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J Chem Phys. 2006 Dec 28;125(24):244506.

Simulation studies of ionic liquids: orientational correlations and static dielectric properties.

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Department of Computational Biological Chemistry, University of Vienna, A-1090 Vienna, Austria.


The ionic liquids BMIM+I-, BMIM+BF4-, and BMIM+PF6- were simulated by means of the molecular dynamics method over a time period of more than 100 ns. Besides the common structural analysis, e.g., radial distribution functions and three dimensional occupancy plots, a more sophisticated orientational analysis was performed. The angular correlation functions g(00)110(r) and g(00)101(r) are the first distance dependent coefficients of the pairwise orientational distribution function g(rij,Omega1,Omega2,Omega12). These functions help to interpret the three dimensional plot and reveal interesting insights into the local structure of the analyzed ionic liquids. Furthermore, the collective network of ionic liquids can be characterized by the Kirkwood factor Gkappa(r) [J. Chem. Phys. 7, 911 (1939)]. The short-range behavior (r<10 A) of this factor may be suitable to predict the water miscibility of the ionic liquid. The long-range limit of Gkinfinity is below 1 which demonstrates the strongly coupled nature of the ionic liquid networks. In addition, this factor relates the orientational structure and the dielectric properties of the ionic liquids. The static dielectric constant epsilon(omega=0) for the simulated system is 8.9-9.5. Since in ionic liquids the very same molecule contributes to the total dipole moment as well as carries a net charge, a small, but significant contribution of the cross term between the total dipole moment and the electric current to epsilon(omega=0) is observed.

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