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

Abstract

AbstractBroadband 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, N⋅⋅⋅Hb‐O, 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.