THEORETICAL STUDIES ON ALUMINATE AND SODIUM ALUMINATE SPECIES IN MODELS FOR AQUEOUS SOLUTION: A1(OH)3, AL(OH)4-, AND NAAL(OH)4

Abstract

Ab initio quantum mechanical calculations were performed on Al(OH) 3 , Al(OH)- 4 , NaAl(OH) 4 and related species with varying numbers of explicit water molecules to elucidate the structural, spectral and energetic properties of the possible species. We find that Al(OH) 3 reacts with H 2 O in the gas-phase with an exoergicity of 24.1 kcal/mol to produce Al(OH) 3 H 2 O, which has shorter Al-OH distances, larger Al-OH stretching frequencies, and a 15 ppm larger Al NMR shielding than does Al(OH)- 4 . When the first hydration spheres of these species are included the Al NMR shieldings becomes very similar, but the O and H NMR parameters and the IR and Raman spectra still show significant differences. The hydration energy of Al(OH) 3 H 2 O is determined from a "supermolecule" calculation on Al(OH) 3 H 2 O ... 6H 2 O, whereas that for Al(OH)- 4 is obtained using the supermolecule calculation on Al(OH)- 4 ... 6H 2 O plus an evaluation of the electrostatic Born hydration energy of the supermolecule. The calculated energy change for the acid dissociation reaction, Al(OH) 3 H 2 O ... 6H 2 O --> Al(OH)- 4 ... 6H 2 O+H (super +) , is +297.9 kcal/mol in the gas phase but only +2.3 kcal/mol in aqueous solution, due to the strong hydration of H (super +) and Al(OH)- 4 ... 6H 2 O. Using quantum mechanically calculated entropies for the unhydrated species, Al(OH) 3 H 2 O and Al(OH)- 4 , plus the experimental hydration entropy of H (super +) , the -TDelta S term for this reaction is calculated as about +11.8 kcal/mol. Adding in calculated zero-point energies and room temperature enthalpy corrections gives a free energy change of +0.5 kcal/mol. Thus pKa for the acid dissociation of Al(OH) 3 H 2 O is near zero at room T, and Al(OH)- 4 will be dominant except under very acidic conditions. Properties are also calculated for the bare close-contact ion pair NaAl(OH) 4 and for hydrated forms of both a close-contact and a solvent-separated ion pair, NaAl(OH) 4 ... 10H 2 O and NaAl(OH) 4 ... 11H 2 O. In accord with previous calculations on silicate anions and ion pairs, formation of an unhydrated close-contact ion pair increases the shielding of the Al in Al(OH)- 4 , while reducing the Al-O symmetric stretching frequency. The calculated energy change at 298 K in aqueous solution for the ion pair formation reaction, Na ... 6H 2 O (super +) + Al(OH)- 4 ... 6H 2 O --> NaAl(OH) 4 ... 11H 2 O+H 2 O, is +17.6 kcal/mol, close to the value determined experimentally. After addition of calculated zero-point energies, enthalpy corrections, and calculated entropy changes we obtain a Delta G value of +1.7 kcal/mol for this reaction, giving a log K around -1, consistent with significant ion pair formation. The NaAl(OH) 4 ... 11H 2 O species is a solvent-separated ion pair with full hydration of both its Na (super +) and Al(OH)- 4 . Its calculated Al NMR shielding and Al-O symmetric stretching frequencies are very similar to those for Al(OH)- 4 ... 6 H 2 O, whereas its Na NMR shielding is about 5 ppm smaller than that of Na(OH 2 )+ 6 , although its Na electric field gradient (and consequently its line-width) are larger. Thus it appears that Na NMR may be the best technique for characterizing this ion pair.