Ammonia

Symmetry

In this section we are going to look at different isomers for NH₃ and the effect of symmetry on the structures, energies and optimisation time. In this section we will be useing a 6-31G basis set and the B3LYP method.

• generate an NH₃ molecule in gaussview and optimise it (save your output file)
  • determine the symmetry of your molecule by looking at the calculation summary in Gaussview, save a snapshot of the summary window and include it in your report.
  • the symmetry should be C_3v
• generate another NH₃ molecule in gaussview and make one bond longer than the others by setting it to 1.01Å, and under the general tab, tick ignore symmetry and then run the optimisation
  • save a snapshot of the summary window and include it in your report.
  • the symmetry should be C₁
• now use the file nh3_b3lyp_d3h.txt and run another optimization. First, copy the contents of this file to one in your directory called nh3_b3lyp_d3h.com and open it in Gaussview. You will see an extra pink "atom", this is called a dummy atom, and I have used it to help define the restricted symmetry of this molecule. Do not make any changes to this file, just run it.
  • save a snapshot of the summary window and include it in your report.
  • the symmetry should be D_3h
  • optional If you are interested in knowing how the symmetry is restricted, look at the contents of the input .com file in conjunction with the section on "z-matricies" in the "Exploring Chemistry with Electronic Structure Methods" book.
• now would be a good time to review the difference between the symmetry groups C₁, C_3v, and D_3h. If you are taking the symmetry and spectroscopy course you will also know (or be finding out shortly!) that these symmetry groups are related to one-another, C₁ is a sub-group of C_3v which is a sub-group of D_3h.
• Look over the three optimisation jobs and answer the following questions in your report:
  • has the symmetry made any difference to the final structure obtained?
  • has the symmetry made any difference to the time it took to optimise the geometry?
  • what implication does symmetry have for the time it takes to do a calculation?
  • can a molecule "break symmetry" during an optimisation?
  • what implications does this have if you enter a high symmetry structure to optimise?
  • which is the lowest energy geometry?
  • what is the energy in kJ/mol of the other two isomers above the energy of the lowest energy structure? Hint: take ΔE=E(high energy)-E(low energy), the energy units are reported in atomic units so you will need to convert the energy difference to kJ/mol.
  • is the energy differences between these structures significant? Explain why.

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