Structural, electronic, and vibrational properties of the Ti-O-Ti quantum wires in the titanosilicate ETS-10

The periodic ab initio CRYSTAL code approach has been adopted to perform for the first time the calculation of the vibrational frequencies of the titanosilicate ETS-10, modeled by one-dimensional periodic chains (including positive counterions). The cell parameter and the internal atomic coordinates of the models have been fully optimized at the Hartree-Fock (HF) level imposing a P2(1)/m11 symmetry. The so-obtained geometries well agree with the experimental data coming from single-crystal X-ray diffraction experiments. The HF and B3LYP band gaps (the latter evaluated at the HF geometry) are 14.03 and 5.22 eV, respectively. The analysis of the density of states at both HF and B3LYP levels shows that states coming from the apical oxygen atoms of the TiO6 octahedra are quite confined in a narrow range of energy. Frequencies have been calculated at HF level. The analysis of the obtained normal modes assigns the Raman "725" cm(-1) fingerprint of ETS-10 to a total-symmetric combination of the Ti-O bond stretching involving only the apical oxygen atoms. The analysis of the vibrational modes of fragments of increasing size indicates that, for this mode to be present, the chain of corner-sharing TiO6 Octahedra has to be confined in a siliceous matrix.