THE QUESTION OF BENDING OF THE ALKALINE-EARTH DIHALIDES MX2 (M = BE, MG, CA, SR, BA, X = F, CL, BR, I) - AN ABINITIO PSEUDOPOTENTIAL STUDY

The geometries of the monomeric alkaline earth dihalides MX2 (M = Be, Mg, Ca, Sr, Ba; X = F, Cl, Br, I) have been obtained from ab initio pseudopotential calculations at the HF and (for M = Ca, Sr, Ba) SDCI levels. In most cases the calculated MX distances agree well with experimental data. The angles agree with experimentally observed trends and give the first reliable values for several of these molecules. Relatively large bending energies indicate BaF2, BaCl2, BaBr2, and SrF2 definitely to be nonlinear. At higher computational levels, BaI2, SrCl2, and CaF2 also are found to favor bent minima but have very flat potential energy surfaces with regard to the bending motions. SrBr2 is borderline, while CaCl2, CaBr2, CaI2, and SrI2 as well as the magnesium and beryllium dihalides are linear. The calculated harmonic vibrational frequencies and force constants confirm these trends and are compared critically to experimental IR and Raman data, where available. Previously uncertain or unknown values are predicted. Bending force constants obtained from a polarized ion model correlate to some extent with the ab initio bending force constants, but there are systematic deviations. These point to covalent bonding contributions, i.e. some p-orbital occupancy for Be and Mg and some d-orbital involvement for Ca, Sr, and Ba. Walsh-type diagrams employing Davidson's "internally consistent" MOs indicate the stabilization of the HOMO upon bending to varying degrees to be an inherent feature of ionic Ca, Sr, and Ba MX2 compounds. It is concluded that both d-orbital participation and core-polarization make major contributions to the bending.