The molecular structure of the solvent and the atomic charges for (a) full charge model, IL1 and (b) zero net charge (neutral), IL0 models, respectively. (c) System size effect on the benzene rotational time correlation function, C 2R(t) obtained by changing the system size at a constant benzene concentration; 8 benzenes in 248 ion pairs (green), 27 benzenes in 6696 ion pairs (blue) and 256 benzenes in 15872 ion pairs (purple), respectively.
The rotational time correlation function C 2R(t) using the IL0.5 model: (a) C–H bond of benzene, (b) O–H bond of water, and (c) the long-axis of the cation. The straight lines are exponential fits for the diffusive part of the long-time tail of C 2R(t).
The relaxation time constant for the cation rotation, τ IL, plotted against (a) reciprocal temperature 1000/T (323 ≤T/K< 750) and (b) reciprocal scaled temperature 1000/T scaled for the four charge models, IL1 (blue), IL0.7 (green), IL0.5 (orange) and IL0 (red), respectively. The scaled temperature T scaled was defined as T, T/1.2, T/1.5, T/1.9 for the IL1, IL0.7, IL0.5, and IL0 models, respectively.
Solvent-solvent (a) and solute-solvent (b) radial distribution functions at 400 K. (a) The four charge models are compared in each panel for the cation-cation, cation-anion, anion-anion radial distribution functions. (b) The four charge models are compared in a single panel. The two panels are for benzene-anion and benzene-cation radial distribution functions, respectively. The cation center was chosen as the midway between the two nitrogen atoms of the ring. The benzene center was chosen as the center of mass.
(a) Benzene and (b) water total spectral density function, j(f)total and the relative contribution, α(f) of the fast part, j(f)fast to the total, j(f)total from 323 to 373 K using the IL0.5 model. The two vertical dashed lines indicate the deuterium Larmor frequencies f L 600 and 2f L 600.
Comparison of computational and experimental values for T 1: (a) T 1 600 (filled circles) and T 1 400 (open circles) of benzene as a function of temperature for each of the charge models, IL1, IL0.7, IL0.5 along with the experimental values taken from Ref. 15. (b) The ratio, R of T 1 600 to T 1 400 for benzene as a function of temperature. (c) Water 2H NMR T 1 600 (filled circles), T 1 400 (open circles), and R = T 1 600/T 1 400 (cross symbols) as a function of temperature using the IL0.5 model compared with experiments.
(a) The total spectral density function, j(f)total (b) its frequency width, Δf and (c) the relative contribution, α(f) of the fast part, j(f)fast to the total, j(f)total from 323 to 500 K in the IL0.7 model. The two vertical dashed lines in panels (a) and (c) and the horizontal dashed line in panel (b) indicate the deuterium Larmor frequencies f L 400 and f L 600. In panel (b), Δf at temperatures above 400 K are out of range: Δf = 4.2, 27, and 43 GHz at 420, 450, and 500 K, respectively.
The effect of the cation atomic charges on the (a) benzene and (b) cation rotational time correlation function, C 2R(t) for the vector connecting the C6 and C10 atoms and the relaxation time, τ IL. The blue curve indicates the model by Lopes et al. (IL1 model) and the thick red curve is for an artificial model where each of the 25 atoms of the imidazolium cation has a charge of −0.04e (IL1eq model). For comparison the results for the IL0.5 model are shown by the thin orange curve.
Values of τ benzene and τ water calculated from the MD simulations compared with those reported in our previous experimental study of Ref. 15. For the computational τ benzene and τ water, they were determined by fitting the diffusive part of the long-time tail of the rotational time correlation function C 2R(t) with an exponential function aexp(−t/τ benzene) using the IL0.5 model. For the values taken from Ref. 15, they were reported as “τ 2R” for each solute in the original paper and obtained by analyzing the experimental T 1 at two frequencies based on the Debye model.
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