Low Barrier Methyl Internal Rotations and ¹⁴N Quadrupole Coupling in the Microwave Spectrum of 2,4-Dimethylthiazole

The microwave spectrum of 2,4-dimethylthiazole was recorded using a pulsed molecular jet Fourier-transform microwave spectrometer operating in the frequency range from 2.0 to 26.5 GHz. Torsional splittings into quintets were observed for all rotational transitions due to internal rotations of two inequivalent methyl groups. Hyperfine structures arising from the nuclear quadrupole coupling of the ¹⁴N nucleus were fully resolved. The microwave spectra were analyzed using the modified version of the XIAM code and the BELGI-C_s-2Tops-hyperfine code. The barriers to methyl internal rotation of the 4- and 2-methyl groups were determined to be 396.707(25) cm^–1 and 19.070(58) cm^–1, respectively. The very low barrier hindering the 2-methyl torsion was a challenge for the spectral analysis and modeling, and separately fitting the five torsional species together with combination difference loops was the key for a successful assignment. The methyl torsional barriers were compared with those of other thiazole derivatives, showing the influence of the methyl group position on the barrier height. The experimental results were supported by quantum chemical calculations.