Surprising torsional barrier reduction in the coupled methyl internal rotations of 2,3-dimethylfuran observed by microwave spectroscopy

The microwave spectrum of 2,3-dimethylfuran was investigated using a Fourier-transform microwave spectrometer under supersonic expansion. The molecule possesses two inequivalent methyl internal rotors, causing the splitting of each rotational transition into five torsional species. A total of 337 torsional transitions were assigned and fitted in a global fit with the program XIAM, where accurate and physically meaningful geometry and internal rotation parameters could be deduced. The different torsional species were also fitted separately with the program SFLAMS to validate the assignment. Both the global fit and the separate fits achieved standard deviations close to the measurement accuracy. The barriers to internal rotation of the ortho- and meta-methyl rotors, which are in direct proximity, were determined to be 298.274(12) cm-1 and 237.6891(47) cm-1, respectively. These values are radically lower than the respective barriers of 412.9 cm-1 and 380.5 cm-1 found for the steric-free methyl groups in 2-methylfuran and 3-methylfuran. This observation appears to be driven primarily by electrostatic effects rather than being adequately accounted for by steric effects. The experiments were accompanied by quantum chemical calculations. Benchmarking the rotational constants revealed that the MP2/6-31G(d,p) level of theory might be helpful to guide the microwave spectral assignment of methylfuran derivatives.