Unusual Dimer Structures of the Heavier Alkaline Earth Dihalides:  A Density Functional Study

Abstract

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.

Group 12 Dihalides: Structural Predilections from Gases to Solids

“…We have found computationally in refs. [1] and [64] that, among the Group 2 binary dihalides, the rigid linear monomers (e.g., BeX 2 and MgX 2 ) strongly prefer the D 2h dimer geometry, while the monomers that are bent (e.g. BaF 2 and SrF 2 ) show a strong preference for the C 3v geometry.…”

High-Temperature Gas-Phase Electron Diffraction: Unexpected Dimer Structures Among Metal Halides

“…A recent study [43], however, found that this is true only in those cases where the monomers are linear. It is well known by now, that only the dihalides of the lighter alkaline earth metals are linear [1].…”

Electronic spectroscopy of the alkaline-earth halide cluster Ca2Cl3

“…4,5 There have since been numerous investigations concerning the factors governing the propensity for linear versus bent geometries. 3,[6][7][8] Polarization and d-hybridization appear to play important roles. The ''dimer'' species M 2 X 4 have been computed to have many isomers, generally with two or more halogen atoms bridging the metal centers.…”

“…The ''dimer'' species M 2 X 4 have been computed to have many isomers, generally with two or more halogen atoms bridging the metal centers. 6 Salt clusters with a closed shell ionic framework and outer shell electrons, such as Na 2 F are termed excesselectron clusters. These radial clusters are suitable for spectroscopic investigation, because they possess bound-tobound optical transitions.…”

“…6 They find that the lowest energy form of Ca 2 Cl 4 is a doubly bridged (D 2h ) structure with a planar four-member ring and two Cl Ϫ tail atoms. A triply bridged (C 3v ) isomer, similar to the D 3h structure presented in this work but with the additional Cl atom in a Ca-Cl tail bond, lies only 10 kJ/mol higher.…”