Two Coupled Low-Barrier Large Amplitude Motions in 3,5-Dimethylanisole Studied by Microwave Spectroscopy

The microwave spectrum of 3,5-dimethylanisole was recorded using a pulsed molecular jet Fourier transform microwave spectrometer, covering the frequency range from 2.0 to 26.5 GHz. Splittings from internal rotations of the <i>syn</i>-<i>m</i> and <i>anti</i>-<i>m</i>-methyl groups were observed, analyzed, and modeled using the <i>XIAM</i> and the <i>ntop</i> programs for a data set including 622 rotational lines. The torsional barriers of the <i>syn</i>-<i>m</i> and <i>anti</i>-<i>m</i>-methyl groups were determined to be 58.62367(53) cm^-1 and 36.28449(69) cm^-1, respectively. The low barriers to internal rotation of both methyl groups posed significant challenges for spectral analysis and modeling. The successful assignment was achieved using combination difference loops and separately fitting the five torsional components. Comparing the torsional barriers observed in various toluene derivatives with methyl groups at <i>meta</i>-positions supports the assumption that electrostatic effects contribute more significantly than steric effects in the low-barrier cases of aromatic molecules.