Reorientational dynamics of the constituent ions in a room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), are explored via molecular dynamics simulations, and several features of orientation dynamics are summarized. The anion, [PF6]−, not only exhibits a higher propensity to orientation jumps than the cation, [BMIM]+ but also accesses a wider jump angle distribution and larger peak-angle. Jump and waiting time distributions for both the ions depict power-law dependences, suggesting temporally heterogeneous dynamics for the medium. This heterogeneity feature is further highlighted by the finding that the simulated first rank (ℓ = 1) and second rank (ℓ = 2) average reorientational correlation times reflect a severe break-down of Debye’s ℓ(ℓ + 1) law for orientational diffusion in an isotropic homogeneous medium. Simulated average H-bond lifetime resides between the mean orientation jump and waiting times, while the structural H-bond relaxation suggests, as in normal liquids, a pronounced presence of translational motion of the partnering ions. Average simulated jump trajectories reveal a strong rotation-translation coupling and indicate relatively larger changes in spatial and angular arrangements for the anion during an orientation jump. In fact, a closer inspection of all these results points toward more heterogeneous dynamics for [PF6]− than [BMIM]+. This is a new observation and may simply be linked to the ion-size. However, such a generalization warrants further study.
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Research Article| March 29 2018
Orientational dynamics in a room temperature ionic liquid: Are angular jumps predominant?
Special Collection: Chemical Physics of Ionic Liquids
Ranjit Biswas b)
Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, Block-JD, Sector-III, Salt Lake, Kolkata 700106,
Thematic Unit for Excellence–Computational Materials Science, S. N. Bose National Centre for Basic Sciences, Block-JD, Sector-III, Salt Lake, Kolkata 700106,
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Suman Das, Biswaroop Mukherjee, Ranjit Biswas; Orientational dynamics in a room temperature ionic liquid: Are angular jumps predominant?. J. Chem. Phys. 21 May 2018; 148 (19): 193839. https://doi.org/10.1063/1.5017797
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