Submitted by Catherine / East Tennessee State University on Wed, 11/04/2020 - 13:07
My Notes
Description

Students first learn the basics of WebMO by building and optimizing 2 small molecules. They then calculate and visualize the molecular orbitals of two diatomic molecules (N2 and BF) and observe how going from a homonuclear to heteronuclear molecule changes the shape of different molecular orbitals. 

As written this activity uses the WebMO demo server so no computational chemistry software/licences are required.

Learning Goals

Students will

  • Become familiar with the use of the WebMO user interface to set up and perform calculations
  • Understand how atomic orbital energies affect molecular orbital shape and distribution
  • Predict simple reactivity based on molecular orbital distribution
Equipment needs

Computer, tablet, or phone with internet access

optional - WebMO app 

Implementation Notes

This was used as a homework activity after asynchronous recorded lectures on MO theory in diatomic molecules.

Because not all students were on campus in Fall 2020, this activity uses the WebMO demo server which can be accessed remotely from a web browser. Each individual calculation can be completed in < 30 s. This activity uses the NWChem computational engine, but other software (Gaussian etc.) should give qualitatively the same results.

Students were provided Table 5.2 from Meissler, Fischer, and Tarr (5th ed) for the atomic orbital energies.

Unfortunately at the level of theory used for these calculations you can't easily make comparisons between the experimental atomic orbital energies and the calculated molecular orbital energies in most cases. For example, the calculated energy of the 2p sigma and pi bonding molecular orbitals in N2 are both higher than the experimental atomic orbital energy of the N 2p orbital.

 

 

Evaluation

Evaluation Methods

Students completed the activity outside of class time and were graded on completion and correctness

Evaluation Results

With my foundation level inorganic class, the biggest misconception uncovered with this activity was that the shape of the molecular orbitals in the heteronuclear diatomic was due to electronegativity differences. The students then had a difficult time rationalizing why the 2p sigma MO had more contribution from B.

The second most common error was selecting and describing the wrong molecular orbitals

In the future I would do this as an in-class activity to help guide students/groups who might be having trouble using the software or selecting the correct orbitals in real time.

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