Prabhu Sugosh R, Dutt G B
Radiation & Photochemistry Division, Bhabha Atomic Research Centre , Trombay, Mumbai 400 085, India.
J Phys Chem B. 2014 Aug 7;118(31):9420-6. doi: 10.1021/jp5055155. Epub 2014 Jul 23.
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.
为了了解C2甲基化对溶质旋转的影响，研究了非偶极溶质2,5-二甲基-1,4-二氧代-3,6-二苯基吡咯并[3,4-c]吡咯（DMDPP）和带电溶质罗丹明110（R110）在1-丁基-3-甲基咪唑双（三氟甲基磺酰）亚胺（[BMIM][Tf2N]）和1-丁基-2,3-二甲基咪唑双（三氟甲基磺酰）亚胺（[BMMIM][Tf2N]）中的旋转扩散。在这两种离子液体中，测得的非偶极溶质DMDPP的重取向时间相似，并且遵循具有滑移流体动力学的斯托克斯-爱因斯坦-德拜流体动力学理论。相比之下，由于与离子液体阴离子的特定相互作用，带电溶质R110在[BMIM][Tf2N]中的旋转扩散遵循粘性流体动力学。然而，这项研究的有趣结果是，R110在[BMMIM][Tf2N]中的重取向时间与粘性流体动力学的预测有显著偏差，尤其是在环境温度下。溶质-溶剂边界条件参数Cobs定义为测量的重取向时间与使用具有粘性边界条件的SED理论计算的重取向时间之比，在298K时，R110在[BMMIM][Tf2N]中的Cobs比在[BMIM][Tf2N]中低2倍。随着温度升高，Cobs逐渐增加，最终在348K时与在[BMIM][Tf2N]中获得的值匹配。已经确定，[BMMIM][Tf2N]中C2位置的甲基化关闭了阴离子与阳离子之间的主要氢键相互作用，但增加了库仑相互作用。由于离子液体阳离子与阴离子之间增强的离子间相互作用，R110与[Tf2N]之间的特定相互作用减弱，导致溶质旋转更快。然而，对于DMDPP来说，这种影响并不明显，因为它与这些离子液体的阳离子或阴离子都没有特定的相互作用。
