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

Abstract

The microwave spectra of thiophene and 2-acetylthiophene were recorded in the frequency range from 2 to 40 GHz using two molecular jet Fourier transform microwave spectrometers. For 2-acetylthiophene, two conformers...

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

“…3 also shows that the other rotor oscillates with an angle of less than 11, indicating no coupling between the two rotors. The acetyl oscillation frequently observed in similar molecules 23,24,45 is also absent.…”

“…Only the most stable of the two expected conformers was observed in the microwave spectrum of 2A5MT, confirming the higher energetic stability predicted for the syn form compared to the anti-form. The preference for the syn-conformer has already been established in other thiophene derivatives such as 2-thiophenecarboxaldehyde, 61,62 2-acetylthiophene, 45 and the isomer 2A4MT 24 of 2A5MT. This was explained due to dipole-dipole interactions between the positively charged sulphur atom in the thiophene ring and the negatively charged oxygen of the carbonyl group, 24,45,61 which also holds true for 2A5MT, as shown in the 3.…”

“…The preference for the syn-conformer has already been established in other thiophene derivatives such as 2-thiophenecarboxaldehyde, 61,62 2-acetylthiophene, 45 and the isomer 2A4MT 24 of 2A5MT. This was explained due to dipole-dipole interactions between the positively charged sulphur atom in the thiophene ring and the negatively charged oxygen of the carbonyl group, 24,45,61 which also holds true for 2A5MT, as shown in the 3. The grey sections were not scanned.…”

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

“…The conjugated π-double-bond system is extended by the carbonyl group C=O. The methyl group can be directly attached to the aromatic ring in the case of a formyl substituent as in 5-methyl furfural (3) [134], or be involved in the substituted moiety in the case of an acetyl substituent such as in acetyl thiophene (1) [132] or acetyl furane (2) [133]. A two-top molecule is also given in this section, 2-acetyl-5-methylfuran (4) [86], combining a methyl rotor attached on the aromatic ring as in 5-methyl furfural (3) and a methyl rotor in the acetyl moiety as in acetyl furan (2) for comparison.…”

“…We found the same situation while comparing the molecules in Figure 12. In anti-2acetyl-4-methylthiophene (6) [194] (−14.8 cm −1 compared to anti-2-acetylthiophene (1) [132]), syn-2-acetyl-5-methylthiophene (7) [195] (−28.4 cm −1 compared to syn-2-acetylthiophene (2) [132]), syn-2-acetyl-5-methylfuran (8) [86] (−27.1 cm −1 compared to syn-2-acetylfuran (3) [133]), the acetyl methyl and ring methyl group lie on a straight chain (marked by a dotted line in Figure 12), and the torsional barrier of the acetyl methyl rotor decreases more than it does for anti-2-acetyl-5-methylfuran (9) (−12.0 cm −1 compared to anti-2-acetylfuran (4)). In anti-2-acetyl-3-methylthiophene (5) [193], the additional methyl group at the 3-position of the ring makes the molecule more globular than anti-2-acetylthiophene (1).…”