Dissociative Water Adsorption on Gas-Phase Titanium Dioxide Cluster Anions Probed with Infrared Photodissociation Spectroscopy

Gas-phase complexes of water on small titanium oxide clusters are model systems to examine the molecular-level mechanism of dissociative water adsorption at defect sites on bulk titania surfaces. Here, we report infrared photodissociation (IRPD) spectra for [(TiO₂)_n(D₂O_m)]⁻ clusters with n = 2–4 and m = 1–3; the clusters are tagged with weakly-bound D₂ so that only single photon absorption is required for photodissociation. Vibrational features are reported in the spectral windows of 400–1200 and 2600–3000 cm^{− 1} _, capturing both fingerprint cluster modes and O–D stretching modes. The IRPD spectra are interpreted with the aid of ωB97X-D/aug-cc-pVDZ density functional theory calculations. We conclusively assign the IRPD spectra of the n = 2, m = 1,2 and n = 3, m = 1–3 clusters to global minimum-energy structures containing dissociatively adsorbed water. We also provide insight into the more complicated spectroscopy of the n = 4 clusters, which show possible contributions from a kinetically trapped reactive intermediate in addition to the global minimum-energy isomer. From this work, we can draw conclusions about the size dependence and site-specificity of (TiO₂)_n ⁻ cluster reactivity.