Temperature effect on intermolecular structure of liquid water

Abstract

The temperature dependence of the Fourier Transform Infrared (FTIR) spectra of liquid water (H2O) and heavy water (D2O), using total internal reflectance cells with crystals of zinc selenide (ZnSe) and germanium (Ge), is interpreted as a composite of three overlapping bands whose areas reflect the concentrations of forms of water in a dynamic, three-way equilibrium. The overall observation, that the experimental profile shifts intensity from the low frequency (traditionally, more strongly hydrogen-bonded) to the high frequency (more weakly-bonded) sides, is reproduced by the deconvolution program (Bandfit) to within 95% or better in most cases. For the equilibrium A⇆B, B⇆C, and A⇆C (A the most stable, B the less stable, and C the least stable) the △ H values are 0.98± 0.22, 0.62± 0.28, and 1.57± 0.16 kcal/mol, respectively for water (independent of crystal). The corresponding values for D2O are 1.37± 0.25, 0.68± 0.30, and 2.07± 0.30kcal/mol, respectively. Hess's law is obeyed, within experimental error, for both isotopic species, and the ratios of the corresponding △ Hs for D2O/H2O are either unity (B ⇆ C), or the square root of the mass ratio Md/Mh (A ⇆ B and A⇆C), indicating a primary isotope effect.