Gas phase vibrational spectroscopy of cold (TiO₂)_n^- (n = 3-8) clusters

We report infrared photodissociation (IRPD) spectra for the D₂-tagged titanium oxide cluster anions (TiO 2) n - with n = 3-8 in the spectral region from 450 to 1200 cm^-1. The IRPD spectra are interpreted with the aid of harmonic spectra from BP86/6-311+G∗ density functional theory calculations of energetically low-lying isomers. We conclusively assign the IRPD spectra of the n = 3 and n = 6 clusters to global minimum energy structures with C_s and C₂ symmetry, respectively. The vibrational spectra of the n = 4 and n = 7 clusters can be attributed to contributions of at most two low-lying structures. While our calculations indicate that the n = 5 and n = 8 clusters have many more low-lying isomers than the other clusters, the dominant contributions to their spectra can be assigned to the lowest energy structures. Through comparison between the calculated and experimental spectra, we can draw conclusions about the size-dependent evolution of the properties of (TiO 2) n - clusters, and on their potential utility as model systems for catalysis on a bulk TiO₂ surface.