25 Mar 2014

'Sophomore' symmetry: Computational analysis

Lab Experiment

Submitted by Chip Nataro, Lafayette College
Categories
Description: 

Having been inspired by a number of wonderful LOs, I introduced group theory in my 'sophomore' inorganic class this spring. In addition to learning to determine the point group of a molecule, students were taught how to construct a qualitative MO diagram though the use of LGOs. While this course can be taken with or without the laboratory component, it seemed only natural to include a lab on this material. A previous lab had introduced the students to computational methods for geometry optimization. This is a natural extension of that lab and a very nice means of visualizing the material being presented in lecture.

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Learning Goals: 

Students should be able to

  1. Draw molecules and assign the point group
  2. Use a character table to derive generator functions
  3. Determine LGOs for their molecules
  4. Construct a qualitative MO diagram
  5. Use DFT calculations to generate a MO diagram
Equipment needs: 

Computers with the ability to do DFT calculations

Implementation Notes: 

The calculations should not take too long since the molecules are small.

 

The molecules covered in class are BH3, H2O and PF5. A detailed examination of these can be found in the 'Sophmore' symmetry: Lecture materials LO that can be found in the related activities.

 

 

Time Required: 
3 hrs
Evaluation
Evaluation Methods: 

The lab report had four questions that needed to be addressed.

The MO diagrams constructed using LGOs assessed using the following criteria.

  • Assignment of a point group to the molecule.
  • Assignment of the generator functions.
  • Assignment of the LGOs including determining which orbitals could hold lone pair.
  • The qualitative MO diagram constructed by the students.

 Inclusion of the MO pictures from Spartan ’10 was a point of emphasis.

The final two questions asked the students to compare the computational and LGO results. They were also required to make comparisons between similar molecules. In particular, their ability to notice similarity in orbital shapes, the distribution of MOs and the relative energy of MOs was emphasized.

Evaluation Results: 

In general, students did a very good job with the first two questions. They were able to generate qualitative MO diagrams using LGOs quite effectively. They did struggle a little with the tetrahedral molecules and the three equivalent T2g orbitals. But they did come to my office to ask about that when they had trouble. They also struggled a little with the assigning the lone pair of ammonia. Again, I was able to clarify this when they came to my office with questions.

 

I was disappointed in their ability to compare and contrast their results. None of the groups considered looking at orbital energies from the DFT calculations as a point worthy of consideration. Perhaps that is due to the qualitative aspect of using LGOs. That is a point I will have to emphasize in the future.

Creative Commons License: 
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