Communication through the furan ring: the conformational effect on the internal rotation of 5-methyl furfural studied by microwave spectroscopy

Abstract

Internal rotation is a fundamental motion of methyl groups that provides important insights into the molecular physics of isolated molecules. The barrier heights of such large amplitude motions are highly sensitive to their molecular and electronic environment. To date, it is still not possible to accurately determine these values using quantum chemical calculations. To probe the effect of molecular conformations on the barrier heights of substituted furan rings, the molecular jet Fourier transform microwave spectrum of 5-methyl furfural was recorded in the frequency range from 2.0 to 40.0 GHz. Quantum chemical calculations yielded two conformers with a trans and a cis orientation of the formyl group, which were both observed in the experimental spectrum. Torsional splittings due to the internal rotation of the methyl group were resolved and analyzed. The experimental spectrum is reproduced with standard deviations close to the experimental accuracy, yielding sets of highly accurate rotational and internal rotation parameters. The results, especially the V3 potentials, are compared to quantum chemical calculations and discussed within the scope of the current literature of other methyl substituted furans, where the methyl group is in close proximity of the furan oxygen atom. The present work provides an accurate evaluation of the different case studies and highlights the bottlenecks and future options of the currently available theoretical techniques.

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

“…7). Compared to the observation found for 2-acetylfuran ( 5 ) and its related molecules furfural ( 4 ), 66 methyl furfural 67 and 2-acetyl-5-methylfuran ( 6 ), 68 we found that the heteroatom in the ring significantly affects the conformational stability. The syn -conformer of 2-acetylfuran ( 5 ) is much higher in energy than the anti -conformer.…”

The heavy atom structures and 33S quadrupole coupling constants of 2-thiophenecarboxaldehyde: insights from microwave spectroscopy

“…The negative charges on the oxygen atoms of furfural (2) stabilizes the anti-conformer, while the syn-form is favored by the opposite charges on oxygen and sulfur or selenium atoms in 2TPC (1) and 2SeC (4) [67], as visualized also in Figure 8 with the charge data obtained from NBO calculations. A further example to support this argument is the stability of the anti-form in two furfural derivatives, methyl furfural (5) [68] and 2-acetyl-5-methylfuran ( 6) [69], where the anti-conformer (called cis in ref. [68]) is also 1.1 kJmol 1 and 4.19 kJmol 1 , respectively, lower in energy than the syn conformer (called trans in ref.…”

“…A further example to support this argument is the stability of the anti-form in two furfural derivatives, methyl furfural (5) [68] and 2-acetyl-5-methylfuran ( 6) [69], where the anti-conformer (called cis in ref. [68]) is also 1.1 kJmol 1 and 4.19 kJmol 1 , respectively, lower in energy than the syn conformer (called trans in ref. [68]).…”

“…[68]). We note that in the case of methyl furfural (5), a methyl substitution (electron donor group) in the 5-position of the ring decreases the energy difference between the two conformers from about 3.4 kJmol 1 in the case of furfural (2) [60] to 1.1 kJmol 1 [68].…”

The Scent of Maibowle – π Electron Localization in Coumarin from Its Microwave‐Determined Structure

“…The basis set was chosen to be either Dunning's cc-pVTZ [6] or Pople's 6-311+G(d,p) and 6-311++G(d,p) [7]. From our own experiences [8][9][10][11] and studies in the literature [12][13][14][15], the MP2/6-311G++(d,p) level of theory has proven its reliability in calculating rotational constants of aromatic-ring containing molecules for assignment purposes. Therefore, quantum chemical calculations at this level were carried out with the Gaussian 16 program [16] and the ab initio rotational constants A = 2819.5 MHz, B = 880.0 MHz, C = 670.7 MHz were used as starting values to predict the microwave spectrum of coumarin.…”