Ion-molecular Crystals as Inclusion Compounds: Van der Waals Contacts in Ionic Carbonates

Among Cambridge substances, which always include “organic”carbon, i.e., carbon contained in molecules or molecular ions,ion-molecular crystals (particularly, carbonates or alkaline-earthand alkaline metals) are of special interest. Some of such carbonatescontain pronounced Van der Waals (vdw) contacts between oxygenatoms of carbonate groups. Thus, agglomerates (layers or columns)formed by contacting carbonate groups arise (columns beingconnected in a frame), and in the cavities between these agglomeratescations are situated. The structure looks like an inclusion compound,the carbonate groups being hosts and the cations being guests. Todiscuss this question we used the refined value of vdw radius of oxygenwhich was obtained by statistical treatment of 8200 crystal structuresfrom Cambridge Structural Database (CSD) and is equal to 1.53 Å for the O?O contacts. Hence the length of a normal supporting vdw contact O?O is 3.06 ± 0.15 Å. Such contacts are present in MgCO₃ and CaCO₃ (calcite) and in α-Na₂CO₃. There are contacts O?O that are even shorter (2.74–2.81 Å)in CaCO₃ (aragonite), SrCO₃, BaCO₃ and Li₂CO₃.So it is possible to suppose the existence of specific (partly covalent)contacts O?O in these substances. In β-Na₂CO₃, all forms of K₂CO₃, Rb₂CO₃ and Cs₂CO3 the vdw contacts of CO₃ groups are absent; therefore they are ionic (not inclusion) compounds.