VCD spectroscopy under cryogenic conditions: Of matrix effects andphotochemistry of chiral molecules
Merten, C.1
1christian.merten@ruhr-uni-bochum.de, Ruhr-Universität Bochum, Organische Chemie II,
Universitätsstraße 150, Bochum, Germany

Vibrational Circular Dichroism (VCD) spectroscopy measures the small difference in theabsorption of left- and right circular polarized infrared light by a chiral sample. It allows for theunambiguous assignment of absolute configurations by comparison of experimental andcomputationally predicted spectra.1 It is also highly sensitive to even very subtle differences instructures, such as conformational changes induced by solute-solvent interactions,2 and weutilize this conformational sensitivity to probe intermolecular interactions of interest in
catalysis3 and supramolecular chemistry.4-6
In this talk I will focus on our activities to combine VCD spectroscopy with the matrix-isolation
technique as sample preparation method for the isolation of small molecules and reactiveintermediates. Highlighting some recent results, we show that trapping chiral molecules in solidrare gas matrices can help us understand problems faced in the interpretation of solution phase
spectra. These challenges in the analysis are, for instance, flat potential energy surfaces7 or
rapidly rearranging photoisomerization products.8 Matrix effects were found to occasionallymake the interpretation of MI-VCD spectra quite challenging,9 and we also implemented a setup
to record MI-VCD spectra in para-hydrogen matrices. Comparison with experimental VCD
spectra recorded in liquified rare gases helped us to reveal further matrix-effects in MI-VCD
spectra and to benchmark computational approaches for the prediction of anharmonic VCDintensities.10-111) Merten, C.; Golub, T. P.; Kreienborg, N. M., Absolute Configurations of Synthetic Molecular Scaffolds from Vibrational CD
Spectroscopy. J. Org. Chem. 2019, 84, 8797-8814; 2) Merten, C., Modelling solute–solvent interactions in VCD spectra analysis
using the micro-solvation approach. Phys. Chem. Chem. Phys. 2023, 25, 29404-29414. 3) Merten, C., Recent Advances in theApplication of Vibrational Circular Dichroism Spectroscopy for the Characterization of Asymmetric Catalysts. Eur. J. Org.
Chem. 2020, 2020, 5892-5900. 4) Kemper, M.; Engelage, E.; Merten, C., Chiral Molecular Propellers of Triarylborane
Ammonia Adducts. Angew. Chem. Int. Ed. 2021, 60, 2958-2962. 5) Weirich, L.; Merten, C., Induced VCD and conformational
chirality in host–guest complexes of a chiral ammonium salt with crown ethers. Phys. Chem. Chem. Phys. 2021, 23, 1830018307. 6) Müller, C.; Scholten, K.; Engelage, E.; Merten, C., Synthesis and VCD Spectroscopic Characterization of a Series ofAzacryptands from a Chiral Valine-based Derivative of Tris(2-aminoethyl)amine (TREN). Chem. Eur. J. 2023, 29,e202302126. 7) Pollok, C. H.; Merten, C., Conformational distortion of α-phenylethyl amine in cryogenic matrices - a matrix
isolation VCD study. Phys. Chem. Chem. Phys. 2016, 18, 13496 - 13502. 8) Pollok, C. H.; Riesebeck, T.; Merten, C.,
Photoisomerization of a Chiral Imine Molecular Switch Followed by Matrix-Isolation VCD Spectroscopy. Angew. Chem. Int.
Ed. 2017, 56, 1925-1928. 9) Grassin, C.; Pollok, C. H.; Kreienborg, N. M.; Merten, C., Matrix effects in MI-VCD spectra of
two chiral oxiranes and their potential microscopic origin. Phys. Chem. Chem. Phys. 2023, 25, 31995-32001. 10) Kreienborg,N. M.; Bloino, J.; Osowski, T.; Pollok, C. H.; Merten, C., The vibrational CD spectra of propylene oxide in liquid xenon: aproof-of-principle CryoVCD study that challenges theory. Phys. Chem. Chem. Phys. 2019, 21), 6582-6587. 11) Kreienborg, N.
M.; Yang, Q.; Pollok, C. H.; Bloino, J.; Merten, C., Matrix-isolation and cryosolution-VCD spectra of α-pinene as benchmark
for anharmonic vibrational spectra calculations. Phys. Chem. Chem. Phys. 2023, 25, 3343-3353.