Solid Memory:  Structural Preferences in Group 2 Dihalide Monomers, Dimers, and Solids

The link between structural preferences in the monomers, dimers, and extended solid-state structures of the group 2 dihalides (MX₂:  M = Be, Mg, Ca, Sr, Ba and X = F, Cl, Br, I) is examined theoretically. The question posed is how well are geometric properties of the gas-phase MX₂ monomers and lower order oligomers “remembered” in the corresponding MX₂ solids. Significant links between the bending in the MX₂ monomers and the D_2h/C_3v M₂X₄ dimer structures are identified. At the B3LYP computational level, the monomers that are bent prefer the C_3v triply bridged geometry, while the rigid linear molecules prefer a D_2h doubly bridged structure. Quasilinear or floppy monomers show, in general, only a weak preference for either the D_2h or the C_3v dimer structure. A frontier orbital perspective, looking at the interaction of monomer units as led by a donor−acceptor interaction, proves to be a useful way to think about the monomer−oligomer relationships. There is also a relationship between the structural trends in these two (MX₂ and M₂X₄) series of molecular structures and the prevalent structure types in the group 2 dihalide solids. The most bent monomers condense to form the high coordination number fluorite and PbCl₂ structure types. The rigidly linear monomers condense to form extended solids with low coordination numbers, 4 or 6. The reasons for these correlations are explored.