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The enthalpy of hydrogen bonding of amines in methanol and water


K. Zaitseva1, A. Tukhbatulina1, M. Varfolomeev1 and B. Solomonov1

1Kazan Federal University, Russia

Keywords: hydrogen bonding in water, amines
property: solution enthalpy
material: water, methanol

Aqueous solutions attract the interest of researchers in different branches of science. Besides the importance of aqueous solutions in life sciences, their physicochemical properties are very interesting from a theoretical point of view. The study of thermodynamic properties of amines is of importance due to their common presence in nitrogen-containing enzymes and receptors. As far as the active centers of solute molecules are bonded with water-solvent, taking part in various chemical reactions in water should cause a disruption of hydrogen bonds (HB) between active centers and water molecules. Therefore, the reactive ability of active centers will depend on the energy of solute-water HB. There are a number of methods for estimation the HB enthalpy of water containing complexes in gas phase and inert solvent. However, there are arguments to suppose the enthalpy of HB complexes in bulky water not to be equal to the enthalpy of equimolar complexation.

In present work the approach to estimate the HB enthalpies of amines and pyridines in water was proposed. Analysis of an interaction of aliphatic alcohols with water and methanol revealed the equality of HB energy for one OH-group of water and its nearest structural analog - methanol. It allowed making an assumption about equality the HB enthalpies of proton acceptors with water and methanol. The method of semi-adiabatic solution calorimetry was used to obtain the solution enthalpies of amine series (isomeric primary, secondary, tertiary amines, pyridines, quinoline) in water and methanol. Measurements were carried out at neutral pH at infinite dilution. Solution enthalpies of aliphatic amines in water were adjusted properly by their ionization process. Using method [1] we calculated nonspecific solvation enthalpies in these systems. It was found that experimental and calculated enthalpies of alkanes and aromatic compounds for water and methanol are not equal. The difference specifies the contribution of hydrophobic effect. If the assumption about the equality of HB enthalpies of proton acceptors in water and methanol is true, the enthalpy of hydrophobic effect should be determined by the difference between the hydration enthalpy, nonspecific hydration enthalpy and HB enthalpy in methanol. Final step of our work was the determination of HB enthalpy of solutes with methanol and water associates (cooperativity effect).

References
  1. B. Solomonov, V. Novikov, M. Varfolomeev, N. Mileshko, J. Phys. Org. Chem. 18, 49 (2005)

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