Hydrogen Bonding and Molecular Geometry in Isolated Hydrates of 2-Ethylthiazole Characterised by Microwave...

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, N⋅⋅⋅H_b-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 ∠(H_b⋅⋅⋅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.