Pseudorotation in PCl5
A classic problem in inorganic
chemistry is visualizing ligand positional changes through Berry pseudorotation.
In the expert builder, construct the standard trigonal bipyramidal structure
for PCl5. Your structure should have D3h symmetry. Now go to "constrain
angle" and constrain one equatorial Cl-P-Cl angle (which should be 120
degrees). Go to "Properties" and make this a dynamic constraint, with
11 steps and the angle varying from 120 degrees to 180 degrees. Set
this up for an "energy profile" calculation with PM3 theory and submit
the job, which should run quickly.
Open the ".profile" file corresponding to your job name. Animation should
show the ligand exchange process. For best visualization, use the "align"
function to align motion relative to two atoms of the P-Cl bond that
is not moving in the process. Use the spreadsheet to add relative energies
and then plot these. You should see a smooth curve with a barrier of
2.5 kcal/mol for ligand exchange.
Alternatively, create a model for PCl5 with C4v symmetry by choosing
a pentacoordinate phosphorous. Optimize this structure with PM3 theory
and calculate the vibrational frequencies. You should see a single imaginary
mode with a frequency of -53 cm-1. By symmetry, this corresponds to
the transition state for pseudorotation. Increase the amplitude to 2.5
- 3.0 Angstroms and make a "list". Visualizing the list will show how
ligands exchange in the pseudorotation process. Each end of the process
shows a trigonal bipyramid.