Knowledge Bank

The infrared spectra of $\mathrm{<mrow\; data-mjx-texclass="ORD">40</mrow>}CaF2,44CaF2,86SrF288SrF2$ and $BaF2$ trapped in solid krypton matrices at $20\circ K$ are reported. From precise measurements of the shifts of the vibrational modes on isotopic substitution upper and lower limits for the apex angles of these molecules are found to be $140\circ$ for $CaF2,108\circ$ for $SrF2$, and a reanalysis of the earlier data for $MgF2$ gives $185\circ$. For $BaF2$ the apex angle has been estimated at $100\circ$. Using a valence force field a set of force constants have been obtained from the measured fundamental frequencies, as well as approximate values for the anharmonic corrections to these vibrational modes. The statistical entropies calculated from the vibrational frequencies and molecular geometries are in excellent agreement with the thermodynamic measurements.