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 (MX(2): 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 MX(2) monomers and lower order oligomers "remembered" in the corresponding MX(2) solids. Significant links between the bending in the MX(2) monomers and the D(2)(h)()/C(3)(v)() M(2)X(4) dimer structures are identified. At the B3LYP computational level, the monomers that are bent prefer the C(3)(v)() triply bridged geometry, while the rigid linear molecules prefer a D(2)(h)() doubly bridged structure. Quasilinear or floppy monomers show, in general, only a weak preference for either the D(2)(h)() or the C(3)(v)() 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 (MX(2) and M(2)X(4)) 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 PbCl(2) 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.