Geometry of OHO interactions in the liquid state of linear alcohols from ab initio molecular dynamics simulations.

Monohydroxy alcohols are strongly associating liquids with the hydrogen bonding associated with the presence of the hydroxyl group having a significant influence on properties. Here we determine the geometry of the hydrogen bond in linear alcohols, methanol to pentanol, arising from intermolecular OHO interactions, from ab initio molecular dynamics trajectories by plotting the intermolecular non-bonded OHO and OO distances, and the ∠HOO (θ) angles for every possible pair of alcohol molecules in the ensemble. Two regions separate out in the scatter-plot; the one with short OHO and OO intermolecular distances and almost linear ∠HOO angles may be identified as the region where the intermolecular OHO geometry would be favorable for hydrogen bonding. We find that the geometry of the hydrogen bond arising from intermolecular OHO interactions in liquid alcohols shows little change with an increase in size of the alkyl group. This observation is in direct contrast to that in the crystalline state where marked departures in the HOO angle from linearity are seen with an increase in the alkyl chain on going from methanol to pentanol.