Structure organization in liquid and gas phase 6



Is the structural feature of the solid phase maintained in the liquid phase?

Several indications indicate that indeed 1,3-dialkylimidazolium possess analogous structural patterns in both the solid and liquid phase and to some extent even in the gas phase.
Note that although significant randomness in organization is necessary to describe the structure of a liquid, in most of the cases there is only 10-15% volume expansion on going from the crystalline to the liquid state and the ion-ion or atom-atom distances are similar in both the solid and liquid states. Furthermore, while long-range order is lost on going from the crystal to the liquid, similarities remain as a consequence of the Coulombic forces between cations and anions of the imidazolium ionic liquids. It is clear that the long-range Coulomb interactions in ionic organic liquids can lead to longer spatial correlations than those in comparable van der Waals organic liquids.
The intrinsic strength of the hydrogen bonds between imidazolium cation and anions follows the order CF3CO2 ->BF4 ->PF6 ->BPh4 -, as showed by IR and NMR Spectrometry studies.

    This structural pattern is confirmed by several studies:6,21
  • IR Spectrometry
  • NMR Spectrometry
  • H,H-NOESY experiments
  • Recoil Spectrometry
  • Neutron Diffraction Analysis
  • X-Ray and Neutron Scattering studies
  • Raman Spectrometry
  • Optical Heterodyne-Detected Optical Kerr Effect (OHD-OKE)
  • Mass Spectrometry

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Figure 4. Infrared spectra of some 1-n-butyl-3-methylimidazolium

IR Spectrometry

The IR spectra of these materials in both the solid and liquid phase are almost identical and show the characteristic C-H…..X hydrogen bonding bands in the region 3100-3200 cm-1 pertaining to the interaction of H2, H4 H5 of the imidazolium cations with the anions (Figure 4).

NMR Spectrometry 

The hydrogen bonding system between the cation and anion has been also evidenced by NMR studies of pure liquid organo-aluminate imidazolium molten salts. The imidazolium H chemical shifts change with the concentration of AlCl3 (Lewis acidity of the liquid).







Figure 4.
Infrared spectra of some 1-n-butyl-3-methylimidazolium ionic liquids showing the C-H…F stretching frequencies (3100- 3200 cm-1).

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