Conformational Effect on the Large Amplitude Motions of 3,4-Dimethylanisole Explored by Microwave Spectroscopy

The microwave spectrum of 3,4-dimethylanisole, a molecule containing three methyl groups allowing for internal rotation, was recorded using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range from 2.0 to 26.5 GHz. Quantum chemical calculations yielded two conformers with an  anti and a syn configuration of the methoxy group, both of which were assigned in the experimental spectrum. Torsional splittings due to the internal rotations of two methyl groups attached to the aromatic ring were resolved and analyzed. The rotational-torsional transitions could be reproduced to measurement accuracy, yielding well-determined rotational and internal rotation parameters. The torsional barriers of the methyl groups at the meta and para position were deduced to be 430.00(37) and 467.90(17) cm^-1, respectively, for the syn-conformer. The respective values for the anti-conformer are 499.64(26) and 533.54(22) cm^-1. A labeling scheme for the G₁₈ group written as the semidirect product ( C₃ ^I ⊗ C₃ ^I) (× C _s was introduced.