Dissociative Water Adsorption by Al<sub>3</sub>O<sub>4</sub><sup>+</sup> in the Gas Phase

We use cryogenic ion trap vibrational spectroscopy in combination with density functional theory (DFT) to study the adsorption of up to four water molecules on Al<sub>3</sub>O<sub>4</sub><sup>+</sup>. The infrared photodissociation spectra of [Al<sub>3</sub>O<sub>4</sub>(D<sub>2</sub>O)<sub>1–4</sub>]<sup>+</sup> are measured in the O–D stretching (3000–2000 cm<sup>–1</sup>) as well as the fingerprint spectral region (1300–400 cm<sup>–1</sup>) and are assigned based on a comparison with simulated harmonic infrared spectra for global minimum-energy structures obtained with DFT. We find that dissociative water adsorption is favored in all cases. The unambiguous assignment of the vibrational spectra of these gas phase model systems allows identifying characteristic spectral regions for O–D and O–H stretching modes of terminal (μ<sub>1</sub>) and bridging (μ<sub>2</sub>) hydroxyl groups in aluminum oxide/water systems, which sheds new light on controversial assignments for solid Al<sub>2</sub>O<sub>3</sub> phases.