Hydroxyl Orientations in Gibbsite and Bayerite.

摘要

The hydrogen positions in bayerite and gibbsite (both Al(OH)3) have been determined by minimizing the electrostatic energy as a function of hydroxyl orientation for a fixed O-H distance. The hydrogen positions in gibbsite are known from an accurate X-ray refinement and one of the H-H distances is short (2.122 A). The point charge model (fully ionized atoms) separates these hydrogens resulting in deviations from the observed OH orientations of approximately 18 deg. Several models with reduced charges on the ions were refined with no improvement in the hydroxyl orientations. The addition of a modifying function to simulate sp3 hybridization of the oxygens resulted in hydrogen positional parameters which are within two standard deviations (X-ray) of the X-ray refinement for 16 of the 18 positional parameters. Hydrogen positions have been determined for all six geometrically possible hydroxyl orientations and the correct one has the lowest electrostatic energy. There are three geometrically possible hydroxyl orientations for bayerite and all three have been refined. The model with the lowest energy has, between the layers, one hydroxyl nearly normal to the hydroxide layer, (001), participating in a single hydrogen bond, a second hydroxyl inclined at 58.3 deg. to (001) participating in a single hydrogen bond and a third hydroxyl in the surface of the hydroxide layer participating in a bifurcated bond.