Geometry of Hydrogen Bonds in Liquid Ethanol Probed by Proton NMR Experiments

Quantitative information of hydrogen bonding is crucial to our understanding of the structure and properties of associated liquids. Here, we outline a simple procedure to establish the geometry of hydrogen bonds in liquid ethanol using proton nuclear magnetic resonance (NMR) spectroscopy. We do so by exploiting differences in proton chemical shift values, that originate from the secondary isotope effect, to distinguish the methyl and hydroxyl group protons of CH₃CH₂OH from those of the deuterated CH₃CD₂OH in the ¹H NMR spectra of mixtures of the two. This has allowed us to measure the ratios of the inter- to intramolecular distances between methyl to hydroxyl and methylene to hydroxyl protons using one-dimensional (1D) transient nuclear Overhauser effect NMR measurements as a molecular ruler. We model liquid ethanol by ab initio molecular dynamics simulations and identify all possible pairs of ethanol molecules in the ensemble that satisfied the NMR-determined inter- to intramolecular distance ratio criteria. For these pairs of ethanol molecules, we find the mean value of the hydrogen bonding distance, r_OH···O, to be 1.93 Å and the value of the ∠HO···O angle to be 13.3°, thus effectively establishing the geometry of hydrogen bonds in liquid ethanol. An interesting observation that emerges from our study is the linear correlation between hydrogen bond distances and angles in ethanol.