Hydrogen Atoms in Boehmite: A Single Crystal X-Ray Diffraction and Molecular Orbital Study

Abstract

The crystal structure of natural boehmite, AIOOH, from Tveidalen, Langesundsfjorden, Nor- way, with a = 3.693(1), b = 12.221(2) and c = 2.865(1) ,~, was refined from single crystal X-ray diffraction data (MoKa; maximum 20 = 120 ~ to an Rw value of 0.032 in space groups Amam and A21am. In both cases the proton was found to occupy positions of the type (x, 0.038, 0), where x = 0.104 and 0.396, about 0.75/~ from the donor oxygen atom. This arrangement corresponds to the presence of chains of asymmetric hydrogen bonds with random polarity parallel to the a axis, and indicates that the effective space group is Amam. No significant electron density was observed either at the center (0, 0, 0) or at the position (I/4, y, 0), corresponding to symmetric and bifurcated hydrogen bonds, respectively, between the layers of oc- tahedra. Calculations of the total energy of a representative portion of the boebmite structure as a function of hydroxyl orientation, using semiempirical molecular orbitals of the Hiickel type, are in agreement with the observed electron density distribution and suggest a preference for (I) sp 3, rather than sp z, hybridization of the donor oxygen atom orbitals, and (2) asymmetric, rather than symmetric hydrogen bonds. Disorder of the hydrogen atom positions within individual hydrogen bond chains is shown to be much less likely than disorder between adjacent fully coherent chains.