Microwave spectroscopic and quantum chemical investigations on aromatic five-membered heterocycles with a carbonyl substituent

  • Mikrowellenspektroskopische und quantenchemische Untersuchungen an aromatischen fünfgliedrigen Heterocyclen mit einem Carbonyl Substituenten

Dindic, Christina; Nguyen, Ha Vinh Lam (Thesis advisor); Lüchow, Arne (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022

Abstract

In this dissertation a combination of molecular jet Fourier transform microwave spectroscopy and quantum chemical calculations was employed to analyse the structural characteristics and internal dynamics, in particular methyl internal rotation, of five thiophene and one furan derivative, all of them containing one substituent with a carbonyl group. The molecular targets are 2-propionylthiophene, 2-acetylthiophene, three isomers of 2-acetylmethylthiophene, and 2-acetylfuran. For 2-propionylthiophene two conformers, syn and anti, were identified in the spectrum with torsional barriers of 807 cm−1 and 865 cm−1 for the syn- and the anti-conformer, respectively. The planarity of the molecule was controversial in calculations at different levels of theory. Therefore, three 13C-isotopologues in the propionyl group and the 34S-isotopologue in the ring were measured, enabling a partial rs structure determination of the most stable syn-conformer. This structure determination confirmed that the molecule is not planar, as there is a small, but non-zero tilt angle between the ethyl group and the thiophene ring. For 2-acetylthiophene two conformers were also identified in the spectrum, alongside the 34S-isotopologue of both conformers, all 13C-isotopologues of the energetically more stable syn-conformer, and some 13C-isotopologues of the anti-conformer. The barriers of internal rotation were determined to be 330 cm−1 and 296 cm−1 for the syn- and the anti-conformer, respectively. They remained almost constant for all isotopologues of a conformer. 2-acetylfuran was investigated to study the effects of the heteroatom in the ring compared to 2-acetylthiophene. A syn- and an anti-conformer with barriers to internal rotation of 240 cm−1 and 320 cm−1 were identified in the spectrum alongside several isotopologues with essentially unchanged barriers. A comparison with the sulphur analogue, 2-acetylthiophene, showed, that the heteroatom in the ring significantly influences the conformational stability. Furans favour the anti-conformation, while in thiophenes the syn form is more stable due to the different electrostatic interactions between the heteroatom in the aromatic ring and the oxygen atom of the acetyl group. While 2-propionylthiophene, 2-acetylthiophene and 2-acetylfuran are molecules featuring internal rotation of only one methyl rotor, the three isomers of 2-acetylmethylthiophene are two-top molecules. They were investigated to study effects of multiple rotors on the microwave spectra and the influence of a methyl group attached to the aromatic thiophene ring on the methyl internal rotation of 2-acetylthiophene. Results of the present thesis together with those of previous investigations on thiophene and furan derivatives have shown that the negative mesomeric and inductive effects of the carbonyl moiety in the acetyl group cause a decrease of the torsional barrier of the methyl rotor attached to the ring (called the ring methyl top), while a methyl group causes an increase due to its positive inductive effect when substituted at the same position. Steric interactions can influence these effects. The conformation of the acetyl group barely influences the torsional barrier of the ring methyl top. The acetyl methyl group possesses a unique sensitivity to the structure of the moiety attached at the other side of the carbonyl bond. From the results of the present thesis the so-called "thiophene class" was established, which collects all molecules featuring an acetyl group connected to a thiophene ring at the other side of the carbonyl bond. It was shown that the torsional barriers of the acetyl methyl group are always around 300 cm−1, serving as a very reliable estimation for future investigations.

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