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 recordedusinga pulsed molecular jet Fourier-transform microwave spectrometer operatingin the frequency range from 2.0 to 26.5 GHz. Torsional splittingsinto quintets were observed for all rotational transitions due tointernal rotations of two inequivalent methyl groups. Hyperfine structuresarising from the nuclear quadrupole coupling of the (14)Nnucleus were fully resolved. The microwave spectra were analyzed usingthe modified version of the XIAM code and the BELGI-C ( s ) -2Tops-hyperfine code. The barriers tomethyl internal rotation of the 4- and 2-methyl groups were determinedto be 396.707(25) cm(-1) and 19.070(58) cm(-1), respectively. The very low barrier hindering the 2-methyl torsionwas a challenge for the spectral analysis and modeling, and separatelyfitting the five torsional species together with combination differenceloops was the key for a successful assignment. The methyl torsionalbarriers were compared with those of other thiazole derivatives, showingthe influence of the methyl group position on the barrier height.The experimental results were supported by quantum chemical calculations.