Hydrogen-bond cooperativity is an effect when hydrogen bonding is influenced by the

Glycol−ether compounds such as 2-methoxyethanol (CH3OCH2CH2OH) and 2-butoxyethanol [CH3(CH2)3OCH2CH2OH] form both intra- and intermolecular hydrogen bonds. Using Fourier transform infrared spectroscopy, we have measured extent of intra- and intermolecular hydrogen bonding in these compounds dissolved in n-hexane at varying concentrations and temperatures. Intramolecular hydrogen bonds are present at all conditions, whereas intermolecular bonds appear at higher concentrations. Using lattice-fluid-hydrogen-bonding theory, equilibrium constants for the formation of intra- and intermolecular hydrogen bonds are determined. The results show that the equilibrium constant for intermolecular bond formation is approximately 6 times the intramolecular equilibrium constant for 2-methoxyethanol systems at 35 °C. Experiments at higher temperature, 45 °C, with 2-methoxyethanol show less hydrogen bonding as expected due to higher thermal energy. Due to steric hindrance, 2-butoxyethanol has a lower degree of hydrogen bonding than 2-methoxyethanol at the same temperature and concentration.

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.

Hydrogen bonding plays an important role in thermodynamic properties of polar fluids. Existing equations of state that include h-bonding cannot accurately predict the phase beha®ior for polar fluids. In the theories for h-bond-chain forming molecules, h-bonding strength is considered a constant at a gi®en temperature and pressure. Infrared spectroscopy and ab initio calculations show that the h-bonding strength depends on whether or not the molecule was pre®iously h-bonded at other sites. When an h-bond is formed between an already hydrogen-bonded species and a free species, the second h-bond has different energetic characteristics from the primary h-bond. In the case of l -alkanol self-h-bonding, the equilibrium constant for the second h-bond is ten times that for the primary h-bond. This phenomenon called h-bond cooperati®ity was incorporated in a lattice