Vibrational mode frequencies of H _4SiO _4, D _4SiO _4, H _6Si _2O _7, and H _6Si _3O _9 in aqueous environment, obtained from ab initio molecular dynamics

摘要

We report the vibrational properties of H _4SiO _4, D _4SiO _4, H _6Si _2O _7, and H _6Si _3O _9 in aqueous solution at 300K and 1000K, 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 1145cm ~(-1) in solution at 300K, opposed to about 930cm ~(-1) in solution at 1000K. The frequency of the δSiOH vibration also depends on environment, shifting from 1145cm ~(-1) in solution to about 845cm ~(-1) in the gas-phase. We found both in the mode-decomposition approach and in multiple-configuration NMA that the H _6Si _2O _7 dimer shows a vibrational mode at about 790cm ~(-1), which we consider to be responsible for a hitherto unexplained shoulder of the monomer Raman band at 770cm ~(-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.