2-Propionylthiophene: planar, or not planar, that is the question

The long-standing ambiguity of the molecular planarity when an alkyl group is attached on a system with conjugated double bonds is a great challenge for both experiments and theory. This also holds for the case of 2-propionylthiophene (2PT) where a propionyl group is attached at the second position of the planar, aromatic thiophene ring. Results from quantum chemistry at the MP2 level of theory, showing that in the two conformers syn-Cs and anti-Cs the ethyl group of the propionyl moiety is slightly tilted out of the thiophene ring plane, conflict with those from the other methods, stating that the ethyl group is in-plane with the thiophene ring. In the microwave spectrum, both syn-Cs-2PT and anti-Cs-2PT were observed, and their geometry parameters such as the rotational and quartic centrifugal distortion constants were precisely determined. The experimental heavy atom skeleton obtained by isotopic substitutions revealed a tiny, but non-zero tilt angle of the ethyl group out of the thiophene plane, thereby convincingly confirming the non-planarity of 2-propionylthiophene. The results were further supported by internal defects calculated from the experimental rotational constants. Finally, splittings arising from the internal rotation of the terminal methyl group were analysed, yielding torsional barriers of 807.3(11) cm-1 and 864.5(88) cm-1 for the two observed conformers, respectively.