B3LYP simulation of the full vibrational spectrum of 45S5 bioactive silicate glass compared to ν-silica

The IR spectrum of the Bioglass 45S5 (of composition 46.1 SiO2, 24.4 Na2O, 26.9 CaO. and 2.6 P2O5 mol %) has been simulated by means of periodic ab initio B3LYP calculations. The initial glass structure unit cell envisaging 78 atoms was generated through a melt quench process by means of classical molecular dynamics simulations. The molecular mechanics optimized unit cell has then been fully reoptimized (both unit-cell parameters and internal coordinates) at B3LYP level in a periodic approach using Gaussian basis sets of double-xi quality by means of the CRYSTAL06 code. Although long-range structural properties cannot be modeled by using this ab initio approach because of the intrinsic amorphous nature of the glass, the quantum mechanical simulation proved to be extremely effective in predicting and analyzing the vibrational features of this biomaterial. The effect of modifiers Na and Ca cations on the network dynamics has been assessed by comparing the 45S5 vibrational spectrum with that of amorphous nu-SiO2, both simulated at the B3LYP level. A detailed assignment of each spectral peak to the corresponding vibrational mode of the two glasses has been addressed and the role of cation modifiers on the vibrational features has been analyzed in greater details. This may be useful to support further IR and Raman investigations of the bulk structure of these materials.