A dispersive QCL-VCD spectrometer for studies on asymmetric catalyst

在各种研究中，我们表明，振动圆二色谱（VCD）光谱，红外光谱的手性版本，可以深入了解不对称催化剂和催化剂-底物复合物的构象偏好。由于VCD光谱记录了左和右圆偏振光的差分吸收，因此无法表征显示总吸收的光谱区域。这样的总吸收率有规律地发生，例如当必须加入过量的底物以形成催化剂-底物（小的结合常数）或当必须使用高吸收性溶剂时。因此，我们最初的研究仅限于可分离的稳定物种，例如具有大结合常数的氢键簇或以高产率原位产生的中间体。在这个项目中，我们提出了一个色散的VCD光谱仪，利用量子级联激光器（QCL）作为明亮的光源。使用该仪器，可以在很大程度上克服总吸光度的限制。配备了广泛可调的QCL，分散QCL-VCD仪器将使我们能够在更现实的催化剂-底物比（允许研究具有小结合常数的系统）和更广泛的溶剂（甲苯，Et 2 O，甚至水成为可能）中表征不对称催化剂。明亮的QCL-VCD有望实现比FT-VCD更高信噪比的测量，从而实现更快的测量。反应监测将成为可能，因此，专门的光谱分析方法将获得中间体的VCD光谱特征，这些中间体在复杂的反应混合物中只存在少量的产率。

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.