Chiral analysis of molecules with multiple chiral centers using chiral tag rotational spectroscopy

Pate, Brooks

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https://hdl.handle.net/2142/104388 Copy

Description

Title

Chiral analysis of molecules with multiple chiral centers using chiral tag rotational spectroscopy

Author(s)

Pate, Brooks

Contributor(s)

• Evangelisti, Luca
• West, Channing
• Mayer, Kevin J.
• Sonstrom, Reilly E.

Issue Date

2019-06-17

Keyword(s)

Non-covalent interactions

Abstract

One major challenge in analytical chemistry is the quantitative determination of the ratio of all stereoisomers in a molecule with multiple chiral centers. The analysis is especially challenging if there are no reference samples available for calibration. In general, a molecule with N chiral centers has 2$^{N}$ stereoisomers. There are 2N-1 distinct structures, the diastereomers, that can be distinguished by traditional rotational spectroscopy. Each diastereomer exists in two non-superimposable mirror images – the enantiomers. Menthone is a simple case of a molecule with two asymmetric carbons. The two diastereomers are known as menthone and isomenthone and each is chiral. The full analysis of the stereoisomers of several commercial samples has been performed using chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW). To determine the diastereomer ratio it is necessary to determine all conformers of the molecule with appreciable population in the pulsed jet sample. The present work expands on previous efforts to assign the lowest conformers of the menthone and isomenthone by analyzing two additional conformers of isomenthone.[1] For a commercial sample where the diastereomer ratio was reported in the Certificate of Analysis, we obtained a menthone and isomenthone composition of 82.8+/-0.9% and 17.2+/-0.9% compared to 81% and 19% in the COA. The rotational spectroscopy analysis uses dipole moment components from quantum chemistry and did not use any other calibration. The enantiomeric excess (EE) of menthone and isomenthone was determined by chiral tagging with 3-butyn-2-ol. The identities of the chiral tag complexes produced in the pulsed jet expansion were determined using computational methods calculated at the B3LYP D3BJ / def2tzvp level of theory. All commercial samples had high enantiopurity of menthone, and isomenthone. One sample had an EE determination reported in the CoA and, for that sample, the EE determination from chiral tag rotational spectroscopy is in agreement with the reported value: Chiral Tag: EE=90.5+/-0.3% and CoA: EE=90%. The current results show major deviations from the previous study of the EE of menthone by three-wave mixing rotational spectroscopy.[2] [1] Schmitz, D.; Shubert, V.A.; Betz, T.; Schnell, M., “Exploring the conformational landscape of menthol, menthone, and isomenthone: a microwave study”, Frontiers in Chemistry 3, 1-13 (2015). [2] Shubert, V.A., Schmitz, D, Schnell, M., “Enantiomer-sensitive spectroscopy and mixture analysis of chiral molecules containing two stereogenic centers – Microwave three-wave mixing of menthone”, J. Mol. Spectrosc. 300, 31-36 (2014).

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

text

Language

eng

Permalink

http://hdl.handle.net/2142/104388

DOI

https://doi.org/10.15278/isms.2019.MH10

Copyright and License Information

Copyright 2019 Brooks Pate

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