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)(D2Om)](-) clusters with n = 2-4 and m = 1-3; the clusters are tagged with weakly-bound D-2 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 omega B97X-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.