Probing Ionic Crystals by the Invariom Approach: An Electron Density Study of Guanidinium Chloride and Carbonate

Abstract

A comparative study of two guanidinium salts, chloride and carbonate, is carried out to test the performance of the invariom approach for ionic crystals. Although treating them as formed by isolated ions with no charge transfer between them, the invariom approach provides features of interionic contacts that are amazingly similar to those obtained from conventional charge density analysis of high-resolution X-ray diffraction data, thus emerging as an easy way towards reliable description of chemical bonding peculiarities in ionic crystals.

Molecular Electrostatic Potentials from Invariom Point Charges

“…Af ull charget ransfer of one electron was appliedt oc ationic sunitinib and the malate anion by giving ad ifferent target value for the electroneutrality correction;t he chargew as spread between all atoms in both molecules, respectively.I th as been found that this rather crude approximation leads to surprisingly good results. [64] Equally important,f ull charge transfer in sunitinib is also supported by the experimental charge-density study, [77] in whicht he charges of the refinedm ultipoles suggest ac harget ransfer of 1.05 e between the two molecular ions and only an overall neutrality constraint wasu sed (a more detailed discussion is provided in the Supporting Information). We suspect that full charge transfer becomes less accurate in cases in which mobileh ydrogen atoms are absent or if anions such as I 3 À are involved, for which electronegativity differences between cation and polyanion become small.…”

“…We next compared the ESPs from aspherical invariom scattering factors, from the new invariom point charges, and from experimental least-squares adjusted multipole populations. [64] Equally important,f ull charge transfer in sunitinib is also supported by the experimental charge-density study, [77] in whicht he charges of the refinedm ultipoles suggest ac harget ransfer of 1.05 e between the two molecular ions and only an overall neutrality constraint wasu sed (a more detailed discussion is provided in the Supporting Information). [49] Both crystal structures contain severalm olecules in the asymmetricu nit, which can be identified by using the chargeassignment program;e ach was adjusted to be neutral.…”

“…Full charge transfer is ar ather crude approximation, but agrees with chemical intuition and has been shown to give surprisingly good results for organic molecules. [64] This is further investigated in Section 3.2.4. To obtain "true" molecular charges, studies that are more demanding are required.…”

“…For modeling ions, [64] charge transfer can be taken into account (which requires user input). Because it is difficult to know the "real" molecular charge when only molecular coordinates are available, ap ractical solution has to be provided.…”

New Polymorphs of the Phase-Change Material Sodium Acetate

“…Table 2 lists invariom scattering-factor names, the respective local-atomic site symmetry and the model compounds that provide the local chemical environment from which these scattering factors were computed. For the sodium ion, full charge transfer was assumed [17,18]. Invariom refinement relies on the Stewart-Hansen-Coppens multipole model [19,20] and can also be applied to disordered structures [21] like sodium acetate.…”