Two methyl internal rotations of 2-acetyl-4-methylthiophene explored by microwave spectroscopy and quantum chemistry

Christina Dindić , Mike Barth , Ha Vinh Lam Nguyen (1)

1 LISA (UMR_7583) - Laboratoire Interuniversitaire des Systèmes Atmosphériques

The microwave spectrum of 2-acetyl-4-methylthiophene (2A4MT) was recorded in the frequency range from 2 to 26.5 GHz using a molecular jet Fourier transform microwave spectrometer, revealing two conformers, syn and anti. Both methyl groups in the molecule, the acetyl methyl and the ring methyl groups, undergo internal rotation, causing resolvable splittings of all rotational transitions into quintets. The torsional barriers determined for the acetyl methyl and the ring methyl rotors are 324.919(94) cm^-1 and 210.7181(61) cm^-1 for the syn conformer; the respective values for anti-2A4MT are 281.201(17) cm^-1 and 212.9797(41) cm^-1. The experimentally deduced rotational constants and torsional barriers are compared to values obtained from quantum chemical calculations. The barriers to methyl internal rotation are also compared to those of related molecules in order to establish a "thiophene class" concerning the acetyl methyl torsion.

Domaines: Planète et Univers [physics]

Christina Dindić, Mike Barth, Ha Vinh Lam Nguyen. Two methyl internal rotations of 2-acetyl-4-methylthiophene explored by microwave spectroscopy and quantum chemistry. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy [1994-..], 2022, 280, ⟨10.1016/j.saa.2022.121505⟩. ⟨insu-03749600⟩