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

Abstract

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.