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

The microwave spectrum of 2-acetyl-5-methylthiophene (2A5MT) was recorded using a molecular jet Fourier transform microwave spectrometer working in the frequency range of 2 to 26.5 GHz. The spectrum was assigned to the syn-conformer of the molecule while that of anti-2A5MT was not observable. For the assignment of the spectrum of 2A5MT, adequate spectral analysis skill and quantum chemical benchmarking helped to significantly reduce the time required for recording survey scans. The rotational and centrifugal distortion constants were determined with high accuracy. The experimental values of the rotational constants are compared to those derived from quantum chemical calculations in the course of ongoing benchmarking effort. Splitting of each rotational transition into quintets due to internal rotations of the acetyl methyl and ring methyl groups could be resolved and analysed to yield barriers to internal rotations of 301.811(41) cm-1 and 157.2612(13) cm-1, respectively. These values are compared to those found in other thiophene and furan derivatives in order to understand the electronic effects transmitted through aromatic rings, as well as how different heteroatoms affect torsional barriers. The acetyl methyl group features torsional barriers of around 300 cm-1 if a thiophene derivative is attached at the other side of the carbonyl group. This finding allows the establishment of the so-called "thiophene class" for the acetyl group containing ketones.

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

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.

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

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 with a syn and an anti orientation of the S1-C2 and C6O bonds (with respect to the C2-C6 bond) were identified, and the syn-conformer was more stable. The spectra of the 34S- and 13C-isotopologues of syn-2-acetylthiophene were also assigned, and the semiexperimental equilibrium structure could be determined. Compared to thiophene, at the substitution position, the S1-C2 and C2C3 bond lengths both increase by about 0.007 Å, and the bond angle S1-C2C3 decreases by 0.06°, noticeably larger than the experimental uncertainties. A-E torsional splittings were observed due to internal rotation of the methyl group hindered by a barrier height of 330.187(35) and 295.957(17) cm-1 for the syn-conformer and the anti-conformer, respectively. Geometry and internal rotation parameters are compared with those of related thiophene derivatives, as well as those of furan and 2-acetylthiophene to gain a better understanding of structure determination in the presence of methyl internal rotation.

Microwave Spectrum of Two-Top Molecule: 2-Acetyl-3-Methylthiophene.

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-1 and 321.813(64) cm-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.

Equilibrium Structure in the Presence of Methyl Internal Rotation: Microwave Spectroscopy and Quantum Chemistry Study of the Two Conformers of 2-Acetylfuran.

For 2-acetylfuran, quantum chemistry predicted and proton magnetic resonance study reported two conformers, anti and syn, differing in the position of the carbonyl group with respect to the O1-C2 bond of the furan ring. The microwave spectrum of the title molecule was recorded in the frequency range from 2 to 26.5 GHz using a molecular jet Fourier transform microwave spectrometer, confirming the presence of both conformers. Spectroscopic parameters such as the rotational and centrifugal distortion constants could be determined with high precision. The spectra of all 13C- and 18O-isotopologues of the energetically more favorable anti-conformer could be assigned, allowing the experimental determination of bond lengths and bond angles from the heavy atom substitution rs and the semi-experimental equilibrium reSE structures. Splittings arising from the internal rotation of the acetyl methyl group could be resolved for both conformers as well as for all assigned isotopologues, from which the barrier to methyl internal rotation was determined. The torsional barrier is largely invariant at around 319 cm-1 in the parent species of anti-2-acetylfuran and its isotopologues, showing that though isotopic substitution greatly influences the rotational properties of the molecule and causes a different microwave spectrum, its effect on the methyl torsion is negligible. On the other hand, conformational effects play a decisive role, as the torsional barrier of 239.780(13) cm-1 found for syn-2-acetylfuran differs significantly from the value for anti-2-acetylfuran. The results are compared and discussed with other methyl-substituted furan derivatives and acetyl group containing ketones for a better understanding of different effects influencing molecular geometry parameters and methyl internal rotations.

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

The long-standing ambiguity of the molecular planarity when an alkyl group is attached to a system with conjugated double bonds is a great challenge for both experiments and theory. This also holds true 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- and anti-2PT 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- and anti-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. This conclusion was further supported by the inertial 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 806.94(54) cm-1 and 864.5(88) cm-1 for the two observed conformers, respectively.

Conformational transformations of sulfur-containing rings: 2-methyltetrahydrothiophene gas-phase structures.

Stable conformations of five-member rings with the prototype cyclopentane are well-known to exist as twist or envelope structures and are of general interest in chemistry. Here, we report on the conformational analysis of the sulfur-containing ring 2-methyltetrahydrothiophene studied by a combination of molecular beam Fourier transform microwave (MB-FTMW) spectroscopy and quantum chemistry. Two twist conformers were observed, whereby highly accurate molecular parameters could be determined. In addition, the (34) S-isotopologue of the most stable conformer was assigned in natural abundances. Geometry optimizations were performed at different levels of theory and the calculated rotational constants were compared with experimental values. Two transition states optimized at the MP2/6-311++G(d,p) level using the Berny algorithm could illustrate the intramolecular conversion between both conformers.