Hydrogen Bonding and Molecular Geometry in Isolated Hydrates of 2‐Ethylthiazole Characterised by Microwave Spectroscopy

Abstract

Broadband microwave spectra of the isolated 2‐ethylthiazole molecule, and complexes of 2‐ethylthiazole···H2O and 2‐ethylthiazole···(H2O)2 have been recorded by probing a gaseous sample containing low concentrations of 2‐ethylthiazole and water within a carrier gas undergoing supersonic expansion. The identified conformer of the isolated 2‐ethylthiazole molecule and the 2‐ethylthiazole sub‐unit within each of 2‐ethylthiazole···H2O and 2‐ethylthiazole···(H2O)2 have C1 symmetry. The angle that defines rotation of the ethyl group relative to the plane of the thiazole ring, (S‐C2‐C6‐C7), is ‐98.6(10)° within the isolated 2‐ethylthiazole molecule. Analysis of molecular geometries and non‐covalent interactions reveals each hydrate complex contains a non‐linear primary hydrogen bond between an O‐H of H2O and the nitrogen atom while the O atom of the water molecule(s) interacts weakly with the ethyl group. The (Hb···N‐C2) parameter, which defines the position of the H2O molecule relative to the thiazole ring, is found to be significantly greater for 2‐ethylthiazole···H2O than for thiazole···H2O. The distance between the O atoms is determined to be 2.894(21) Å within the dihydrate complex which is shorter than observed within the isolated water dimer. The primary hydrogen bond within 2‐ethylthiazole···(H2O)2 is shorter and stronger than that in 2‐ethylthiazole···H2O as a result of cooperative hydrogen bonding effects.