Sugosh Prabhu - Rotational Diffusion of Nondipolar and Charged Solutes in Alkyl-Substituted Imidazolium Triflimides: Effect of C2 Methylation on Solute Rotation

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      Publication Details (including relevant citation   information):

      J. Phys. Chem. B, 2014, 118 (31), pp 9420–9426

      DOI: 10.1021/jp5055155

      Publication Date (Web): July 8, 2014


      Rotational diffusion of a nondipolar solute   2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole   (DMDPP) and a charged solute rhodamine 110 (R110) has been   investigated in 1-butyl-3-methylimidazolium   bis(trifluoromethylsulfonyl)imide ([BMIM][Tf2N]) and   1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide   ([BMMIM][Tf2N]) to understand the influence of the C2   methylation on solute rotation. The measured reorientation times   of the nondipolar solute DMDPP are similar in both the ionic   liquids and follow Stokes–Einstein–Debye hydrodynamic theory with   slip hydrodynamics. In contrast, rotational diffusion of the   charged solute R110 in [BMIM][Tf2N] obeys stick   hydrodynamics due to specific interactions with the anion of the   ionic liquid. Nevertheless, the intriguing result of this study   is that the reorientation times of R110 in   [BMMIM][Tf2N] deviate significantly from the   predictions of stick hydrodynamics, especially at ambient   temperatures. The solute–solvent boundary condition parameter   Cobs, which is defined as the ratio of the   measured reorientation time to the one calculated using the SED   theory with stick boundary condition, for R110 is lower by a   factor of 2 in [BMMIM][Tf2N] compared to   [BMIM][Tf2N] at 298 K. Upon increasing the   temperature, Cobs gradually increases and   eventually matches with that obtained in [BMIM][Tf2N]   at 348 K. It has been well established that methylation of the C2   position in [BMMIM][Tf2N] switches off the main   hydrogen-bonding interaction between the anion and the cation,   but increases the Coulombic interactions. As a consequence of the   enhanced interionic interactions between the cation and anion of   the ionic liquid, specific interactions between R110 and   [Tf2N] diminish leading to the faster rotation of the   solute. However, such an influence is not apparent in case of   DMDPP as it does not experience specific interactions with either   the cation or the anion of these ionic liquids.

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