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

Abstract

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)(H 2 O) 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(H 2 O) 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.

Quantum Chemical Studies of Mononuclear Zinc Species of Hydration and Hydrolysis

“…There were hydrogen bonds between water H and hydroxide O in the inner-sphere of the Zn hydrolysis species. Compared with Li(OH)(H 2 O) n (n ) 1-4), 27 hydrogen bonds of the zinc hydrolysis species were weaker since HOH--OH distances (1.830-2.833 Å) were bigger than those of Li (1.59-1.69 Å) because the distances of Zn-OH 2 and Zn-OH were bigger than 2). The Zn-OH bonds in Zn(OH) 3and Zn(OH) 4 2were electrostatic only because the OH --OHrepulsions were much stronger 14 than those in Zn(OH)(H 2 O) n + (n ) 0-3) and Zn(OH) 2 (H 2 O) n (n ) 0-2), which led to the expanded Zn-OH distance and accordingly no covalent interaction.…”

“…There were hydrogen bonds between water H and hydroxide O in the inner-sphere of the Zn hydrolysis species. Compared with Li(OH)(H 2 O) n ( n = 1−4), hydrogen bonds of the zinc hydrolysis species were weaker since HOH- -OH distances (1.830−2.833 Å) were bigger than those of Li (1.59−1.69 Å) because the distances of Zn−OH 2 and Zn−OH were bigger than those of Li−OH 2 and Li−OH although both ions have similar radii (Zn 2+ , 0.60 Å; Li + , 0.59 Å). In addition, the outer-sphere water molecules could also provide O and H to form hydrogen bonds with water H or hydroxide O of the inner-sphere, whose distances were from 1.550 to 1.877 Å indicating strong interactions.…”

Plane-Wave Density Functional Theoretic Study of Formation of Clay-Polymer Nanocomposite Materials by Self-Catalyzed in Situ Intercalative Polymerization

“…15 However, the most important source of Brønsted acidity is widely believed to be dissociated water molecules in the hydration sphere of the interlayer exchangeable cations. [16][17][18][19] The acidity depends on the water content of the clay and, to a lesser extent, upon whether the layer charge arises mainly from substitution in the octahedral or tetrahedral layers. Acidity is maximized when the water content of the clay is low (i.e.…”

Single Ion and Dimerization Studies of the Al(III) Ion in Aqueous Solution

“…The geometry of hydrated metal ions has usually been investigated using static first principles calculations in the gas phase and/or in aqueous solution using a continuum model for water. [33][34][35][36] Even though these theoretical studies can provide important information, they do not capture the impact of the solution dynamics nor the impact of intermolecular hydrogen bonding interactions in bulk solution on the predicted geometries of metal ions. Specifically, the structures of the octahedral Al(H 2 O) 6…”

“…On the other hand, theoretical and computational chemistry studies have provided valuable insights into the structure of hydrated metal ions. The geometry of hydrated metal ions has usually been investigated using static first principles calculations in the gas phase and/or in aqueous solution using a continuum model for water. − Even though these theoretical studies can provide important information, they do not capture the impact of the solution dynamics nor the impact of intermolecular hydrogen bonding interactions in bulk solution on the predicted geometries of metal ions. Specifically, the structures of the octahedral Al(H 2 O) 6 3+ and the tetrahedral Al(OH) 4 − ions in solution (which form at low and high pH values, respectively) have been studied extensively using both experimental and theoretical techniques. − It is known that the octahedral Al(H 2 O) 6 3+ ion undergoes hydrolysis through deprotonation and produces Al ion species with smaller coordination numbers with an increasing pH value.…”