Bifurcated hydrogen bonds in water

Abstract

AbstractA new model is proposed for liquid water, based on recent studies of the Raman spectra as a function of temperature. It features two kinds of hydrogen bonds: in addition to the well known linear hydrogen bonds (LHB), predominant in cold water, there are some bifurcated hydrogen bonds (BHB) between three water molecules. These three‐centered bonds, mainly electrostatic, have an average energy of 2.5 kcal mol−1, about half of that of the LHB. Their characteristic OH stretching frequency is around 3420–3440 cm−1, compared with 3220 cm−1 for the LHB. In addition, there are a few percent of nearly free OH groups, i.e. non‐hydrogen‐bonded (NHB), with a stretching frequeney of ca 3615 cm−1. Reorientation between these three states occurs through rotation of the H2O molecules about one of their OH bonds. Thus, on melting, one third of the hydrogen atoms move out of the tetrahedral framework to occupy interstitial sites in the open ‘shafts’ characteristic of the ice structure. As in the case of ice, Fermi resonance between the ν1 and 2ν2 levels has little impact on the vibrational spectra of liquid water and iee. The hybrid molecule, HDO, does not form LHB with H2O or D2O, hence its simpler spectra. The BHB model can account much better than previous models for the thermodynamic anomalies of water.