Encouraging Student Engagement by Using a POGIL Framework for a Gas-Phase IR Physical Chemistry Laboratory Experiment
What can an IR spectrum tell you about a molecule?
A version of the classic rotationally resolved infrared (IR) spectrum of a diatomic molecule experiment has been developed using the POGIL framework to more fully engage students in the collection, modeling, analysis, and interpretation of the data. An analysis of the experimental protocol reveals that the POGIL approach actively engages students in scientific practices. The student learning objectives for this laboratory experiment are to (1) develop an energy level diagram and relate that diagram to rotational–vibrational spectra; (2) identify, describe, and interpret the molecular constants that can be extracted from gas-phase IR spectra; (3) discover the impact of spectral resolution on precision of molecular constants derived from the spectral data; and (4) use data to evaluate and refine quantum mechanical models.
What is this experiment about?
In this experiment, students collect and interpret IR spectra of a diatomic molecule (usually HCl or CO) with varying resolution. Students develop an energy level diagram and relate it to the rotational-vibrational spectra. Data is used to evaluate and refine rotational and vibrational quantum models and use the models to extract molecular parameters including rotational constant and bond length. The lab presumes that the students are familiar with the fundamental ideas of spectroscopy (i.e. the energy of the radiation matches the energy change in the molecule being studied) and have had prior exposure to IR spectroscopy (likely in an organic chemistry course).
What do students do?
After collecting gas phase IR spectra of a small molecule at several different resolutions, students then try an initial mathematical model to fit the data and then work through the process of refining that model. Students consider the effect of resolution on the uncertainties of extracted parameters. There are four additional optional cycles that focus on isotopes, overtones, and statistical mechanics. What equipment and supplies will you need? Data sets (HCl and CO) are available if an instructor wants to do this as a dry lab. For instructors collecting data, an IR instrument and a gas sample will be needed. There are two data analysis template options that are provided: Excel and Jupyter notebook.
What makes this experiment a physical chemistry experiment?
Students use mathematical models to generate ro-vibrational energy diagrams and then use experimental data to update their diagrams and mathematical models. Students also acquire data at different spectral resolutions, enforcing the connection between resolution and the precision of the reported molecular constants. This experiment expands students’ understanding of how IR spectroscopy can be used beyond their previous classes where IR is used mainly for functional group identification.
And what makes it a POGIL-PCL experiment?
Students sketch a predicted IR spectrum and then go on to predict the effect of resolution on the spectrum. Students work in groups to collect spectra at different resolutions and then pool their data as a class. In groups and as a class, students are prompted to work through the process of refining model equations used to fit the spectra, using residuals to evaluate the fit. Students can then use the model to determine molecular parameters like the rotational constant and bond length, learning what information can be extracted from an IR spectrum.
Reference
Jordan P. Beck and Diane M. Miller
J. Chem. Educ. 2022, 99, 12, 4079–4084 https://doi.org/10.1021/acs.jchemed.2c00314
The Instructor’s Handbook with implementation details, sample data, and expected answers is available through the POGIL-PCL project.