Vibrational mode frequencies of H4SiO4, D4SiO4, H6Si2O7, and H6Si3O9 in aqueous environment, obtained from ab initio molecular dynamics.

ABSTRACT

We report the vibrational properties of H(4)SiO(4), D(4)SiO(4), H(6)Si(2)O(7), and H(6)Si(3)O(9) in aqueous solution at 300 K and 1000 K, obtained from the combination of ab initio molecular dynamics (MD) and a mode-decomposition approach. This combination yields vibrational subspectra for selected vibrational modes at finite temperatures. We also performed normal-mode analysis (NMA) on numerous configurations from the same MD run to sample the effect of the variable molecular environment. We found good agreement between both approaches. The strongest effect of temperature is on the SiOH bending mode δSiOH, which is at about 1145 cm(-1) in solution at 300 K, opposed to about 930 cm(-1) in solution at 1000 K. The frequency of the δSiOH vibration also depends on environment, shifting from 1145 cm(-1) in solution to about 845 cm(-1) in the gas-phase. We found both in the mode-decomposition approach and in multiple-configuration NMA that the H(6)Si(2)O(7) dimer shows a vibrational mode at about 790 cm(-1), which we consider to be responsible for a hitherto unexplained shoulder of the monomer Raman band at 770 cm(-1) in dilute silica solutions. Our results demonstrate the importance of temperature and solvation environment in calculations that aim to support the interpretation of experimental Raman spectra of dissolved silica.