Alternative Structures for SF4
SF4 exhibits an unusual "see-saw" geometry.
This can be rationalized by VSEPR theory by first noting that the best way to arrange the sulfur's five electron pairs is in a trigonal bipyramid, and second by postulating that the non-bonded electron pair takes up more space than the bonded pairs. What VSEPR theory does not tell us is whether there are alternative "stable" structures for SF4, for example, trigonal-base-pyramid and square-planar structures.
Spartan may be used to calculate equilibrium structures for different SF4 geometries, and then to calculate vibrational frequencies. Energy minima have only "real" frequencies, meaning that the energy surface is curved upward in all directions, while the presence of one or more "imaginary" frequencies means that the energy surface is curved downward in one or more directions. Only energy minima can exist.
Build three different forms of SF4. Click on , and then on the Expert tab at the top of the model kit. Select S from the Periodic Table and trigonal bipyramid () from the list of hybrids. Click anywhere on screen. Click on and then click on one of the equatorial free valences to delete it. Switch to the entry model kit (click on the Entry tab at the top of the expert model kit) and add four fluorines to your structure. Click on .
Select New Molecule from the File menu and re-enter the expert model kit. Trigonal bipyramid sulfur is still selected. Click on screen. Click on and then click on one of the axial free valences. Enter the entry model kit and add four fluorines. Do not click on , as Spartan's minimizer will lead to a tetrahedral geometry.
Select New Molecule one last time and bring up the expert model kit. Select square planar () from the list of hybrids and click on screen. Bring up the entry model kit, add four fluorines and click on . Click on .
Scroll through the three structures using the the and keys at the bottom left of the screen. If all is in order, bring up the Calculations dialog (Setup menu) and specify calculation of equilibrium geometry using the Hartree-Fock 3-21G model. Check Freq. (frequency) to the right of "Compute". Click on Submit at the bottom of the dialog and name the job "sulfur tetrafluoride isomers".
After all three calculations have completed (several minutes), bring up the Vibrations dialog (Display menu), and look for imaginary frequencies.
You will see that only "see-saw" SF4 is an energy minimum. Trigonal-based pyramid and square planer structures have imaginary frequencies.
Repeat your calculations for XeF4, examining both the square-planar and "see-saw" structures.