Getting Ready for the Hydrogen Evolution Reaction : The Infrared Spectrum of Hydrated Aluminum Hydride-Hydroxide HAlOH⁺(H₂O)_n-1, n = 9–14

Hydrated singly charged aluminum ions eliminate molecular hydrogen in a size regime from 11 to 24 water molecules. Here we probe the structure of HAlOH⁺(H₂O)_n−1, n = 9–14, by infrared multiple photon spectroscopy in the region of 1400–2250 cm⁻¹. Based on quantum chemical calculations, we assign the features at 1940 cm⁻¹ and 1850 cm⁻¹ to the Al−H stretch in five- and six-coordinate aluminum(III) complexes, respectively. Hydrogen bonding towards the hydride is observed, starting at n =12. The frequency of the Al−H stretch is very sensitive to the structure of the hydrogen bonding network, and the large number of isomers leads to significant broadening and red-shifting of the absorption of the hydrogen-bonded Al−H stretch. The hydride can even act as a double hydrogen bond acceptor, shifting the Al−H stretch to frequencies below those of the water bending mode. The onset of hydrogen bonding and disappearance of the free Al−H stretch coincides with the onset of hydrogen evolution.