Theoretical study of the conformational equilibrium of 1,4-dioxane in gas phase, neat liquid, and dilute aqueous solutions
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The conformational equilibrium of 1,4-dioxane in the gas phase, in the pure liquid, and in aqueous solution has been studied by means of the Average Solvent Electrostatic Potential from Molecular Dynamics (ASEP/MD) method and the Integral Equation Formalism for the Polarizable Continuum Model (IEF-PCM). The dioxane molecule was described at the DFT(B3LYP)/aug-cc-pVTZ level. In the three phases, the equilibrium is almost completely shifted toward the chair conformer, with populations of the twist-boat conformers lower than 0.01 %. The equilibrium is dominated by the internal energy of the molecule, as the solute–solvent interaction free energies are very similar in the three conformers considered (chair, 1,4 twist-boat, and 2,5 twist-boat). In the pure liquid, where the dioxane–dioxane interaction is dominated by the Lennard-Jones term, the structure is characteristic of a van der Waals liquid. However, the decrease in the C–H distance from gas phase to solution, the increase in the C–H vibrational frequencies, and the presence of a shoulder in the O–Haxial pair radial distribution function point to the presence of a weak C–H–O hydrogen bond. The analysis of the occupancy maps of water oxygen and hydrogen atoms around the 1,4-dioxane molecule confirms this conclusion. Contrary to what is found in small water-dioxane clusters, in the liquid, there is a preference for oxygen atoms to interact with axial hydrogen atoms to form C–H–O hydrogen bonds. Comparison of ASEP/MD and IEF-PCM results indicates that including specific interactions is very important for an adequate description of the solute–solvent interaction; however, the influence of these interactions does not translate in changes in the relative stability of the conformers because it cancels out when energy differences are calculated.
Keywords1,4-Dioxane ASEP/MD QM/MM Conformational equilibrium Solvent effect
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