Temperature Dependent Crystal Structure Analyses and Ion Volume Determinations of Organic Salts

Abstract

The crystal structures of nine organic salts of which seven are ionic liquids (ILs), comprising imidazolium, ammonium, piperidinium, and sulfonium cations, were determined. The occurrence of short contacts in the compounds' crystal structures was quantified with Hirshfeld surface analyses. Temperature dependent crystal structure data for several organic salts were recorded systematically in the temperature range from 100 K to room temperature. The thermal behavior of the cell constants was linear and anisotropic for almost all compounds. The accuracy of the molecular volume (Vm) – the key quantity in volume based thermodynamics (VBT) – was investigated in terms of its temperature dependency and the statistical error of the constituent ion volumes (Vm+/–). The former effect turned out to be at most on the same order of magnitude as the errors of Vm+/–. The same analysis was applied to the recently proposed new internally consistent molecular ion volumes (Vm, new+/–). For this purpose, a set of overall 40 Vm, new+/– values of typical polyatomic ions constituting ILs was established. In general, it is recommended to use as many salts as possible and as many temperature dependent data as available for the derivation of new Vm+/– and Vm, new+/– values.

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

“…Certainly, there is a volume overlap due to the existence of short contacts. However, these are slightly below the sum of the van der Waals radii (< 0.5 in the presented compounds [18] ). Therefore, the volume overlap is rather low and below the standard deviations as given in Table 2.…”

“…[51] The standard deviations of V vdw,r + /À are highly reduced (<4.5 %) compared to V m + /À and V m,new + /À . [18] The average standard deviation is " s = 1.2 3 , corresponding to " s perc = 1.1 %. This is contrasted against " s perc (V m +/À ) = 11 % and " s perc (V m,new +/À ) = 5 % of typical ILs constituting polyatomic ions [18] or s perc (V m [3] Thus, the ions' van der Waals volumes do not suffer from statistical average over different crystal structures or error propagation.…”

“…[19] As an outcome of this study it was suggested to average over several crystal structures to account for temperature and statistical effects to obtain representative V m + /À , V m,new + /À values. [18] Furthermore, it was shown by exemplary standard deviations (s) of V m + /À that V m,new + /À can result in large errors of the predicted viscosity (h). + /À could reduce Dh, s perc was still significant (24-29 % of h exp ).…”

“…[17] Recently, we analyzed the accuracy V m with respect to the temperature (T) dependency of V cell and the statistical error of the constituent ions (V m + /À , V m,new + /À ). [18] V m,new + /À are the newly derived internally consistent volumes. [19] As an outcome of this study it was suggested to average over several crystal structures to account for temperature and statistical effects to obtain representative V m + /À , V m,new + /À values.…”

“…Newly obtained crystal structures and the thermal expansion of all compounds have been reported before. [18] This manuscript is organized as follows: First, a radii-based approach for the calculation of V vdw (hereafter V vdw,r ) of organic salts from a given crystal structure is introduced. An ion volume partitioning scheme for V vdw,r is presented in the next section and various ion van der Waals volumes (V vdw,r + /À ) of ions typically constituting an IL are established.…”