Association energies: Ammonia-Borane

Introduction

Hydrogen is going to be a 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.

Ammonia-borane is also interesting because it is an acid-base complex. What is the energy of the dative bond? Formally the nitrogen atom donates two electrons to the boron. Of course this is not what really happens, the word "formally" is very important, and refers to a "counting" mechanism used to understand and predict bonding in a range of molecules, in reallity the electrons are shared between the boron and nitrogen.

Calculations

• to compute a reaction energy, which in this case is the dative bond energy, we need the energy of the reactants and products. Your reactants are NH₃ and BH₃ and your product is NH₃BH₃. You have already completed the calculations for BH₃. Thus you need to compute NH₃ and NH₃BH₃.
• When computing energies the reactants and products must be computed at exactly the same level, this includes basis set and method.
• optimise molecules of NH₃ and NH₃BH₃ at the B3LYP/6-31G(d,p) level and carry out a frequency analysis to ensure you have a minimum.
• as a hint you can create a molecule of NH₃BH₃ by using the alkyl chain fragment library and creating a molecule of ethane, then replace one carbon with nitrogen, and the other carbon with boron. Fragment libraries are covered in the help manual for gaussview.

  

• now to determine the energy of the bond, we take the energy of the molecule relative to the energy of the dissociated fragments. Total energies are reported in AU (atomic units) while relative energies are reported in kJ/mol. Record the energy in AU (atomic units, as reported in gaussview) of the following individual molecules in your wiki
  • E(NH₃)=
  • E(BH₃)=
  • E(NH₃BH₃)=
• then report the association energy first in AU and then converted to kJ/mol ΔE=E(NH₃BH₃)-[E(NH₃)+E(BH₃)]
• Many people new to calculations can make an error at this stage. It is very important in quantum chemistry to keep asking yourself if your result is sensible. Rubbish in is rubbish out, and you may have inadvertently made a mistake somewhere. So look at your number, is it a sensible value? How do you know what a sensible value is? (Hint: this is a bond energy, so what "ballpark" value should it have?)
• Based on your energy calculation is the B-N dative bond weak, medium or strong? What comparison have you made to come to this conclusion?
• OPTIONAL ethane and aminoborane are isoelectronic, yet the C-C and N-B bonds are very different. Optimise ethane and compute the NBO charges on the Carbon atoms, compute the NBO charges on the Nitrogen and Boron of aminoborane. Are these different? How does this relate to the character of the bond? How could you chemically manipulate the charges on the Carbon or Nitrogen or Boron atoms? You could carry out a few calculations to test the effects of your predictions. Say we wanted to weaken the N-B bond, how could we do this? We could computationally test your predictions before attempting any (complex) synthesis and characterisation. This is one of the very powerful aspects of computational chemistry.

Check your wiki

• Have you added the key data for NH₃?
• Have you added the key data for NH₃BH₃?
• Have you included your energy calculations?
• Have you answered the bond strength questions?

Top