文摘
In this work, we have performed molecular dynamics (MD) simulations to compare the structural and dynamical properties of three ionic liquids (ILs), 1-ethyl-3-methyl-imidazolium tetrafluorborate ([EMI+][BF4–]), 1,1′-dimethyl-4,4′-bipyridinium bis(tetrafluorborate) ([VIO2+][BF4–]2), and 1,1′-dimethyl-4,4′-bipyridinium bis(trifluoromethylsulfonyl)imide (bistriflimide in short) ([VIO2+][Tf2N–]2), aiming to discover the influence of ion rigidity on the physical properties of ILs. [VIO2+] is more rigid than [EMI+], and [BF4–] is more rigid than [Tf2N–]. [VIO2+][BF4–]2 has an anion distribution different from the other two by the higher and sharper peaks in the cation–anion radial distribution functions, reflecting a close-packed local structure of anions around cations. [VIO2+][BF4–]2 and [VIO2+][Tf2N–]2 have similar dynamics much slower than [EMI+][BF4–], and [VIO2+][Tf2N–]2 shows a more isotropic molecular distribution than [VIO2+][BF4–]2 and [EMI+][BF4–]. Additionally, we have simulated two modified viologen-based ILs to reinforce our interpretations. We conclude from the above simulation results that the rigidity of anions influences the alignment of cations and that the rigidity of cations shows a large obstacle to their rotational capacity. Moreover, we have observed a slower diffusion of [VIO2+][BF4–]2 due to the electrostatic correlations, which stabilizes the ion-cage effect.
