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This activity has students make observations on a series of metal complexes with varying geometries and electronic structures (tetrahedral vs octahedral, d7 vs d0). They are then led to develop hypotheses to explain the differences in their spectra based on crystal field theory and their understanding of selection rules. Finally, students inspect theoretical calculations to determine whether their hypotheses based on observation are supported by theory.
| Attachment | Size |
|---|---|
| Optimization and TD-DFT files for MnO4- | 1.19 MB |
| Optimization for CoCl4 | 915.1 KB |
| TD-DFT for CoCl4 | 1.27 MB |
| Cube files for NTOs in CoAqua complex (doublet) | 617.68 KB |
| Cube files for NTOs in CoAqua complex (quartet) | 1.78 MB |
| ICA_CFTandSpectroscopy_2026_StudentHandout.docx | 22.4 KB |
1. Students will relate spectroscopic features of various metal complexes to their geometry and electronic structure.
2. Students will refine their understanding of spectroscopic selection rules.
3. Students will develop an an understanding for how theory can be used to support hypotheses based on observation.
Solutions of [CoCl4]2-, [Co(OH2)6]2+, and [MnO4]- (approximately 0.1 M)
Spectrometer (recommended, but optional)
Some notes for the instructor:
- The calculations here are at a relatively low level of theory. As such, there are some notable deviations in the calculated spectra from the experimental spectra. This is particularly true for the permanganate anion which requires a more sophisticated approach to more closely reproduce experimental observations. However, the relative intensities, relative energies, and orbitals involved are generally in line with expectation and work well for this level of discussion. A brief explanation to students may help with their understanding of the incongruencies.
- I make solutions of [CoCl4]2-, [Co(OH2)6]2+, and [MnO4]- at the same concentrations (roughly 0.1 M). At this concentration it is still easy enough for students to visually distinguish the relative intensity for each absorption and make some qualitative observations. It can also be useful to use a spectrometer if you have a portable one (in my class we used a fiber optic cable attached to an emission spectrometer to view difference spectra as we shined a light source on the solutions to qualitatively determine the absorption wavelength). Alternatively, this could be a fun exercise to carry out in the lab or pre-collected spectra could be distributed.
- ORCA files (optimization and TD-DFT output) for these complexes . It includes both the doublet and quartet spin states for the [Co(OH2)6]2+ complex. These can be used as is with a visualizer, such as ChimeraX. Alternatively, students could carry out these calculations on their own. (I ran these calculations on my personal laptop. Run in parallel with 4 CPUs, the tetrahedral complexes take about 30 minutes [B3PW91 def2-SVPD] while the octahedral complexes are closer to an hour.) I have included .cube files for the Natural Transition Orbitals which can be projected or distributed. The discussion about spin state and relative energy can be modified or ignored depending on your preference, but I think it provides a nice moment to discuss the limitations of theory and why it is important to validate results experimentally.