Structures and internal dynamics of ketones studied by microwave spectroscopy and quantum chemistry

  • Strukturen und interne Dynamik von Ketonen untersucht mittels Mikrowellenspektroskopie und Quantenchemie

Andresen, Maike; Lüchow, Arne (Thesis advisor); Cuisset, Arnaud (Thesis advisor); Schmitt, Michael (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021. - Dissertation, Université Paris-Est, 2021

Abstract

In this dissertation, diverse ketones containing an acetyl methyl group were studied by microwave spectroscopy with the main focus on the effects of internal rotations. As a result a “barrier class system” was introduced, which firmly links the barrier to internal rotation of the acetyl methyl group to the characteristic structure of the molecule. The acetyl methyl barrier thus functions as a “structural detector”. The analysis was supplemented by quantum chemical calculations of molecular parameters. For a series of methyl n-alkyl ketones, including pentan-2-one, hexan-2-one, heptan-2-one and octan-2-one, at least two conformers could be identified in the spectrum of each of these aliphatic ketones: one with C1 and one with Cs symmetry. The barriers to internal rotation of the acetyl methyl group of the C1 conformers were determined to always be about 240 cm−1. In case of the Cs conformers, the barrier heights are found to be approximately 180 cm−1. These conformers are hence members of the “C1” and “Cs class”, respectively. Only for hexan-2-one, a third conformer showing another specific C1 structure and a barrier height of 182 cm−1 was assigned. Furthermore, spectral splittings caused by the internal rotation of the methyl group at the end of the respective alkyl chain were observed. Here, the barriers are about 1000 cm−1 for each conformer. E-3-Penten-2-one and 3-methyl-3-buten-2-one are α,β-unsaturated ketones and members of the “mesomeric class”. They exhibit complicated splitting patterns in the spectrum arising due to the internal rotation of the acetyl methyl group, as well as of the methyl group attached to the propenyl or isopropenyl group, respectively. For E-3-penten-2-one, two conformers were assigned, one with antiperiplanar (ap) and one with synperiplanar (sp) structure. In case of 3-methyl-3-buten-2-one, only the ap conformer could be identified. The acetyl methyl torsional barriers range from 350 cm−1 to 520 cm−1, while they are about 600 cm−1 for the propenyl and isopropenyl methyl groups. In the microwave spectrum of 3-methylbutan-2-one, a branched ketone, additional splittings beside those due to internal rotation were observed. They are probably caused by a tunnelling motion between two enantiomeric versions of the molecule. Therefore, 3-methylbutan-2-one is a member of the “tunnelling class”.

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