Aromaticity

You are all very familar with benzene as an aromatic compound. But how does this relate to the MO picture of molcular bonding? What happens when we exchange some of the "C-H" units by isoelectronic fragments containing main group elements. In this project you should investigate the conformers, vibrations and MOs of benzene and its analogues.

Important For the projects just presenting information is not sufficient, you must interpret your results. (After all this is the key component in all research projects!). The calculations for this project are straight forward, and thus most of the assigned marks are for interpretation.

  • Use the full basis set 6-31G(d,p) on all atoms
  • optimise, and carry out a frequency analysis to confirm you have a minima of a molecule of benzene. Include relevent key informtion in your wiki. Then compute the MOs and carry out an NBO charge analysis (be sure you have NBO charges, not Mulliken!). Benzene is going to be your "reference" molecule for comparison.
  • Draw the "center" part of a MO diagram for benzene, include both sigma and pi orbitals. Use your MO calculation! Don't try to construct the MOs from fragments, but interprete the MOs from your calculation. Almost all textbooks include only the pi orbitals when they discuss benzene, but as you can see there are many sigma orbtials and they are interleved with the pi MOs. You are "deconstructing" your MOs into a LCAOs. If in doubt please check with a demonstrator that you are doing the right thing. Hint1: a ring is just a chain with either end joined-up.
  • How do these MOs relate to the common conception of aromaticity? Hint 2: aromaticity relates to delocalisation. Hint 3: aromaticity relates to the total pi electron density, and MOs contain formally only 2 electrons each.
  • Boratabenzne replaces a "C-H" with "B-H" but to be isoelectronic an extra charge needs to be added, so the molecule is negatively charged. Conversely pyridinium is the cationic form of pyridine where "C-H" is replaced by "N-H" but to be isoelectronic one charge needs to be removed. Optimise, carry out a frequency analysis, compute the MOs and carry out an NBO charge analysis of a molecule of boratabenzene and pyridinium

  • Borazine is the boron,nitrogen substituted analogue of benzne. Optimise, carry out a frequency analysis, compute the MOs and carry out an NBO charge analysis of a molecule of borazine.

  • Compare the charge distribution accross this series of molecules, placing images side by side is not sufficient, tabulate and discuss the charges. The key words here are "compare" and "discuss" just presenting the tables is not sufficient, you must interpret your results.
  • Compare and contrast the MOs of benzene with those of the analogues.
    • Pick 3 different MOs (not core, and not unoccupied, but any of the valence occupied MOs). For example you could pick the pi totally bonding MO. If you don't know the difference between core and non-core orbtials please ask a demonstrator!
    • Present a comparison across the series for benzene, boratabenzne, pyridinium and borazine for each MO. What effect do these substitutions have on the MOs? Things you could consider are:
      • the LCAOs contributing to the MOs
      • the energy of the MOs
      • the ordering of MOs
      • the degeneracy of the MOs
      • what are the similarities, what are the differences between these MOs?
      • any other aspect of the MO you think may be interesting or important
  • Discuss the effect these substitutions would have on the full MO diagram (you do not need to construct the diagrams)
  • optional Comment on or analyse or describe some aspect of these systems which you found interesting.
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