Events & Seminars > Event Details


4:00 pm
Room 304, Chemistry Building

Unpacking the Symmetric Stretch of CCl4: The Confluence of Undergraduate Research and Pedagogical Advancement in Physical Chemistry Lab


Professor Steve Mayer
University of Portland

Hosted by: Professor Andreas Gahlmann

When developing a research program at a predominately undergraduate institution, one must carefully select projects that have potential to contribute to the body of scientific knowledge, yet can be performed by undergraduates. Our most recent work from the summer of 2014 was an investigation of the Raman spectrum of the symmetric stretching vibration (ν1) of carbon tetrachloride in the liquid phase. Reports in the literature over the last eighty years clearly show four of the five more abundant isotopomers at 440 – 470 cm-1.  At the lower energy end of this spectrum, additional intensity due to isotopomeric contributions from the symmetric stretch for v=1→2 (hotbands) partially overlaps the prominent v=0→1 features, and accounts for about 18% of the integrated intensity at 295 K in agreement with theory. When these two patterns are modeled and subtracted from the experimental spectrum, a feature underlying almost exactly the C35Cl4 (v=0→1) band at 462.5 cm-1 becomes apparent.   We propose that this feature is the ν34 difference band.  Observations at lower temperatures, and of the combination bands, and the polarized Raman spectra are consistent with this hypothesis.

While working on this manuscript, I saw an opportunity to adapt our work to advance the teaching of Raman spectroscopy, which is often missing from the undergraduate physical chemistry laboratory curriculum. Carbon tetrachloride is the ideal molecule for an introductory vibrational spectroscopy experiment and the symmetric stretch vibration contains fine structure due to isotopic variations of the molecule according to C35Clx37Cl4-x. I will discuss simple theoretical predictions of the fine structure, calculation of relative Raman differential scattering cross sections and discussion of the inherent asymmetry in the ν1 mode resulting from the different isotopes of chlorine. In addition, I will discuss our investigation of the contributions from both the v=0 to v=1 excitation and the v=1 to v=2 excitation as well as the calculation of spectral contributions from these excited state vibrational transitions.