Hydrogen bonding with aromatic rings

Thermodynamic effects of hydrogen bonding are important in determining the phase behavior in polar fluids. While efforts to understand “strong” donor and “strong” acceptor combinations (H‐bonds involving a formation energy more negative than −20 kJ/mol such as alkanol‐alkanol H‐bonding have been substantial, weak H‐bonds involving double bonds or aromatic rings as proton acceptors) have largely been ignored. Using FTIR spectroscopy H‐bonding between alcohol donor and aromatic ring‐containing acceptor molecules was studied including l‐hexanol and cyclohexanol at low concentrations where self‐association is negligible (strong donors). H‐bonding “weak” acceptors studied include toluene and m‐xylene. Clear spectroscopic evidence existed for the formation of H‐bonds between the alcohol and aromatic molecules, which are much weaker than conventional “strong donor”–“strong acceptor” H‐bonds. Using quantitative FTIR measurements, the percentage of H‐bonded alcohol molecules over a range of aromatic concentrations was determined. Ab initio calculations also showed that alcohol–aromatic H‐bonds are much weaker than alcohol–alcohol H‐bonds. This H‐bonding, though weak, will contribute significantly to the chemical potential of the molecules.