Coupled large amplitude motions: The effects of two methyl internal rotations and 14N quadrupole coupling in 4,5-dimethylthiazole investigated by microwave spectroscopy

The molecular jet Fourier-transform microwave spectrum of 4,5-dimethylthiazole has been recorded between 2.0 and 26.5 GHz, revealing torsional splittings arising from two inequivalent methyl internal rotations with relatively low hindering barriers and nitrogen quadrupole hyperfine structures. Two global fits with 1009 hyperfine components of 97 rotational transitions involving 315 torsional lines were performed using the program XIAM and BELGI-C-s-2Tops-hyperfine, an extended version of the BELGI-C-s-2Tops code which includes the effect of the N-14 quadrupole coupling, giving a root-mean-square deviation of 399.8 kHz and 4.2 kHz, respectively. Compared to the monomethyl substituted thiazole derivatives, the barriers to internal rotation are drastically lower. This is also in contrast to chemical intuition which suggests high barriers due to steric hindrance. Because of the strong interaction between the methyl groups, strong top-top couplings in both the potential energy and kinetic parts of the Hamiltonian were observed. (C) 2020 Elsevier B.V. All rights reserved.