The two-top molecule 3-penten-2-one: Acetyl methyl torsion in α,β-unsaturated ketones

The LAM of the Rings: Large Amplitude Motions in Aromatic Molecules Studied by Microwave Spectroscopy

“…Therefore, a V 3 potential is dominant; however, the torsional barrier is still affected, being very low. The same explanation can be made for the oxygen atom and the nitrogen atom in 2-methyloxazole (11) [107], or the NH group and the nitrogen atom in 2-methylimidazole (21) [114]. Probably, the electronic surroundings of a nitrogen atom is more similar to that of a sulfur atom than that of an NH group or an oxygen atom.…”

“…The number of studies significantly decreases if multiple methyl internal rotors are present in the molecule. While for two-top cases we can still find some dozens of investigations, for example 3-penten-2-one [21], N-acetyl alanine methyl ester [22], and N-methylacetamide [23], only a handful of molecules featuring three methyl rotors were reported in the literature [24][25][26]. We found only one four-top molecule, 2,3,4,5tetramethylthiophene, in an on-going study [3,27].…”

Determination of the semiexperimental equilibrium structure of 2-acetylthiophene in the presence of methyl internal rotation and substituent effects compared to thiophene

“…7)). Concerning the establishment of a possible “thiophene class” in the categorisation of ketones containing an acetyl methyl group, 7,9 the results obtained from 2AT are very promising. However, since there are currently only two data points, 2AT and 2-acetyl-3-methylthiophene, 65 where the syn -conformer is not observable in the latter molecule, a conclusive classification is not attempted at this stage.…”

“…4–7 The torsional barrier increases significantly to about 600 cm −1 in acetophenone derivatives where a phenyl ring is attached to the carbonyl group 8 or to 430 cm −1 for the antiperiplanar conformers and 350 cm −1 for the synperiplanar conformers of α,β-unsaturated ketones. 9 Recently, we investigated 2-acetylfuran where we observed a barrier height of 320 cm −1 for the anti -conformer and 240 cm −1 for the syn -conformer; 10 both values do not fit in any of the categories classified by Andresen et al 7,9 This suggests that if an aromatic heterocycle is involved in conjugation with the carbonyl group, special classes might be required. Therefore, we are interested in 2-acetylthiophene (2AT, illustrated in Fig.…”

“…On the other hand, 2AT will yield a data point as the prototype molecule towards establishing a possible “thiophene class” to predict methyl torsional barriers for acetyl group containing ketones. 7,9…”