with water in aprotic solvents on the S-VW parameter. The cooperativity factor of hydrogen bonding is calculated as the ratio of C=O frequency shifts in multi-particle complex to the binary complex. The strengthening of hydrogen bonds is up to 90%. The quantum-chemical calculations are made to obtain the geometries, energies and frequencies of binary and tertiary complexes of water with acetone. The influence of cooperative effect on the bonding energies in complexes with two adjacent hydrogen bonds compared with a complex having one bonding site is shown. Experimentally determined cooperativity coefficients of hydrogen bonding in water solution are found to agree with the results of quantum-chemical calculations for tertiary complexes of acetone with water dimer. This accordance demonstrates the attenuation of the cooperative effect from tertiary complexes to real clusters in water environment.