Molecular models and selected molecular orbital surfaces and slices were calculated with Spartan for HF, LiH, CO2, XeF2, and BF3, and the results were used by students in an in-class activity (covering several class sessions) to answer a series of questions.
By examining the calculated molecular orbitals on Spartan and comparing these with MO energy diagrams, a student will:
- Gain experience predicting the atomic orbital contributions to a given molecular orbital.
- Discover, in some cases, the unanticipated role of sp mixing.
- Interpret the implications of molecular orbital occupation on bond polarity and bond order.
- Consider the meaning of 3 center, 2 electron bonds in hypervalent molecules such as XeF2.
- Explain the bonding and Lewis acidity of BF3 from the perspective of molecular orbitals.
Computers with Spartan molecular modeling software
Spartan models, selected molecular orbital surfaces and slices were calculated in advance for the students using an appropriate level of theory. Students were already familiar with the use of Spartan to visualize molecular orbitals from a previous conference activity (a one hour session) examining the MOs of N2 and CO. The detailed “help sheet” and handout for this conference activity is attached below. Although many of the details in these handouts are particular to our computer lab, I left them in to show the level of detail that I provide for students in the instructions.
I have assigned these same questions as problem set questions in previous years (usually no more than one per year), but in this iteration, I assigned all three as an extensive in-class activity for students to complete while I was away at the ACS meeting. I anticipated a minimum of 3 hours for this activity to compensate for 3 missed class sessions. In my understanding, this was mostly accurate for time required, although some students spent longer.
The textbook that I was using in this particular year was Housecroft & Sharpe, 3rd edition. I usually point them to the appropriate MO diagrams in their text to compare to the Spartan results. Obviously, the MO diagrams vary from text to text, and it is helpful to look at these in advance in case you need to change the hints at all on various questions.
A solutions key is posted for this learning object under “Problem Sets.” You must have approved “Faculty” status on VIPEr to access these.
This activity was “spot-graded” meaning that I looked carefully at all of the answers for several questions but skipped over others. This is my new solution to too much grading. On this assignment, I “spot-graded” the LiH/HF question and the CO2 part of question 2
The following results are out of 24 students that completed the assignment. On the LiH/HF question: 6 out of 24 students scored 85% or above, 11 out of 24 students scored 67–84%, and 7 students scored below 66%. The most common problem was misunderstanding the composition of the LiH molecular orbitals, particularly the role of sp mixing on Li.
On the CO2 part of question 2, 16 of 24 students scored above 90%, 4 students scored 67–85%, and 4 student scored below 66%. The main problems were identifying the labels and types of molecular orbitals (bonding, nonbonding, antibonding, gerade, ungerade, etc.).