The 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 three-fold 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.

The microwave spectra of two dimethylfluorobenzene isomers, 2,6-and 3,4-, were measured using two pulsed molecular jet Fourier transform microwave spectrometers operating in the frequency range from 2.0 to 40.0 GHz with the goal of quantifying the steric effects on the barriers to internal rotation of the two methyl groups. Due to the internal rotations of two equivalent methyl groups in 2,6-dimethylfluorobenzene a (26DMFB), all rotational transitions split into quartets, while quintets appear for 3,4-dimethylfluorobenzene b (34DMFB) because its two methyl groups are inequivalent. The splittings were analyzed and modeled to deduce a torsional barrier of 236.7922(21) cm −1 for the two methyl groups in 26DMFB and of 456.20(13) cm −1 and 489.78(15) cm −1 for the methyl groups at the meta and para positions, respectively, in 34DMFB. Each torsional species was fitted separately using odd power order parameters to check the correctness of the assignment. For both isomers, a global fit obtained with the program XIAM c has achieved a standard deviation close to the measurement accuracy. The experimental results are compared with the predicted values obtained by quantum chemical calculations and those of other toluene derivatives.