title: Raman spectroscopic study on the hydration structures of tetraethylammonium cation in water.
abstract: Conformational changes of tetraethylammonium ion (Et4N+) in aqueous solution have been studied by Raman spectroscopy as functions of pressure and concentration. The difference in the partial molar volume (Delta V(tg x tg-->tt x tt)) between the trans-gauche x trans-gauche (tg x tg) and trans-trans x trans-trans (tt x tt) conformers of Et4N+ ion has been calculated from the pressure dependence of the relative Raman intensity ratio between the conformers. We discuss about the Delta V(tg x tg-->tt x tt) with two contributions from the molecular and hydration volumes. Delta V(tg x tg-->tt x tt) is found to be negative, and this is mainly due to the large molecular volume contribution. The value of Delta V(tg x tg-->tt x tt) becomes smaller with increasing R probably due to the hydration volume contribution. In view of the pressure and concentration dependences, the water molecules around Et4N+ ions at R = 19 (R= moles of water/moles of salt) mainly form the hydrophobic hydration. The hydrophobic hydration may prefer the tt x tt conformer to the tg x tg conformer. On the other hand, the hydration structure at R = 4 no longer forms the hydrophobic hydration and changes to another type of hydration, where the medium H-bond component is the dominant species. The increase of the ion-ion interactions with decreasing R induces large fractions of the tg x tg conformer.
authors.affiliation: Department of Applied Chemistry, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan. take214@nda.ac.jp
authors.forename: Takahiro
authors.initials: T
authors.lastname: Takekiyo
authors.forename: Yukihiro
authors.initials: Y
authors.lastname: Yoshimura
mesh_terms: D002412:Cations / Q000737:chemistry; D008968:Molecular Conformation; D013059:Spectrum Analysis, Raman; D019789:Tetraethylammonium / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
chemical_list: D002412:Cations; D014867:Water; D019789:Tetraethylammonium