Two equivalent methyl internal rotations in 2,5-dimethylthiophene investigated by microwave spectroscopy

Abstract

The microwave spectrum of 2,5-dimethylthiophene, a sulfur-containing five-membered ring with two conjugated double bonds, was recorded in the frequency range from 2 to 40 GHz using molecular beam Fourier transform technique. Highly accurate molecular parameters were determined. A labeling scheme for the group G36 written as the semi-direct product (C(I)(3) × C(I)(3)) ⋊ C(2v) was introduced.

Interplay Between Microwave Spectroscopy and X‐ray Diffraction: The Molecular Structure and Large Amplitude Motions of 2,3‐Dimethylanisole

“…A potential curve was calculated, where the corresponding dihedral angle d = ff(C 6 ÀC 1 ÀO 14 ÀC 15 ) was varied in a grid of 108, while all other geometry parameters were optimized. [10][11][12] However, for some previously studied molecules containing aromatic rings such as OMA, [3] 2,5-dimethylthiophene, [6] and phenetole, [13] harmonic frequency calculations yielded one imaginary vibrational mode, which is a bending vibration of the phenyl ring. [9] In almost all of our recent investigations, the MP2/6-311 + +G(d,p) level was applied.…”

“…Since decades, this is proven by the fact that many analysis methods have been developed to determine the three-dimensional structure of a molecule. Only a very limited number of microwave spectroscopic investigations on similar aromatic systems with two internal rotors are reported in the literature, e. g. 2,5-dimethylfurane, [5] 2,5-dimethylthiophene, [6] 2-acetyl-5-methylfuran, [7] and dimethylbenzaldehyde. In the gas phase, molecular jet Fourier transform microwave (FTMW) spectroscopy is the most suitable tool for this purpose.…”

Methyl Internal Rotation in the Microwave Spectrum of o‐Methyl Anisole

“…If not stated otherwise, the MP2/6–311++G(d,p) level of theory is used. We choose this level, because it yields quite reasonable results for some other molecules containing aromatic rings like phenetole, 2,5‐dimethylthiophene, and 2‐acetyl‐5‐methylfuran …”

The Structure and Torsional Dynamics of Two Methyl Groups in 2‐Acetyl‐5‐methylfuran as Observed by Microwave Spectroscopy

“…While the steric influence is rather local, the electronic contribution can arise from quite distant sources in the molecule, especially when conjugated double bonds and/or aromatic systems are involved. Predicting torsional barriers is challenging because chemical intuition often fails and quantum chemical calculations are still rather inaccurate . On the other hand, microwave spectroscopy yields highly accurate torsional barriers.…”