Microwave Spectrum of Two‐Top Molecule: 2‐Acetyl‐3‐Methylthiophene

Abstract

The microwave spectrum of 2-acetyl-3-methylthiophene (2A3MT) was recorded in the frequency range from 2 to 26.5 GHz using a molecular jet Fourier transform microwave spectrometer and could be fully assigned to the anti-conformer of the molecule, while the syn-conformer was not observable. Torsional splittings of all rotational transitions in quintets due to internal rotations of the acetyl methyl and the ring methyl groups were resolved and analyzed, yielding barriers to internal rotation of 306.184(46) cm À 1 and 321.813(64) cm À 1 , respectively. The rotational and centrifugal distortion constants were determined with high accuracy, and the experimental values are compared to those derived from quantum chemical calculations. The experimentally determined inertial defect supports the conclusion that anti-2A3MT is planar, even though a number of MP2 calculations predicted the contrary.

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

“…For syn -2AT, we found a barrier height of about 330.2 cm −1 for the parent species, which remains almost constant for all isotopologues. A slightly lower, yet similar value of about 296 cm −1 was obtained for anti -2AT as well as its 34 S isotopologue, which is close to the value of 306.2 cm −1 found for the anti -conformer of 2-acetyl-3-methylthiophene 65 (see (Fig. 7)).…”

“…2. This behaviour of the MP2/6-311++G(d,p) approximation was frequently observed in ketones, such as n -alkyl ketones, 4–7 2-propionylthiophene, 51 2-acetyl-3-methylthiophene, 65 pinacolone, 69 and diethyl ketone. 70 However, since the barrier between these configurations is not significant (essentially lower than the first vibrational state), the S1–C2–C6O7 fragment should be considered as quasi-planar in this particular prediction.…”

“…The microwave spectra of two other thiophene derivatives related to 2AT (molecule ( 2 ) in Fig. 7), 2-thiophenecarboxaldehyde ( 1 ) 61 and 2-acetyl-3-methylthiophene ( 3 ) 65 , have been reported in the literature. All three molecules exhibit two stable conformers with the S–C bond and the carbonyl group in anti or syn position, with the syn -conformer being more stable (see Fig.…”

“…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.…”

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

“…The furan and thiophene derivatives illustrated in Figure 11 all possess a methyl group attached to the ring and an acetyl methyl group. (1) anti-2-acetyl-3-methylthiophene [193], (2) 2-acetyl-4-methylthiophene [194], (3) syn-2-acetyl-5methylthiophene [195], (4) 2-acetyl-5-methylfuran [86].…”