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.

Automated summary

Infrared multiple photon spectroscopy was used to probe the structure of hydrated singly charged aluminum ions. The Al-H vibration frequency is highly sensitive to the hydrogen bonding network, and the hydride can act as a double hydrogen bond acceptor. The onset of hydrogen bonding and disappearance of the free Al-H stretch coincides with the onset of hydrogen evolution.