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

Abstract

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 anti-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 XIAM and BELGI-C_s-hyperfine codes, yielding a barrier to methyl internal rotation of 107.0901(7) cm⁻¹ and the quadrupole coupling constants ${\chi }_{aa}$ = −3.545(13), ${\chi }_{bb}-{\chi }_{cc}$ = −1.563(24) and $\left|{\chi }_{ab}\right|$ = 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 ${\chi }_{cc}$ 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.

GRAPHICAL ABSTRACT

KEYWORDS:

• Rotational spectroscopy
• internal rotation
• nuclear quadrupole coupling
• microwave spectroscopy

Acknowledgments

S.K. thanks the HRMS committee of the 27^th Colloquium on High Resolution Molecular Spectroscopy, Cologne, 2021, for the opportunity to present the results of this work in an oral contribution.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was financially supported by the Agence Nationale de la Recherche ANR (project ID ANR-18-CE29-0011). I.K. thanks the Programme National ‘Physique et Chimie du Milieu Interstellaire’ (PCMI) of CNRS/INSU for supporting partly this work.