Covalency of the Hydrogen Bond in Ice: A Direct X-Ray Measurement

E. D. Isaacs, A. Shukla, P. M. Platzman, D. R. Hamann, B. Barbiellini, C. A. Tulk
1999 Physical Review Letters  
Periodic intensity variations in the measured Compton profile anisotropies of ordinary ice Ih correspond to distances of 1.72 and 2.85 Å, which are close to the hydrogen bond length and the nearestneighbor O-O distance, respectively. We interpret this result as direct evidence for the substantial covalent nature of the hydrogen bond. Very good quantitative agreement between the data and a fully quantum mechanical bonding model for ice Ih and the disagreement with a purely electrostatic
more » ... ctrostatic (classical) bonding model support this interpretation and demonstrate how exquisitely sensitive Compton scattering is to the phase of the electronic wave function. [S0031-9007(98)08227-1] PACS numbers: 71.15.Cr, 32.80.Cy, 61.10.Eq Hydrogen bonds play a crucial role in determining many of the distinctive properties of water and biological complexes. In particular, in ice, hydrogen forms two distinct types of bonds with neighboring oxygen. The shorter (1.00 Å) covalent bond is a typical molecular s bond between the oxygen and hydrogen. It has been appreciated since Pauling [1] that the longer (1.75 Å) so-called hydrogen bond is most probably partly covalent. Even so, a microscopic quantitative understanding of the hydrogen bonds covalent, or quantum mechanical, character remains experimentally untested and controversial [2] . In this paper we describe high momentum transfer inelastic (Compton) x-ray scattering studies of the hydrogen bond in ice Ih. In particular, we have measured Compton profile anisotropies which are exceptionally sensitive to the phase of the electronic wave function and therefore to the covalency of the hydrogen bond. Periodic intensity variations in the anisotropy reveal two distances, one of 1.72 Å, near the hydrogen bond length of 1.75 Å, and another at 2.85 Å, close to the nearest-neighbor O-O distance of 2.75 Å [3]. The presence of these two dominant lengths in the Compton profile anisotropy is interpreted as the first direct experimental evidence for the substantial covalent character of the hydrogen bond. Very good quantitative agreement between the data and a fully quantum mechanical bonding model for ice Ih [4] and the disagreement with a purely electrostatic (classical) bonding model are strong support for this interpretation. Ice is a molecular solid in which the intermolecular bonding consists primarily of hydrogen bonds. In particular, in ordinary ice ͑Ih͒ the oxygen atoms sit on a lattice of two interpenetrating hexagonal close-packed structures with space group P6 3 ͞mmc (see Fig. 1 ) [5] . The molecular orientations are such that one hydrogen atom lies along the axis joining each of the neighboring, tetrahedrally coordinated oxygen atoms. As determined by neutron (in D 2 O) [6] and x-ray diffraction [3], the two O-H distances on a given axis are approximately 1.00 Å for the covalent bond and 1.75 Å for the hydrogen bond, the bond energies being 4.8 and 0.29 eV, respectively [3]. The molecular orientations are correlated in such a way as to maintain this arrangement (obeying Bernal-Fowler ice rules), but do not have long-range order [1, 7] . Moreover, the 50% concentration of hydrogen bonds relative to covalent bonds, the simplicity of the molecules, and the coherency of the ordered structure make the ice Ih an ideal system in which to probe the covalent nature of the hydrogen bonds. Unfortunately, neither neutron nor x-ray diffraction is sensitive to the extended wave function of the electrons in the hydrogen bond. Very high momentum transfer inelastic x-ray scattering, so-called Compton scattering, has been shown to be an ideal probe for measuring the Fourier transform of the FIG. 1(color) . Crystal structure of Bernal-Fowler ice Ih. Red (white) balls give the positions of the oxygen (hydrogen). The crystallographic c-axis is in the vertical direction. 600 0031-9007͞99͞82(3)͞600(4)$15.00
doi:10.1103/physrevlett.82.600 fatcat:h2n6dearpbcg7f23tss7m4tmc4