Multinuclear magnetic resonance studies of the hydrolysis of aluminium(III). Part 8. Base hydrolysis monitored at very high magnetic field

Abstract

New studies of base-hydrolysed aluminium(III) solutions were made using high-field ¹H and ²⁷Al n.m.r. spectroscopy. The increased sensitivity of the modern equipment has allowed data to be obtained at concentrations as low as 1 × 10^–3 mol dm^–3, some of it quantitative, and the ¹H n.m.r. spectra of solutes transferred to non-aqueous solvents has provided new structural data. The results enable it to be stated quite unequivocally that the tridecameric cation [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ is the principal species formed at lower concentrations, and is virtually the only one formed below 20 × 10^–3 mol dm^–3. At concentrations higher than this, and at intermediate levels of hydrolysis, an oligomeric mixture is formed with a stoicheiometry close to {[Al(OH)_2.5]^1/2+}_n and which could be composed of species such as [Al₂(OH)₅]⁺, [Al₃(OH)₈]⁺, [Al₄(OH)₁₀]²⁺, and [Al₅(OH)₁₃]²⁺. Partial deuteriation produces isotopic splitting of the proton resonances of bound water which allows these to be distinguished from the resonances of OH-bridge protons. An indication of fine structure due to hydrogen–deuterium substitution on AlO₆ fragments is also obtained which favours the more condensed structures and this is in accord with the ease with which the oligomer is converted into the tridecamer on simple dilution. It is now certain that the dihydroxo-bridged dimer found in the solid state is not a significant constituent of these solutions. Oxygen-17 n.m.r. data are also briefly reported.