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

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.