Proof-of-concept for background-free ‘chiral nonlinear’ Raman scattering spectroscopy and its application for mechanistic understanding of chiral Brønsted acid-assisted asymmetric organocatalysis

• Applicant: Vikas Kumar, Ph.D.
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 511625800

Asymmetric organocatalysis which has been identified as one of the ten emerging technologies in chemistry in 2019 at 100th anniversary of founding of IUPAC, presents an economical and environment friendly method to convert achiral educts into chiral products in the presence of a chiral organic catalyst. The development of these important reactions requires a proper screening of chiral organocatalysts, a mechanistic understanding of the reaction and its pathways involving chiral molecules. However, the observation of chirality with chemical specificity on a natural timescale of these reaction remains challenging due to lack of a fast and versatile genuine enantioselective spectroscopic technique. This project proposes the experimental realization of a ‘fast’ chiral nonlinear Raman spectroscopy technique called optical heterodyne-detected Raman-induced Kerr effect - Raman optical activity (OHD-RIKE-ROA) spectroscopy. OHD-RIKE-ROA spectroscopy, based on a ‘nonlinear Raman effect’ called RIKE, has been theoretically predicted to provide enantio-selectively the background-free vibrational spectrum of chiral molecules, however, it has not yet been experimentally realized. It is expected to have high detection sensitivity down to millimolar concentrations and acquisition time below 1 minute. Consequently, it can be applied in a time-resolve manner for mechanistic study of an asymmetric organocatalysis. In this project, the developed OHD-RIKE-ROA spectroscopy will be applied to study a particular chiral Brønsted acid-assisted asymmetric organocatalysis. In particular, it will be utilized for characterization of the ‘active site’ of the catalyst in this reaction where catalyst-loading dependent ‘catalyst-catalyst’ and ‘catalyst-substrate’ molecular interactions will be targeted. The observed OHD-RIKE-ROA spectra will be characterized by DFT calculated Raman optical activity spectra. Furthermore, it will also be employed to understand the observed solvent-dependent enantioselectivity in this reaction. The successful completion of the project would avail a highly sensitive, background-free ‘chiral nonlinear’ Raman scattering spectroscopy technique for studying various asymmetric reactions.

• DFG Programme: Research Grants