The structures of neutral rhodium carbonyl clusters containing two, four, and six rhodium atoms have been known for some time. In a recent paper (J. Am. Chem. Soc. 2008, 130, 2126-2127.) the structures of the monocation rhodium carbonyl clusters were investigated via infrared multiphoton dissociation spectroscopy (IR-MPD), comparing experimental results to those predicted from computational modeling. These are ideal problems to apply simple group theory tools to predict the distinguishing features in the IR spectrum of each possible structure.
By working on these problems in small groups, a student will gain experience and confidence applying the tools of symmetry and group theory to the spectroscopic characterization of metal carbonyl complexes. Specifically, a student will practice:
- Assigning the point group symmetries of a variety of multinuclear rhodium carbonyl clusters ranging from relatively easy high symmetry groups to more challenging examples
- Applying the tools of group theory to predict the number of IR-active CO stretching frequencies in the bridging and terminal CO regions of the spectrum for a particular structure
Access to Jmol or another structure viewing program that enables the students to rotate these structures interactively can be very helpful, particularly for the more challenging point group assignments. The pdb files for the different structures are included below. Clicking on one of the pdb files will open the text file in a new window. Save the file with a .pdb extension using the "Save page as..." command (right click or control-click on a Mac).
30-60 minutes depending on how many structures are to be completed
I follow up this in-class activity with a problem set question asking for the same type of analysis for two more structures from the paper. This was very useful for making sure that each student knew how to do these problems on their own rather than relying on other stronger classmates from their in-class group. The problem set question can be viewed by VIPEr users with Faculty status.
The biggest challenge for most students was assigning the point groups! It turns out this took at least 20 minutes of discussion before I started giving them big hints so they could move on to the analysis. Some students are confused by when to do a complete normal mode analysis and when to use a smaller basis set to determine the initial representation. Or they may determine the correct representation for CO stretches, but then mistakenly subtract off the representations for translational and rotational motion.
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A recent paper on iron carbonyl clusters is another good source for follow-up questions since IR spectroscopy is a key characterization technique in this paper. There is also some interesting NMR data in this paper.
Cristina Femoni, Maria Carmela Iapalucci, Giuliano Longoni, Stefano Zacchini,* and Salvatore Zarra, “New Findings in the Chemistry of Iron Carbonyls: The Previously Unreported [H4-nFe4(CO)12]n– (n = 1, 2) Series of Clusters, Which Fills the Gap with Ruthenium and Osmium,” Inorg. Chem. 2009, 48, 1599-1605.