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

Abstract

The molecular-beam Fourier transform microwave spectrum of 2-acetyl-5-methylfuran is recorded in the frequency range 2-26.5 GHz. Quantum chemical calculations calculate two conformers with trans or cis configuration of the acetyl group, both of which are assigned in the experimental spectrum. All rotational transitions split into quintets due to the internal rotations of two nonequivalent methyl groups. By using the program XIAM, the experimental spectra can be simulated with standard deviations within the measurement accuracy, and yield well-determined rotational and internal rotation parameters, inter alia the V potentials. Whereas the V barrier height of the ring-methyl rotor does not change for the two conformers, that of the acetyl-methyl rotor differs by about 100 cm . The predicted values from quantum chemistry are only on the correct order of magnitude.

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

“…The planar moment of inertia P cc =−2 ( I c − I a − I b )=13.396 u Å 2 of OMA confirms that the heavy atom skeleton is planar with two pairs of hydrogen atoms out of plane. These values are very similar as that found in other planar molecules containing two methyl groups such as the trans and cis conformers of 2‐acetyl‐5‐methylfuran ( P cc =12.938 and 13.160 u Å 2 , respectively) and the syn and anti conformers of 3,4‐dimethylbenzaldehyde ( P cc =12.904 and 12.938 u Å 2 , respectively) …”

“…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 …”

“…We choose this level, because it yields quite reasonable results for someo ther molecules containinga romatic rings like phenetole, [10] 2,5-dimethylthiophene, [17] and 2-acetyl-5-methylfuran. [18] Because the rotations of two methyl groups aboutt he C 4 À C 16 andt he O 11 ÀC 12 bond by varying the dihedral angles b = ff(C 5 ,C 4 ,C 16 ,H 19 )a nd g = ff(C 3 ,O 11 ,C 12 ,H 14 ), respectively,( for atom numbering see Figure 1) do not create new conformations, the conformational preferences of OMA are solely determined by the rotationo ft he methoxy group about the O 11 ÀC 3 bond by varying the dihedral angle a = ff(C 12 ,O 11 ,C 3 ,C 4 ). Ap otential curve was calculated where a was varied within a1 0 8 step width while all other geometry parameters were optimized.T he calculated energies wereparameterized using aFourierexpansion with the corresponding coefficients given in Ta ble S1 in the Supporting Information.…”

Sensing the Molecular Structures of Hexan‐2‐one by Internal Rotation and Microwave Spectroscopy

“…There are also some microwave spectroscopic studies on ketones with LAM(s) reported in the literature, i.a. methyl vinyl ketone, methyl isobutyl ketone and methyl neopentyl ketone, or some unsaturated ketones such as allyl acetone, and 2‐acetyl‐5‐methylfuran . The torsional barrier height of the acetyl methyl group in these ketones varies in a wide range from about 170 cm −1 to 440 cm −1 .…”

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

“…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.…”