Influence of π-electron conjugation outside the aromatic ring on the methyl internal rotation of 4-methyl-5-vinylthiazole

The microwave spectrum of 4-methyl-5-vinylthiazole was measured using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range from 2.0 to 26.5 GHz. Only the <i>anti</i>-conformer was observed. Due to internal rotation of the methyl group, doublets containing an A and an E torsional species were found for all rotational transitions. Hyperfine structures arising from the ¹⁴N nuclear quadrupole coupling were resolved. The complex spectral patterns were analysed and fitted using the <i>XIAM</i> and <i>BELGI-C_s-hyperfine</i> codes, yielding a barrier to methyl internal rotation of 107.0901(7) cm⁻¹ and the quadrupole coupling constants χaa = −3.545(13), χbb− χcc = −1.563(24) and |χab| = 1.76(11) MHz. The experimental results were supported by data from quantum chemistry. Some calculations using the MP2 method yielded non-planar structures, where the vinyl group is tilted out of the thiazole ring plane. The inertial defect and the χcc constant were compared with those obtained for other aromatic molecules to support the planarity of 4-methyl-5-vinylthiazole. A comparison of the methyl torsional barrier with other thiazole derivatives suggests that electrostatic effects caused by the π-conjugated system of thiazole extended by the vinyl group are the reason of the low barrier of the methyl group.