Bond-lengths, bond angles and transition barrier in ice Ih by neutron scattering

Abstract

Although several studies^1,2 of ordinary ice (Ih) have established its nearly perfect oxygen arrangement and the statistical distribution of hydrogen among two symmetrically equivalent sites (the ‘half-hydrogen’ model³), several fundamental problems remain. The O—H bond length found in ice Ih is considerably larger than in other polymorphs of ice determined precisely^4,5, and many arguments have been put forward against the value observed^5,6. Similarly, the tetrahedral H—O—H bond angle in ice Ih differs considerably from the value observed in the vapour phase, and this led Chidambaram⁷ to propose a bent hydrogen bond model which further splits the atom positions, and which has not yet been verified. Finally, the mean shape of the double potential governing the atom distribution and the barrier governing the mobility still remain to be evaluated. We show here how high-precision, short-wavelength neutron diffraction data from ice Ih can be used to evaluate the molecular structure and atomic density distribution at 60 K. The unusually long O—H distance is confirmed, but there is no evidence for bent hydrogen bonds. Indeed the hydrogen atom density distribution is well described by a librational motion of the hydrogen atom around the oxygen. The mean barrier height between the ‘half-hydrogen’ atom positions was obtained from the scattering density as 0.012 eV, which implies that the zero point motion is of high importance for the proton exchange at low temperatures.