The microwave spectrum of 2-methylthiazole: 14N nuclear quadrupole coupling and methyl internal rotation

The rotational spectrum of 2-methylthiazole was recorded using two pulsed molecular jet Fourier transform microwave spectrometers operating in the frequency range of 2-40 GHz. Due to the internal rotation of the methyl group, all rotational transitions were split into A and E symmetry species lines, which were analyzed using the programs XIAM and BELGI-C_s-hyperfine, yielding a methyl torsional barrier of 34.796 75(18) cm^-1. This value was compared with that found in other monomethyl substituted aromatic five-membered rings. The ¹⁴N quadrupole coupling constants were accurately determined to be χ_aa = 0.5166(20) MHz, χ_bb - χ_cc = -5.2968(50) MHz, and χ_ab = -2.297(10) MHz by fitting 531 hyperfine components. The experimental results were supplemented by quantum chemical calculations.