The strength of hydrogen bonds in liquid water and around nonpolar solutes

The unusual properties of liquid water are usually attributed to hydrogen b onding. A longstanding question is whether the continuum of strengths of hy drogen bonds in water can be usefully simplified to two states: 'intact' an d 'broken'. We show that such a simplification is justified by two very dif ferent computational models of water. We then show that there is a unique v alue of the free energy (Delta G), enthalpy (Delta H), and entropy (Delta S ) for breaking a hydrogen bond in pure water that gives quantitative agreem ent with both Raman spectra and the known heat capacity of water: Delta G = 480 cal mol(-1), Delta H = 1.9 kcal mol(-1), and Delta S/k = 2.4. Breaking a water/water hydrogen bond in the first solvation shell around Argon, a n onpolar solute, leads to Delta G 620 cal mol(-1), Delta H = 2.4 kcal mol(-1 ) and Delta S/k = 3.0. A prediction, not yet tested experimentally, is that the hydrophobic heat capacity should decrease dramatically in superceded w ater.