Current Projects: Vibrational Assignments
So far, we have synthesized a total of eight separate model compounds, shown in the figures on the right. The goal of this portion of our project is to publish complete vibrational assignments for each of these molecules. The different ligands present in the molecules result in differences in the electronic properties of the di-iron active site. These differences show up in the vibrational spectra, so by better understanding the vibrational spectra, we can get a better understanding of the electronic characteristics of these models.
The vibrations in these molecules are also very useful probes of changes during reactions, particularly ultrafast processes. While the C≡O and C≡N stretches are commonly used as probes in the IR spectra, similar probes in the Raman spectra may be useful. These compounds exhibit extremely strong Raman scattering, especially from the Fe(CO)3 bending and Fe-S stretching and bending motions in the 100-500 cm-1 region.
We have acquired mid-IR, far-IR (THz), and Raman spectra of each of the compounds. Vibrational assignments for 1 and 2 have already been published. Additional vibrational assignments are under way. The vibrational assignments of 7 and 8 will probably be the next to be completed, as these molecules are uncharged. The charged molecules (3-6) all include a relatively complicated counter-ion (tetraethyl ammonium, N(C2H5)4+) when synthesized according to literature methods. The counter ion introduces a number of vibrational modes, which complicates the analysis.
This portion of the project has resulted in one publication so far:
Stromberg, C. J.; Kohnhorst, C. L.; Van Meter, G. A.; Rakowski, E. A.; Caplins, B. C.; Gutowski, T. A.; Mehalko, J. L.; Heilweil, E. J., Terahertz, infrared and Raman vibrational assignments of [FeFe]-hydrogenase model compounds. Vibrational Spectroscopy 2011, 56 (2), 219-227. http://www.sciencedirect.com/science/article/B6THW-529CNJF-1/2/870e69e634f44cd7d358e0a4a50d24ca