A method to determine the Gibbs energy of specific (hydrogen bonding) interactions of a solute with water is proposed. The energies of hydrogen bonding in bulk water are very difficult to determine by any method. The Gibbs energy of hydration of substances forming hydrogen bonds with water is considered as the sum of contributions due to non-specific interactions, the hydrophobic effect, and specific interactions. The first two terms were found to be rather accurately described by empirical equations. The Gibbs energies of hydrogen bonding of aliphatic amines and pyridines in bulk water are determined, and the results are compared with the energies of their complexes with one molecule of water. The cooperativity of hydrogen bonding is proved in aqueous solutions of amines and pyridines. To use our equations, experimental values of the Gibbs energies of solvation in 'standard' solvents need to be known. The Gibbs energies of solvation of ten amines and pyridines in dimethyl sulfoxide are determined experimentally using chromatographic head space analysis. The tendencies observed for the series of amines and pyridines are in agreement with other studies.
In the present work, the thermochemistry of solution, solvation, and hydrogen bonding of cyclic amides in proton acceptor (B) and proton donor (RXH) solvents were studied. The infinite dilution solution enthalpies of δ-valerolactam, N-methylvalerolactam, ε-caprolactam, and N-methylcaprolactam were measured at 298.15 K. The solvation enthalpies of cyclic amides were calculated based on the measured solution enthalpies and sublimation/vaporization enthalpies from literature. The enthalpies of hydrogen bonding between cyclic amides and proton acceptor and donor solvents were then calculated as a difference between the total solvation enthalpy and the non-specific contribution. The latter was estimated via two different approaches in proton donor and proton accepting solvents. The effect of the cycle size on the strength of hydrogen bonding of the cyclic amides in solution is discussed.
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