title: Thermodynamic stability of hydrogen-bonded systems in polar and nonpolar environments.
abstract: The thermodynamic properties of a selected set of benchmark hydrogen-bonded systems (acetic acid dimer and the complexes of acetic acid with acetamide and methanol) was studied with the goal of obtaining detailed information on solvent effects on the hydrogen-bonded interactions using water, chloroform, and n-heptane as representatives for a wide range in the dielectric constant. Solvent effects were investigated using both explicit and implicit solvation models. For the explicit description of the solvent, molecular dynamics and Monte Carlo simulations in the isothermal-isobaric (NpT) ensemble combined with the free energy perturbation technique were performed to determine solvation free energies. Within the implicit solvation approach, the polarizable continuum model and the conductor-like screening model were applied. Combination of gas phase results with the results obtained from the different solvation models through an appropriate thermodynamic cycle allows estimation of complexation free energies, enthalpies, and the respective entropic contributions in solution. Owing to the strong solvation effects of water the cyclic acetic acid dimer is not stable in aqueous solution. In less polar solvents the double hydrogen bond structure of the acetic acid dimer remains stable. This finding is in agreement with previous theoretical and experimental results. A similar trend as for the acetic acid dimer is also observed for the acetamide complex. The methanol complex was found to be thermodynamically unstable in gas phase as well as in any of the three solvents.
authors.affiliation: Institute for Theoretical Chemistry, University of Vienna, Währinger Strasse 17, 1090 Vienna, Austria.
authors.forename: Hasan
authors.initials: H
authors.lastname: Pasalić
authors.forename: Adélia J A
authors.initials: AJ
authors.lastname: Aquino
authors.forename: Daniel
authors.initials: D
authors.lastname: Tunega
authors.forename: Georg
authors.initials: G
authors.lastname: Haberhauer
authors.forename: Martin H
authors.initials: MH
authors.lastname: Gerzabek
authors.forename: Herbert C
authors.initials: HC
authors.lastname: Georg
authors.forename: Tatiane F
authors.initials: TF
authors.lastname: Moraes
authors.forename: Kaline
authors.initials: K
authors.lastname: Coutinho
authors.forename: Sylvio
authors.initials: S
authors.lastname: Canuto
authors.forename: Hans
authors.initials: H
authors.lastname: Lischka
mesh_terms: D000081:Acetamides / Q000737:chemistry; D019342:Acetic Acid / Q000737:chemistry; D002725:Chloroform / Q000737:chemistry; D006536:Heptanes / Q000737:chemistry; D006860:Hydrogen Bonding; D000432:Methanol / Q000737:chemistry; D056004:Molecular Dynamics Simulation; D009010:Monte Carlo Method; D012997:Solvents / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D000081:Acetamides; D006536:Heptanes; D012997:Solvents; D014867:Water; C028618:n-heptane; D002725:Chloroform; C030686:acetamide; D019342:Acetic Acid; D000432:Methanol