Aluminum monomer line-broadening as evidence for the existence of [AlOH]2+ and [Al(OH)2]+ during forced hydrolysis: a 27Al nuclear magnetic resonance study

The Al monomer resonance of 0.05M aluminum nitrate solutions gradually broadens from 7.82 to 89.40 Hz upon forced hydrolysis from an OH/Al molar ratio of 0.0 to 2.2. The broadening is due to increasing amounts of [AlOH] and [Al(OH)] up to 2.24% and 0.11%, respectively. Additional measurements on the hydrolysis with ammonium hydroxide and the dissolution of aluminum sec-butoxide in dilute HCl give similar results. To calculate the linewidth of the monomer resonance, it is assumed that [AlOH] and [Al(OH)] have identical linewidths as the isoelectronic compounds [AlF] and [AlF], respectively. The calculation is based on pK values of 4.99 for [AlOH] and 10.13 for [Al(OH)]. The results of the theoretical calculations agree well with observed linewidth data