A dispersive QCL-VCD spectrometer for studies on asymmetric catalyst

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In various studies, we showed that vibrational circular dichroism (VCD) spectroscopy, the chiroptical version of infrared spectroscopy, can give insights into the conformational preferences of asymmetric catalysts and catalyst-substrate complexes. As VCD spectroscopy records the differential absorbance of left- and right-circularly polarized light, spectral regions showing total absorbance cannot be characterized. Such total absorbance occurs regularly, for instance when excess substrate must be added in order to form the catalyst-substrate (small binding constants) or when a highly absorbing solvent must be used. Therefore, our initial studies were limited to isolable stable species, e.g. hydrogen bonded clusters with large binding constants or intermediates generated in situ in high yields. In this project we propose the implementation of a dispersive VCD spectrometer that takes advantage of a quantum cascade laser (QCL) as bright light source. With this instrument, the limitations of total absorbance can be overcome to a large degree. Equipped with a widely tunable QCL, the dispersive QCL-VCD instrument will allow us to characterize asymmetric catalysts at more realistic catalyst-substrate ratios (allowing studies on systems with small binding constants) and in a wider range of solvents (toluene, Et2O, even water becomes possible). The bright QCL-VCD is expected to enable measurements with higher signal-to-noise than FT-VCD, thereby allowing much faster measurements. Reaction monitoring will become possible, so that specialized spectra analysis methods will give access to the VCD spectral signatures of intermediates, that exist only in small yields in complex reactions mixtures.