Geometries of hydrogen bonds in water-ethanol mixtures from ab initio molecular dynamics simulations.

We outline a simple procedure to determine the geometry of hydrogen bonds between different molecular species in binary mixtures from ab initio molecular dynamics (AIMD) trajectories. Here we determine the geometry of the hydrogen bonds arising from intermolecular OH⋯O interactions between different H-bonded pairs, water-water, ethanol-ethanol and water-ethanol in water-alcohol mixtures at different compositions by plotting the intermolecular non-bonded OH⋯O and O⋯O distances, and the ∠HO⋯O (θ) angles for each of the possible pairs in the ensemble. Two regions separate out in each of the scatter-plots; the one with short OH⋯O and O⋯O intermolecular distances and almost linear ∠HO⋯O angles may be identified as the region where the intermolecular OH⋯O geometry would be favorable for hydrogen bonding. Using the different geometric criteria for each of the three possible H-bonded pairs we estimate the average number of water and ethanol molecules that are hydrogen bonded to a water molecule, and to an ethanol molecule, respectively, at different mole fractions of the mixture. We validate the results from values of the chemical shift of the two OH resonances (water and ethanol) in the proton NMR spectra of the mixtures at different concentrations as these values are known to be sensitive to the local chemical environment of the resonating nuclei.

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, rOH···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.

Distinguishing Intra- and Intermolecular Interactions in Liquid 1,2-Ethanediol by 1H NMR and Ab Initio Molecular Dynamics.

The central OCCO backbone of the 1,2-ethanediol molecule adopts the gauche conformer in the gaseous and crystalline states but exists in conformational equilibrium between gauche and trans in the liquid; an observation that has been attributed to the competition between intra- and intermolecular interactions. Here, we show that the nuclear Overhauser effect (NOE) has the ability to distinguish inter- from intramolecular interactions in liquid 1,2-ethanediol. We do so by exploiting the secondary isotope effect to distinguish the hydroxyl protons of HOCH2CH2OH and the deuterated HOCD2CD2OH in the 1H NMR spectra of mixtures of the two and, in conjunction with ab initio MD simulations, show how the interplay between inter- and intramolecular interactions gives rise to the conformational isomers in the liquid state of 1,2-ethanediol.