30 Jun 2015

Vibrational Modes and IR Spectra using Character Tables

In-Class Activity

Submitted by Karen McFarlane Holman, Willamette University

In this activity, students in my upper-level Inorganic course are given two possible structures of sulfur dioxide, and based on an assessment of given vibrational modes, they determine which of the modes are IR active by two methods: (1) the “Intro Chem” method (determing whether the dipole moment changes for a particular vibrational mode) and (2) using character tables. They compare their assessment to experimental IR absorption peaks, and the students decide which structure is valid. For those of you who teach Raman spectroscopy, it could be included in this LO as well. 

Learning Goals: 

The students will be able to…

• determine the dipole moment (relative direction and magnitude) for each extreme of the vibrational mode.

• assess whether the dipole moment is changing during a particular vibrational mode.

• assess the vibrational mode by carrying out the appropriate operations and assigning characters, i.e. using character tables to determine whether the mode is IR active.

• compare theoretical results from their vibrational mode analysis to experimental IR spectra and deduce the correct structure.

Implementation Notes: 

I have used this activity in three different ways:  In-Class Activity, as a problem in a homework assignment, and (originally) as an exam question.  All three work well, but I now use it as an In-Class Activity because I appreciate that it gives immediate feedback regarding student understanding while I’m teaching this topic.

One issue of note is that depending on how they define their coordinate axis system, they can get either B1 or B2 when analyzing the asymmetric stretch for C2v symmetry. Both give the same result (IR active) so the result will be correct regardless.  I remind them that there are conventions for how coordinate axis systems are defined, but they can define it differently and the same result is revealed.

Time Required: 
15-20 minutes
Evaluation Methods: 

I evaluate the students in real time during the class activity.  They work in pairs and I walk around the classroom and assess their understanding as they explain their answers to each other and me.

Evaluation Results: 

As an exam question in Inorganic, the vast majority of the students were able to do the first part correctly (the “Intro Chem” assessment), and a smaller portion (probably 75%) worked through the character table analysis correctly.  However, I now only use this activity during class because I have found that it’s more effective to assess their understanding through active discussion to identify difficulties they are having.  In addition, I like to use a more challenging problem for a follow-up homework set and exam questions.  I like this problem in class because they can check and verify their answer quickly.

Creative Commons License: 
Creative Commons Licence


I tried to use this today in my inorganic class.  It's an awesome idea and I wanted to create it, but I didn't have time and @Karen Holman had already done it.

we got a little hung up because I tried to embellish it by having the students come up with the Lewis Dot Structure in which SO2 would be linear, but we could not find a way to make that happen.  

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