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We propose a method for calculating the Gibbs energies of hydrogen bonding of solutes with associated solvents via the thermodynamic analysis of experimental values of solvation Gibbs energies. The method is applied to solutions of different proton acceptors in methanol. It is shown that the contribution of hydrogen bonding processes to the solvation Gibbs energy in methanol is in most cases very different in magnitude from the formation Gibbs energy of equimolar complexes of the solute and methanol. We demonstrate the need to include the contributions from solvophobic effects in investigating intermolecular interactions in associated solvents by means of thermodynamic data. [ABSTRACT FROM AUTHOR]

Stability constants, free energies, and enthalpies and entropies of the complexation of L-alanine methyl ester hydrochloride (L-Ala-HCl), L-phenylalanine methyl ester hydrochloride (L-Phe-HCl), and valine methyl ester hydrochloride (L-Val-HCl) with 15-crown-5 (15C5), benzo-15-crown-5 (B15C5), 18-crown-6 (18C6), benzo-18-crown-6 (B18C6), dicyclohexano-18-crown-6 (DC18C6), and dicyclohexano-24-crown-8 (DC24C8) in methanol are reported for 20°C. No significant variation in the stability constants and free energies of complexation is observed, indicating that the various crown ethers are poorly selective in binding the amino acids. However, the nature of the crown ether and the amino acid and their pattern of substitution cause a remarkable variation in the enthalpies and entropies of complexation. This indicates a strong enthalpy–entropy compensation effect. The enthalpy–entropy compensation effect for the crown ether complexes of the amino acid methyl ester hydrochlorides reported herein is compared with that of the crown ethers complexes of the amino alcohols and the free amino acid. It is found that the enthalpy–entropy compensation effect holds equally for the three classes of complexes. [ABSTRACT FROM AUTHOR]