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) polarization functions on the metals and correlation-consistent all-electron basis sets on the halogens were utilized. The monomers of SrF2, BaF2, and BaCl2 were found to be genuinely bent, while CaF2 and SrCl2, although also bent, have extremely flat potential energy surfaces and are better described as quasilinear. The dimers of the heavier alkaline earth difluorides and dichlorides, Ca2F4, Ca2Cl4, Sr2F4, Sr2Cl4, Ba2F4, and Ba2Cl4, were investigated in great detail. Six different isomers were calculated for the strontium and barium dihalide dimers. The typical D 2 h symmetry halogen-bridged structure is the most stable only for the dimers of the lighter dihalides, and it is not a stable structure for the heavier dimers. For these molecules, a triple-bridged C 3 v 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 and the dimer structures for alkaline earth dihalides; for the linear halides, the metal tends to have planar, while for the bent ones, pyramidal coordination in their most stable dimer structure. Not only have our calculations extended information on this class of compounds but they have also considerably improved the agreement between the calculated and the available experimental data.

The molecular structures of SnCl 2 and Sn 2 Cl 4 have been calculated by high-level quantum chemical methods. The global minimum structure found for the tin dichloride dimer is of C s symmetry and has two halogen bridges. This structure is very different from the one suggested earlier by Fields et al. based on their gasphase Raman spectroscopic measurements. Thermodynamic functions for gaseous SnCl 2 and Sn 2 Cl 4 have also been calculated. The minimum-energy geometry of the trimeric molecule, Sn 3 Cl 6 , has a six-membered ring with alternating Sn and Cl atoms in the highly puckered ring. The possible structural changes during evaporation are discussed in some detail.