The conformational preference of 2-ethylthiazole and its weakly bound complexes with water revealed by microwave spectroscopy Charlotte Cummings, Nick Walker Newcastle University, UK Microwave spectroscopy is a powerful technique for studying weakly bound complexes formed between aromatic/heteroaromatic rings and other small molecules. However, there have been limited studies on binary or ternary clusters formed between alkyl-substituted heteroaromatic rings and water. The rotational spectrum of 2-ethylthiazole, 2-ethylthiazole···H 2 O and 2-ethylthiazole···(H 2 O) 2 have been recorded and assigned for the first time using chirped pulse Fourier transform (CP-FTMW) spectroscopy over the frequency range 2-18.5 GHz with the aid of quantum chemical calculations. Potential energy scans performed at several different levels of theory consistently predict one minimum, in which the methyl group of the ethyl side chain is orientated out of the plane defined by the thiazole ring. Experiments performed using argon and neon as carrier gases revealed that only one conformation of this molecule was present within the supersonic expansion, which is consistent with the predictions of quantum chemical calculations. However, this differs from what was previously observed in a microwave spectroscopy study of 2-ethylfuran, 1 in which two conformers of this molecule were assigned. In total, the spectra of six isotopologues of 2-ethylthiazole were assigned, 13 C and 34 S analogues were observed in natural abundance. A partial r s structure determination of this molecule allowed the determination of the tilt angle of the ethyl group. The spectra of the mono- and dihydrate complexes of 2-ethylthiazole were also assigned. In total, the spectra of six isotopologues of the monohydrate complex and three isotopologues of the dihydrate have been assigned. Non-covalent interactions (NCI) and natural bond orbital (NBO) analyses have been performed to provide an insight into the interactions present within each complex. In addition to the relatively strong hydrogen bonds within both complexes, the analyses predicted water will interact with the ethyl group. The position and relative orientation of the water molecule(s) within each complex have been determined through the calculation of intermolecular bond distances and bond angles. Parameters calculated will be compared with similar complexes 2,3 such as thiazole···H 2 O and thiazole···(H 2 O) 2 to evaluate the effect of ethyl-substitution. References 1. H. V. L. Nguyen, J. Mol. Struct. , 2020, 1208 , 127909 2. W. Li, J. Chen, Y. Xu, T. Lu, Q. Gou and G. Feng, Spectrochim. Acta, Part A , 2020, 242 , 118720 3. E. Gougoula, C. N. Cummings, Y. Xu, T. Lu, G. Feng and N. R. Walker, J. Chem. Phys ., 2023, 158 , 114307
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