Ab initio model potential study of the equilibrium geometry of alkaline earth dihalides: MX2 (M=Mg, Ca, Sr, Ba; X=F, Cl, Br, I)

The ground state equilibrium geometry of alkaline earth dihalides MX₂ (M=Mg, Ca, Sr, Ba; X=F, Cl, Br, I) has been optimized at the Hartree-Fock (HF) level using the Cowan-Griffin relativistic ab initio model potential method and a uniformly good, extended, spd valence basis set. The results show that, according to the method, all magnesium dihalides and CaCl₂, CaBr₂, and CaI₂ are linear, SrF₂ and all barium dihalides are bent, and CaF₂ and SrCl₂, SrBr₂, and SrI₂ are quasilinear molecules. The alkaline earth (n-1)d orbitals are shown to be responsible for the bending of the heavier molecules while their (n-1)p orbitals contribute considerably to the final quantitative prediction of the apex angle and the relative stability of the bent structures. Relativistic effects are shown to be very small on the bond distances and vibrational frequencies; they are important on the size of the bending barrier of the bent molecules. The results obtained are compared to previous theoretical studies and provide some insight in the interpretation of the contradictory experimental conclusions.