Created by Margret J. Geselbracht, Reed College (mgeselbr@reed.edu) and posted on VIPEr
(www.ionicviper.org) on May 31, 2009. Copyright Margret J. Geselbracht 2009. This work is licensed
under the Creative Commons Attribution Non-commercial Share Alike License. To view a copy of this
license visit http://creativecommons.org/about/license/.
MOLECULAR ORBITALS AND JAHN-TELLER DISTORTION OF THE XENON TRIFLUORIDE ANION
1. The molecular anion [XeF3]– has been observed in the gas phase in mass spectrometry
experiments, although it has never been isolated in the condensed phase nor structurally
characterized. Recently, computational studies were carried out to predict the structure and
understand the bonding in [XeF3]–. The researchers examined three possible structures,
shown below, one with D3h symmetry and two different distorted models, a T-shaped
molecule (predicted from VSEPR) and the Y-shaped molecule.
F
F
distort
Xe
Xe
F
F
F
F
F
F
Xe F
Xe
F
F
distort
F
T-shaped molecule
Xe
F
F
F
Y-shaped molecule
(a) Start by drawing the -only MO diagram for the undistorted D3h version of [XeF3]–. Only
consider the use of the valence ns and np atomic orbitals on Xe: No d orbitals allowed!
Determine the representation for -bonding, reduce it, draw the well-labeled MO
diagram, fill in the right number of electrons and draw cartoons of all the MOs.
(b) What would be the formal bond order in the [XeF3]– anion?
(c) Now consider the effects of each distortion. Starting with the MOs you have drawn for
the symmetric molecule, draw a correlation diagram to show any energy changes that you
would expect for each molecular orbital, labeling the MOs in the new point group of the
distorted molecule. In other words, do the MOs go down in energy, go up in energy, or
stay the same after the distortion has occurred? Briefly explain the origin of any energy
changes you show. Notice that in the distorted molecules, there are unequal Xe–F bond
lengths. You should do this for each of the two distorted structures.
(d) The D3h triplet state was found to be 60-110 kJ/mol higher in energy than the ground state
structure, which turned out to be the Y-shaped molecule. (The range in energies
depended on the details of the atomic orbital basis sets chosen for the Xe and F atoms.)
Can you explain why there would be such a significant gain in energy upon distortion for
this molecule? If you can, please do so. 