AB-INITIO MOLECULAR-ORBITAL STUDY OF THE ACIDITY OF HYDRATED LITHIUM HYDROXIDE

The proton and water affinities of the Li+ cation are predicted from a b initio molecular orbital theory using Gaussian 90 and Gaussian 92. T hese calculations were undertaken in order to understand the role that the hydrated Li+ cation has in controlling acidity within the clay in terlayers. Proton affinities for hydrated Li(OK) complexes increase wi th increasing degree of hydration but level off above two waters. This results in the highest acidity for the Li+ complex with fewer than tw o waters of hydration. Acidity is controlled by the effective charge o n the Li+ cation. Stabilization of the charge by associated water mole cules contributes to the reduced acidity at higher hydration numbers. Implication of a tight, inner sphere coordination complex is suggested from the calculations. The calculations imply that acidity in Li+ cla ys is relatively independent of the degree of hydration. Comparison wi th experimentally-derived hydration data for smectite clays reveals th at sufficient water exists within the clay layers even at low relative humidities to fully hydrate the Li+ cation. The calculated proton aff inities for Li(OH)(H2O)(n) (298 K) are 238, 252, 251, and 253 kcal/mol for n = 0, 1, 2, and 3, respectively. The calculated water affinities (tendency for a cation to adsorb water for Li(H2O)(n)(+) (298 K) are 29.5, 16.5, 7.0, and 9.5 kcal/mol for n = 1, 2, 3, and 4, respectively . The water affinities obey the same trends as both experimentally det ermined water affinities and those of a recent theoretical paper but a re consistently lower in value.