Main group halides

From 1st and 2nd year lectures you discovered that the BH₃ molecule is electron deficient and prefers to form dimers B₂H₆ which are unusal because of the bridging H-atoms. You also found out that a similar case exists for other group 13 elements. (As well as group II elements, for example Grinard reagents) In this project you will investigate the conformers, vibrations and MOs of Al₂Cl₄Br₂

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 a full basis set 6-31G(d,p) on Al and Cl and a PP LANL2DZdp on Br
• Determine the four possible isomers that can be formed from AlCl₂Br monomers. Identify the symmetry of each isomer.
• compute the energy of the four isomers of Al₂Cl₄Br₂ and determine their energy in kJ/mol relative to the lowest energy conformer
• discuss the position of the Br atoms with respect to the stability of the different conformers
• determine the dissociation energy for the lowest energy conformer into 2AlCl₂Br
• is the product more or less stable than the isolated monomers?
• carry out a frequency analysis for every conformer and present the IR spectra
• discuss the IR spectra with respect to the symmetry of each conformer, why do some spectra have more bands than others?
• an advantage of computational chemistry is that you can calculate the position of bands that are not experimentally active, what is the requirement for an IR band to be active?
• discuss the position and nature of Al-Br stretching vibrations with respect to the terminal or bridging position of the Br atom. Placing images of the spectra side by side is not sufficient, tabulate the key vibrational frequencies (remember that they can reorder, so be sure you are comparing similar modes). The key word here is "discuss" just presenting images and tables is not sufficient, you must interpret your results.
• carry out a MO calcualtion on the lowest energy isomer (only!)
• visualise all the occupied non-core MOs, you do not need to reproduce them all. If you don't know the difference between core and non-core orbtials please ask a demonstrator!
• in your wiki present 5 MOs ranging from highly bonding to highly antibonding (from any of the MOs you have visualised) and describe the interactions occuring in the MOs. Please be detailed, vaugue references such as saying there are "through space" interactions is not sufficient. You can annotate your orbital pictures in chem-draw. Things you might want to consider are:
  • Are AO interactions strong or weak?
  • Are AO interactions bonding or antibonding?
  • Are there through space interactions?
  • How many and what kinds of nodes are there?
  • How delocalised is the MO?
• optional Comment on or analyse or describe some aspect of these systems which you found interesting.