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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.


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