Discussion of the VSEPR model in Section 24.1 suggested a number of failures, in particular, in CaF₂ and SrCl₂, which (according to the VSEPR) should be linear but which are apparently bent, and in SeF^2-₆ and TeCl²⁻₆, which should not be octahedral but apparently are. Are these really failures or does the discrepancy lie with the fact that the experimental structures correspond to the solid rather than the gas phase (isolated molecules)?

a. Obtain equilibrium geometries for linear CaF₂ and SrCl₂ and also calculate vibrational frequencies (infrared spectra). Use the HF/3-21G model, which has actually proven to be quite successful in describing the structures of main-group inorganic molecules. Are the linear structures for CaF₂ and SrCl₂ actually energy minima? Elaborate. If one or both are not, repeat your optimization starting with a bent geometry.

b. Obtain equilibrium geometries for octahedral SeF²⁻₆ and TeCl²⁻₆ and also calculate vibrational frequencies. Use the HF/3-21G model. Are the octahedral structures for SeF²⁻₆ and TeCl²⁻₆ actually energy minima? Elaborate. If one or both are not, repeat your optimization starting with distorted structures (preferably with C₁ symmetry).