Bonding models: Discrete molecules

28 May 2019

Quadruple Bond Acrobatics

Submitted by Lori Watson, Earlham College
Evaluation Methods: 

Students are typically asked one multiple chose or short answer question where they identify which d orbitals are involved in metal-metal quadruple bonding and/or idetify/draw the interaction.  They will also use these concepts in a more applied way in both problem set and exam in depth questions where they must explain particular structural or spectroscopic evidence using, for example, the ligand geometry forced by the eclipsed conformation of the dx2-y2 remaining d orbital.

Evaluation Results: 

Students generally perform very well on the basic identification/d-orbital interaction question that mostly tests recal of the facts.  There is a range of performance on more complex application problems, though students usually correctly identify the role of the quadruple bond orbitals and geometry as a factor.  Common challenges involve misidentification of axes, and an inability to think through how changes to variables like metal identity or oxidation state, or ligand sterics, may further contribute to observed bonding or structural data.

Description: 

Four pairs of students represent quadruple bonding in metal complexes by "forming bonds" with a variety of physical methods involving actions like facing each other while holding hands (sigma bond), touch hands and feet of their partner "above and below" the plane (two pi bonds), touching hands and feet while facing each other (delta bond).  This results in a "Twister"-like pile of students resembling the quadruple bonding interaction

 

Procedure:

1. Ask for 8 volunteers who are comfortable touching each other (holding hands, touching foot to foot)
2. Start with the shortest pair of students, and proceed through all four pairs having them do the following:

  • Sigma bond: have two students face each other at a comfortable distance, holding both hands.  The held hands represent electron density along the internuclear axis.  This is dz2
  • Pi bonds: have two pairs of students form the dxz and dyz bonds by having two students stand behind each of the first pair. They will represent pi electron density above and below the internuclear axis by touching hands together on either side (dxz) or a hand and foot above and below the axis respectively (dyz), where the y axis points toward the ceiling.  Unless your students can levitate, one foot must remain on the floor at all times--so the dxz orbital interaction is challenging, and one "lobe" (represented by the foot stick out toward the back) will not be properly represented.  
  • Delta bond: have the tallest students face each other, one behind each of the previous three students on their side.  Have them spread out their feet and hands at approximate right angles to each other, and then touch both hands palm to palm together above the z axis, and both feet together below th z axis.  To do this, the previous pairs of students will have to move even closer together, and the dxy orbitals will need to "bend" toward each other.  Students will observe that it's difficult to make good contact palm to palm.  Quadruple bonds are weaker!

3. Let the class dissolve into giggles, and then debrief.  How did each group of students have to move? Which orbital was "left out"? How would be expect incoming ligands to bind? Why? Could you have quintuple bonds? (Hint: yes) What would happen if the incoming ligands were too large to be eclipsed? (Hint: will tend to form staggered, triple bonded metal-metal complexes instead).

4. Give the class time to sketch out all four orbitals involved in a metal-metal quadruple bond in their notes.

Learning Goals: 

A student should be able to identify and draw the d orbitals involved in quadruple bonding, including their interactions.  They should be able to explain why quadruple bonds are shorter than corresponding triple bonds and where and which d orbital will be involved in bonding to ligands.

Prerequisites: 
Equipment needs: 

8 willing students who consent to physical contact with each other (holding hands, touching foot to foot).  It works best to begin with the shortest pair of students and proceed toward the tallest pair of students.

Corequisites: 
Implementation Notes: 

This works best when begining with the shortest pair of students and proceeding toward the tallest pair of students.

 

Please see attached pictures for a step-by-step guide to movement.

Time Required: 
5 minutes, plus 5 minutes debrief
21 May 2019

CompChem 04: Single Point Energies and Geometry Optimizations

Submitted by Joanne Stewart, Hope College
Evaluation Methods: 

This exercise usually takes less than a 50 minute class period. Students record their answers directly onto their handouts, and I collect the handouts either at the end of class or at the beginning of the next class.

Evaluation Results: 

Student work is typically complete and correct because they have completed the exercise in class and received feedback as they worked.

Description: 

This is the fourth in a series of exercises used to teach computational chemistry. It has been adapted, with permission, from a Shodor CCCE exercise (http://www.computationalscience.org/ccce). It uses the WebMO interface for drawing structures and visualizing results. WebMO is a free web-based interface to computational chemistry packages (www.webmo.net).

In this exercise, students perform coordinate scans to explore how changes in bond length, bond angle, and dihedral angle can affect molecular energy. The results allow them to visualize the relationship between the geometry change and molecule's energy.

The exercise provides detailed instructions, but does assume that students are familiar with WebMO and can build molecules and set up calculations.

 

Learning Goals: 

Students will be able to:

  1. Calculate and visualize the potential energy surface of a diatomic molecule.
  2. Calculate and visualize the energy changes in a small molecule during bending.
  3. Calculate and visualize the changes in energy when a small molecule undergoes conformational changes.
Equipment needs: 

Students need access to a computer, the internet, and WebMO (with Mopac and Gaussian). 

Corequisites: 
Implementation Notes: 

I use this as an in-class exercise. Students bring their own laptops and access our institution's installation of WebMO through wifi.

Time Required: 
30 minutes
15 May 2019

Lewis Structures

Submitted by Will, Bucknell University
Evaluation Methods: 

A short problem set is assigned with the video

Evaluation Results: 

Most students are able to learn the content in this video independently

Description: 

Part 4 of the Flipped Learning in General Chemistry Series. This video introduces the Lewis structure, which is used to show the connectivity between atoms and the location of valence electrons.

Prerequisites: 
Corequisites: 
Course Level: 
Subdiscipline: 
Learning Goals: 

After watching this video and completing the assigned problems, students should know the octet rule, understand the components of a Lewis structure, and be able to create new Lewis structures from molecular formulas.

Time Required: 
10-15 minutes

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