# Isotopic labeling and reduced mass calculations for IR spectroscopy

## In-Class Activity

Submitted by Adam R. Johnson, Harvey Mudd CollegeI used this as an in class activity but it may work better as a problem set for your class. I had the students read the pertinent chapters of the textbook which go through symmetry and molecular vibrations, including using both stretches and cartesian axes as bases. In class, I divided the students up into four groups. Each group did one of the problems for 30 minutes and during the last 20 minutes of class, they reported out their solution. The students had not seen the Hooke’s law in the textbook so I included it as part of the activity. I also included a handout on applying the group theory to molecular motions.

Attachment | Size |
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a handout that describes the projection operator technique for molecular motion | 111 KB |

the in class activity | 142.05 KB |

A student should be able to use the Cartesian axes as a basis for molecular motion

A student should be able to use a bond vector as a basis for a molecular vibration

A student should be able to, given an IR stretch, predict a stretch after an isotopic substitution

a set of character tables (C2v, C3v and C4v at a minimum) is needed for some of the groups

I did this as an in-class activity on 3/28/2016. I had 15 students, so groups of 3-4 on each of 4 problems. I used problem 2a, 3, and the two related LOs in class.