1: Boron Based Acids

G: Real Molecular Orbitals of BH3

Now we are going to generate the delocalised MOs for BH3, these are the quantitative or computed MOs which we can compare to the qualitative or approximate MOs produced via the MO diagram.

Download and print out this tutorial from 2nd year to remind you how to create the MO diagram for BH3, pdf. Create your own MO diagram of trigonal planar D3h BH3 in chemdraw, and include it in your presentation.

  • From the main menu along the top of the screen (in gaussview) choose "File" and then choose "open", a new window will open with available files. Before selecting a file first check that under "File type" the pull-down menu has "Gaussian Checkpoint Files (*.chk) selected. Navigate to the directory with your checkpoint files in it, this may be in the SCRATCH directory in the G03 directory ... if you can't find it ask a demonstrator.
  • open the checkpoint file associated with your frequency analysis job, it should be called "yourname_bh3_freq.chk" if you followed the instructions properly. Click on the "open" button. The checkpoint file contains a very large amount of information and is simply too large to be stored in a human readable format, however gaussview can read these files and interpret the information for us.
  • a new molecule window should open with your molecule in it
  • From the main menu along the top of the screen (in gaussview) choose "Edit" and then choose "MOs":

  • A new orbital window will open. At this point there won't be any surfaces (ie blue and orange surfaces in the diagram below) just your molecule in the preview part of the window:

  • In the orbital window
    • click on the "Visualise" Tab (1.)
    • under the "HOMO" pulldown menu pick "Occupied" (2.)
    • then click on the button "Update ..." (3.)
    • Now you need to wait, this may take some time depending on the power of the computer you are using. Eventually the program will update, and a surface will apear around your molecule. If nothing appears within 10 minuets get a demonstator to help you.
    • The right hand part of the orbital window contains the various MO energy levels, those with boxes have been computed, and the red filled box shows the MO which is currently pictured.
    • click on a grey box to move the red counter, and look at the new orbital that is displayed
  • work your way through all the avaliable orbitals
  • More MOs can be computed, for example, choose "Current List" in the pull down menu and then type "5-7" in the text box. Then click on the "Update" button ... and wait while the new MOs are generated.
  • You will notice that the unoccupied orbitals are more diffuse than the occupied ones, and that their form can be quite odd (eg MO 6) because of this we generally only look at the occupied and lowest energy unoccupied orbitals.
  • Using the screen capture application take snap-shots of the occupied and LUMO orbitals that feature in your MO diagram.
  • Import both the chemdraw diagram and your MOs into your presentation and display the predicted qualitative MOs next to the quantitative MOs. Are there any significant differences? What does this say about the accuracy and usefulness of qualitative MO theory? Comment on these questions in your presentation.
  • When you are ready close the molecule window move onto the next step
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