The bifurcated hydrogen-bond model of water and amorphous ice

The existence of bifurcated hydrogen bonds (BHB) between three molecules as a major feature of the structure of liquid water was postulated recently to account for the remarkable effect of temperature on the O–H stretching bands in the Raman spectra. As a corollary, there should be two kinds of H⋅⋅⋅O distances in water: one, 1.85 Å, for the well-known linear bonds (LHB), prevalent in cold water, the other, 2.3 Å, for the weaker BHB. This is evident in the neutron diffraction studies of heavy water, which reveal important structural changes with temperature. For instance, the atom pair correlation functions, both in the first-order difference, and the isochoric temperature derivative methods, show two peaks at 1.8 and 2.3 Å, with inverse temperature dependence similar to that of the Raman bands at 3220 (LHB) and 3420 cm−1 (BHB). In the BHB the nearest-neighbor O⋅⋅⋅O distances are the same as in the LHB, but the apex angle is much smaller than the tetrahedral, between 95° and 100°. This allows slightly shorter second neighbor O⋅⋅⋅O distances, and a closer packing of the molecules. The increased average coordination of the H and O atoms creates an imbalance in the stoichiometry of hydrogen bonding. As a result, a few percent of the water molecules are left with one ‘‘free,’’ i.e., nonhydrogen-bonded OH group (NHB). The energy of the BHB is estimated at 2.5 kcal mol−1, i.e., half that of the LHB, and its proportion in the liquid, at nearly 30% at 0 °C. Amorphous ice prepared from the vapor may also contain BHB according to x-ray and neutron diffraction data. The BHB appears as a common feature of hydroxylic compounds; e.g., hydrogen peroxide, alcohols, etc.