Benchmarking acetylthiophene derivatives: methyl internal rotations in the microwave spectrum of 2-acetyl-5-methylthiophene

• Published in: Physical chemistry chemical physics : PCCP Vol. 25; no. 1; pp. 59 - 519
• Main Authors: Dindi, Christina, Nguyen, Ha Vinh Lam
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 21.12.2022
• Subjects: Aromatic compounds; Benchmarks; Carbonyl groups; Carbonyls; Chemical Sciences; Fourier transforms; Frequency ranges; Ketones; Physics; Quantum chemistry; Spectrum analysis
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d2cp03897h

The microwave spectrum of 2-acetyl-5-methylthiophene (2A5MT) was recorded using a molecular jet Fourier transform microwave spectrometer working in the frequency range of 2 to 26.5 GHz. The spectrum was assigned to the syn -conformer of the molecule while that of anti -2A5MT was not observable. For the assignment of the spectrum of 2A5MT, adequate spectral analysis skill and quantum chemical benchmarking helped to significantly reduce the time required for recording survey scans. The rotational and centrifugal distortion constants were determined with high accuracy. The experimental values of the rotational constants are compared to those derived from quantum chemical calculations in the course of ongoing benchmarking effort. Splitting of each rotational transition into quintets due to internal rotations of the acetyl methyl and ring methyl groups could be resolved and analysed to yield barriers to internal rotations of 301.811(41) cm −1 and 157.2612(13) cm −1 , respectively. These values are compared to those found in other thiophene and furan derivatives in order to understand the electronic effects transmitted through aromatic rings, as well as how different heteroatoms affect torsional barriers. The acetyl methyl group features torsional barriers of around 300 cm −1 if a thiophene derivative is attached at the other side of the carbonyl group. This finding allows the establishment of the so-called "thiophene class" for the acetyl group containing ketones. The time required for recording survey scans of the microwave spectrum of the title molecule with two inequivalent methyl internal rotors was significantly reduced with the help of adequate spectral analysis skill and quantum chemical benchmarking.