Unravelling conformational flexibility and chirality in molecules using microwave spectroscopy

Conformational flexibility and chirality play important roles in biochemical and pharmacological processes. Thus, a detailed understanding of molecular structure, intra- and intermolecular forces, and chirality is required to comprehend the interplay between the three-dimensional structure of a biomolecule and its functionality. This study demonstrates the analysis of chiral molecules using the chirped-pulse Fourier transform microwave spectrometer. The first part of the thesis investigates the conformational analysis of flexible molecules with broadband rotational spectroscopy, while the second part showcases the applicability of its chirality-sensitive extension, microwave three-wave mixing, to manipulate and control gas-phase chiral molecules. The conformational landscape and internal rotation of six molecules with increasing structural complexity and flexibility were analyzed based on the recorded rotational spectra in the first part. The high sensitivity of the microwave spectroscopy technique allowed for the gas-phase structure determination of a substituted [2,2]-paracyclophane and α-methoxy phenylacetic acid. In the second part, a general approach using tailored microwave pulses is shown to address the limiting factors: thermal population and spatial degeneracy of the rotational levels involved in the energy level scheme for generating enantiomer-selective population transfer. Additionally, a non-linear and resonant microwave six-wave mixing approach is demonstrated for the manipulation of flexible chiral molecules like benzyl alcohol, whose enantiomers are inseparable using conventional chemical methods. These results showcase the high level of chiral control achievable in gas-phase samples using microwave spectroscopy and pave the way for future experiments aiming for chiral purification and precision measurements. Understanding these events at the molecular level brings us closer to unraveling the functionality of molecules in nature.

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Singh, H., 2024. Unravelling conformational flexibility and chirality in molecules using microwave spectroscopy.
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