Two-ring vibrational modes on silica surfaces investigated via fully coordinated nanoclusters

Vibrational modes of rings containing two silicon atoms and two oxygen atoms, two-rings, as found on a variety of silica surfaces, are modelled with fully connected (SiO2)(12) clusters containing no terminating groups. Such clusters naturally reflect the embedding of surface two-rings in a silica matrix without the need for large calculations of silica surface layers. The relatively small size of the clusters allows us to employ the recommended high levels of theory for vibrational calculations, but are sufficiently large to study a range of two-ring-containing conformations. The calculated spectra for the clusters display many peaks in the experimental window, with some in excellent frequency and intensity agreement with measured bands. The results are discussed with respect to the structural nature of the clusters and the possibility of collective two-ring surface modes. (C) 2003 Elsevier B.V. All rights reserved.