Mini Project

Some suggestions

Some general ideas for things to investigate using simple molecules

• aromaticity and borazine derivatives or pyridine and other group 15 substituted rings
• dative bonds between lewis acids and bases
• how do substituents effect the orbitals of lewis acids OR bases?
• PF₃ and it's hypervalent analogues, and the heavier group 15 analogues
• siloxanes and the Si-O-Si bond

Molecule-electrode Contacts: metal containing diatomic molecules

In year 1 and 2 you built MO diagrams for simple first and second row elements. But what happens when one or two of the elements in a diatomic have dAOs?

• Can you predict the MO diagram? On performing a calculation does it match your expectations? For example would you expect AuS^- or AuO^- to be more stable (work out the energy of dissociation into the atoms). Don't forget the 1- charge, this must be set in the input to your calculations.
• How do the MO diagrams for these two species compare? Au is a soft metal and S^- a soft ligand, will they bind tighter than AuO^-?
• Can you make a better model for an electrode-organic connector? Consider adding more Au atoms, and adding an alkyl chain to the S atom (this will effect the charge on the complex)
• Reference: Molecular Alligator Clips for Single Molecule Electronics. DOI

Metallic Clusters

Small metallic clusters enter into the nano regime, they lie between the gas-phase and solid-state and have properties distinct from both. Like small organic molecules mixed metallic clusters can exhibit electron delocalization and aromaticity. However, the presence of dAOs and relativistic effects introduce additional stabilizing effects. Modern pseudo-potentials can account for most of the relativistic effects (but not spin-orbit coupling) and thus modern computational methods can be used to evaluate the stability of various bonding and structural arrangements.

• compute the energy of various structural arrangements of the mixed metal cluster Na₂Au₂ and determine the most stable geometry. Don't forget that metal clusters are not restricted to the directional bonding found in most organic molecules.
• Investigate the electronic structure of at least one stable conformer.
• you might like to comment on the aromaticity of metallic clusters, this is a ''hot'' (and controversial) topic at the moment DOI )
• Reference: A Photoelectron Spectroscopic and Computational Study of Sodium Auride Clusters, Na_nAu_n- (n = 1-3) DOI

Explore bonding in main group halides

From second year main group chemistry you know that B₂H₆ has two bridging H atoms, and four terminal atoms. You also know that a similar bonding situation holds for Al₂Cl₆. But what happens if there are 4 Cl atoms and 2 Br atoms? Do the Br atoms prefer one end, the bridging positions or is it more stable to have one at either end? What effect do the Br atoms have on the vibrational? What do the orbitals for this kind of molecule look like?

Haloaluminate ionic liquids for Al production

Ionic liquids are important solvents for electrochemical reactions, one of which is the deposition of Al. Normally this reaction has to be carried out at a very high temperature. A solution composed of large organic cations and Cl anions is added to AlCl₃ which then forms a mixture of ions, called an ionic liquid. In this case a chloroaluminate melt with AlCl₄- and Al₂Cl₇- anions. Ionic liquids are important for the purification of metals, because they have much lower melting points and hence less energy is required for the electrochemical deposition.

What is the structure of AlCl₃ and these anionic species? What is the bonding like? Which is more stable: (a) two AlCl₃ molecules and a Cl- anion (b) one AlCl₃ and one AlCl₄- anion (c) one Al₂Cl₇- anion. (compute the energy of each species individually and then add the energies for each combination together and compare the totals, eg 2*E(AlCl₃)+E(Cl-) compared to E(AlCl₃)+E(Cl-)+E(AlCl₄-) compared to E(Al₂Cl₇-). Be sure to use exactly the same level and basis set for each calculation and ensure you have fully optimised your structures. wiki