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Ab Initio Molecular Orbital Study of the Acidity of Hydrated Lithium Hydroxide

1996; American Chemical Society; Volume: 100; Issue: 39 Linguagem: Inglês

10.1021/jp960018p

1541-5740

Christopher L. Marshall, John B. Nicholas, Holmann V. Brand, Kathleen A. Carrado, Randall E. Winans,

Chemical Synthesis and Characterization

The proton and water affinities of the Li+ cation are predicted from ab initio molecular orbital theory using Gaussian 90 and Gaussian 92. These calculations were undertaken in order to understand the role that the hydrated Li+ cation has in controlling acidity within the clay interlayers. Proton affinities for hydrated Li(OH) complexes increase with increasing degree of hydration but level off above two waters. This results in the highest acidity for the Li+ complex with fewer than two waters of hydration. Acidity is controlled by the effective charge on the Li+ cation. Stabilization of the charge by associated water molecules 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+ clays is relatively independent of the degree of hydration. Comparison with experimentally-derived hydration data for smectite clays reveals that sufficient water exists within the clay layers even at low relative humidities to fully hydrate the Li+ cation. The calculated proton affinities 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 determined water affinities and those of a recent theoretical paper but are consistently lower in value.