Hydrogen bonding between water and heterocyclic compounds explored by microwave spectroscopy

Abstract

Microwave spectroscopy is a powerful spectroscopic technique used to study the rotational spectra of molecules and weakly-bound complexes in the gas phase. The assignment of the microwave spectrum allows the determination of rotational constants from which structural information of the molecular species under study can be derived. This technique has also been used extensively to study large amplitude motions and conformations of molecules, as well as to investigate non-covalent interactions when studying weakly-bound complexes. Microwave spectroscopic studies of several alkyl-substituted heteroaromatic rings and their weakly-bound complexes with water are presented within this thesis. The studies have been performed using Chirped Pulse Fourier Transform Microwave spectroscopy, over the 2.0 – 18.5 GHz frequency region, in combination with either a heating reservoir or laser ablation for the study of volatile and non-volatile species respectively. The isolated molecules and hydrate complexes were generated and probed in a supersonic expansion comprised of low concentrations of the target molecule and water in an inert carrier gas. Quantum chemical calculations, Non-Covalent Interaction index and Natural Bond Orbital analyses have been performed alongside the experimental studies. Structural parameters within the complexes of methylthiazole···H2O, 2-ethylfuran···H2O, 2- ethylthiazole···(H2O)n and ethylimidazole···(H2O)n have been precisely determined. It will be shown that each complex contains a comparatively strong O–H···X (X = N or O) hydrogen bond between water and a nitrogen or oxygen atom of the heteroaromatic ring. It will be further shown that weaker interactions between the oxygen atom of water and H, CH3 or C2H5 groups (C–H···O) located on the heteroaromatic ring have important effects. The results reveal a dynamic interplay between the formation of hydrogen bonds between the heterocycle and water, the amplitudes of V3 barriers to internal rotation (of CH3 groups) and conformational preferences (of C2H5 groups) across the series of complexes studied.