Moleuclar Orbitals

  • If you are a chemist you will have already met Atomic Orbitals AOs they are s, p and d. Electrons fill atomic orbitals in pairs. In molecules, we have Molecular Orbitals, which are filled by electron pairs. MOs are built up from the AOs of individual atoms, because the orbitals overlap they become smeared out and so MOs can look a bit strange if you haven't seen them before. MOs are the basis of modern bonding theories and understanding. MOs are something you will study at in your 2nd year of a chemistry degree.
  • Use your open the *.chk file (not the *.log file!) for final optimised NH3
  • Then from the "Tools" pulldown menu, choose "MOs"
  • A MO window should open. In the new window select the "Visualize" tab and then in the box "Add List" type "1a-6a", and click on Update. What will happen next is that gaussview will read information off the checkpoint file and covert this into the molecular orbitals 1-6 for you to visualise. Be patient, this requires quite a bit of number-crunching and may take up to 5min. The larger the number of MOs to compute the longer this will take.

    vlab_wiki_nh3_compute_mos

  • While you wait, we want to set the surfaces to appear as transparent, this makes the MOs easier to interpret as you can see the atomic nuclei. Click on the Gaussview tab (top-right) select "Preferences" a new window will open. In the new window select "Display Format" click on the "Surface" button and then select "Transparent" from the pulldown menu. Once done click the "OK" button.

    vlab_wiki_display_format

  • Once the MOs are computed you should step through each of the MOs from number 1 through to the lowest unoccupied MO (the first MO without electrons in it as we fill the MO levels up from the bottom).
    • the deep MOs determine the bonds that form in a molecule
    • the highest occupied MO and lowest unoccupied MO determine the reactivity
    • these two aspects mean that MOs are a central part of modern chemical understanding

    vlab_wiki_mos

  • You could also look at the higher energy MOs above the LUMO, however their shapes are not as reliable as those of the bonding MOs particularly the higher energy ones. When you get to your quantum mechanics course you will learn that the mathematics of the way we arrive at the un-occupied MOs is not as robust.

  • Congratulations you are now finished, with NH3!

  • Return to the outline to see what you could do next if you have a bit more time
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