Rotational Diffusion of
Nonpolar and Ionic Solutes
in 1‑Alkyl-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imides:
Is Solute Rotation Always Influenced by the Length of the Alkyl Chain
on the Imidazolium Cation?
posted on 2016-02-20, 07:57authored byV. Gangamallaiah, G. B. Dutt
In an attempt to find out whether the length of the alkyl
chain
on the imidazolium cation has a bearing on solute rotation, temperature-dependent
fluorescence anisotropies of three structurally similar solutes have
been measured in a series of 1-alkyl-3-methylimidazolium (alkyl =
methyl, ethyl, propyl, butyl, and hexyl) bis(trifluoromethylsulfonyl)imides.
Solute–solvent coupling constants obtained from the experimentally
measured reorientation times with the aid of Stokes–Einstein–Debye
hydrodynamic theory indicate that there is no influence of the length
of the alkyl chain on the rotation of nonpolar, anionic, and cationic
solutes 9-phenylanthracene (9-PA), fluorescein (FL), and rhodamine
110 (R110), respectively. It has also been noticed that the rotational
diffusion of 9-PA is closer to the predictions of slip hydrodynamics,
whereas the rotation of negatively charged FL and positively charged
R110 is almost identical and follows stick hydrodynamics in these
ionic liquids. Despite having similar shape and size, ionic solutes
rotate slower by a factor of 3–4 compared to the nonpolar solute.
Interplay of specific and electrostatic interactions between FL and
the imidazolium cation of the ionic liquids, and between R110 and
the bis(trifluoromethylsulfonyl)imide anion, appear to be responsible
for the observed behavior. These results are an indication that the
length of the alkyl chain on the imidazolium cation does not alter
their physical properties in a manner that has an effect on solute
rotation.