Establishing Consistent van der Waals Volumes of Polyatomic Ions from Crystal Structures

Based on temperature (T) dependent crystal structure data of seven organic salts, a radii-based scheme for the calculation of the van der Waals volume (V-vdw) is analyzed. The obtained volumes (V-vdw,V-r, r=radius-based) are nearly T independent. An ion volume partitioning scheme is proposed by fixing the anion volumes of [Cl](-), [Br](-), [I](-), [BF4](-), [PF6](-), [OTf](-) and [NTf2](-). The van der Waals volumes (V-vdw,r(+/-)) of 48 ions are established, with low standard deviations (0.2-3.6 angstrom(3), 0.1-4.5% of V-vdw,r(+/-)). The ion volumes are independent of the counterion and one crystal structure already suffices for their derivation. Correlations of the viscosity with V-vdw,V-r via a Litovitz ansatz and our recently derived Arrhenius-type approach prove that these volumes are suitable for the volume-based description and prediction of IL properties. The corresponding correlation coefficient for the latter is R-2=0.86 for 40 ILs (354 data points) in the T range of 253-373 K.