Unusual dimer structures of the heavier alkaline earth dihalides: A density functional study

Density functional theory has been applied to investigate the monomeric and dimeric dihalides of the heavier alkaline earth metals. Quasirelativistic pseudopotentials and large basis sets with uncontracted d (and f) polarizat ion functions on the metals and correlation-consistent all-electron basis s ets on the halogens were utilized. The monomers of SrF2, BaF2, and BaCl2 we re found to be genuinely bent, while CaF2 and SrCl2, although also bent, ha ve extremely flat potential energy surfaces and are better described as qua silinear. The dimers of the heavier alkaline earth difluorides and dichlori des, Ca2F4, Ca2C4, Sr2F4, Sr2Cl4, Ba2F4, and Ba2Cl4, were investigated in g reat detail. Six different isomers were calculated for the strontium and ba rium dihalide dimers. The typical D-2h symmetry halogen-bridged structure i s the most stable only for the dimers of the lighter dihalides, and it is n ot a stable structure for the heavier dimers. For these molecules, a triple -bridged C-3v symmetry structure is the most stable and even other isomers with pyramidal coordination of the metal were found to be stable, although with higher energy. There appears to be a correlation between the monomer a nd the dimer structures for alkaline earth dihalides; for the linear halide s, the metal tends to have planar, while for the bent ones, pyramidal coord ination in their most stable dimer structure. Not only have our calculation s extended information on this class of compounds but they have also consid erably improved the agreement between the calculated and the available expe rimental data.