Microwave Spectrum of Two‐Top Molecule: 2‐Acetyl‐3‐Methylthiophene

• Published in: Chemphyschem 2021-12, Vol.22 (23), p.2420-2428
• Main Authors: Dindić, Christina, Nguyen, Ha Vinh Lam
• Format: Article
• Language: English
• Subjects: Chemical Sciences; density functional calculations; Fourier transforms; Frequency ranges; large amplitude motion; Mathematical analysis; microwave spectroscopy; or physical chemistry; Quantum chemistry; rotational spectroscopy; Theoretical and; thiophene derivatives
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.202100514

The microwave spectrum of 2‐acetyl‐3‐methylthiophene (2A3MT) was recorded in the frequency range from 2 to 26.5 GHz using a molecular jet Fourier transform microwave spectrometer and could be fully assigned to the anti‐conformer of the molecule, while the syn‐conformer was not observable. Torsional splittings of all rotational transitions in quintets due to internal rotations of the acetyl methyl and the ring methyl groups were resolved and analyzed, yielding barriers to internal rotation of 306.184(46) cm−1 and 321.813(64) cm−1, respectively. The rotational and centrifugal distortion constants were determined with high accuracy, and the experimental values are compared to those derived from quantum chemical calculations. The experimentally determined inertial defect supports the conclusion that anti‐2A3MT is planar, even though a number of MP2 calculations predicted the contrary. Torsion‐rotational spectrum of 2‐acetyl‐3‐methylthiophene determined using microwave spectroscopy – quantum chemistry duet. The anti‐conformer, being energetically more stable than the syn‐form, is ultimately the only one observed in the experimental spectrum. Internal rotations arising from two inequivalent methyl groups are analyzed to yield highly accurate torsional barriers whose values are strongly influenced by steric effects.