Steric effects on two inequivalent methyl internal rotations of 3,4-dimethylfluorobenzene

International audienceThe microwave spectrum of 3,4-dimethylfluorobenzene was measured using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range from 2.0 to 26.5 GHz with the goal of quantifying the steric effects on the barriers to internal rotation of the two inequivalent methyl groups. Due to these torsional motions, splittings of all rotational transitions into quintets were observed and fitted with residuals close to measurement accuracy. The experimental work was supported by quantum chemical calculations, and the B3LYP-D3BJ/6-311++G(d,p) level of theory yielded accurate optimized geometry parameters to guide the assignment. The threefold potential values of 456.19(13) cm−1 and 489.77(15) cm−1 for the methyl groups at the meta and para position, respectively, deduced from the experiments are compared with the predicted values and those of other toluene derivatives