Mod:nmr
See also: Laptop,Programs,Organic,Physical,Inorganic
The Exercise -Bonding based
Bonding is intrinsically a quantum mechanical phenomenon, and by analysing the results of QM calculations, one can often obtain profound insight into the properties of molecules.
Suggested Explorations
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:S0002-7863(98)02234-3 10.1021/ja982234c S0002-7863(98)02234-3 )
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.
- The aromaticity of metallic clusters is a hot (and controversial) topic at the moment. Comment on this aspect in any of the structural arrangements you may find (DOI:10.1021/cr030091t )
Reference: A Photoelectron Spectroscopic and Computational Study of Sodium Auride Clusters, NanAun- (n = 1-3) (DOI:10.1021/jp0703511 )
Fuels of the future
Hydrogen is going to be the fuel of the future, however ways of storing hydrogen safely are still on the drawing board. Ideally you want to store hydrogen in a solid matrix eliminating the need for high pressures and improving safety. Ammonia borane (NH3BH3) is being investigated because it has a high hydrogen content and is a stable solid at room temperature. Have a look at this article from Chemistry World (02 July 2008): Borane leads the way to alternative fuels. You can calculate the structure of ammonia borane to explore this issue. Some questions you might want to consider are:
- Is ammonia borane staggered or eclipsed?
- What is the energy difference between these conformers?
- How does this energy difference compare to that of ethane?
- What can you say about the bonding in this compound relative to the organic analogue?
- What is the stability of ammonia-borane relative to the suggested reactants (NH4Cl and NaBH4), which form NH4BH4 and NaCl, then H2 is released.
- Ammonia-borane is isoelectronic with ethane. But whereas ethane has an extremely low melting point, the former melts around 110°C. Can you find an explanation? (Hint: is there a way of modelling the solid state of this species? Are any of the techniques described in Module 3 relevant?)