Browsing by Subject "AUTOXIDATION PRODUCT C6H8O7"

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  • Hirvonen, Viivi; Myllys, Nanna; Kurtén, Theo; Elm, Jonas (2018)
    The role of covalently bound dimer formation is studied using highlevel quantum chemical methods. Reaction free energy profiles for dimer formation between common oxygen-containing functional groups are calculated, and based on the Gibbs free energy differences between transition states and reactants, we show that none of the studied two-component gas-phase reactions are kinetically feasible at 298.15 K and 1 atm. Therefore, the catalyzing effect of water, base, or acid molecules is calculated, and sulfuric acid is identified to lower the activation free energies significantly. We find that the reactions yielding hemiacetal, peroxyhemiacetal, alpha-hydroxyester, and geminal diol products occur with activation free energies of less than 10 kcal/mol with sulfuric acid as a catalyst, indicating that these reactions could potentially take place on the surface of sulfuric acid clusters. Additionally, the formed dimer products bind stronger onto the pre-existing cluster than the corresponding reagent monomers do. This implies that covalent dimerization reactions stabilize the existing cluster thermodynamically and make it less likely to evaporate. However, the studied small organic compounds, which contain only one functional group, not able to form dimer are products that are stable against evaporation at atmospheric conditions. Calculations of dimer formation onto a cluster surface and the clustering ability of dimer products should be extended to large terpene oxidation products in order to estimate the real atmospheric significance.
  • Myllys, Nanna; Ponkkonen, Tuomo; Passananti, Monica; Elm, Jonas; Vehkamäki, Hanna; Olenius, Tinja (2018)
    The role of a strong organobase, guanidine, in sulfuric acid-driven new-particle formation is studied using state-of-the-art quantum chemical methods and molecular cluster formation simulations. Cluster formation mechanisms at the molecular level are resolved, and theoretical results on cluster stability are confirmed with mass spectrometer measurements. New-particle formation from guanidine and sulfuric acid molecules occurs without thermodynamic barriers under studied conditions, and clusters are growing close to a 1:1 composition of acid and base. Evaporation rates of the most stable clusters are extremely low, which can be explained by the proton transfers and symmetrical cluster structures. We compare the ability of guanidine and dimethylamine to enhance sulfuric acid-driven particle formation and show that more than 2000-fold concentration of dimethylamine is needed to yield as efficient particle formation as in the case of guanidine. At similar conditions, guanidine yields 8 orders of magnitude higher particle formation rates compared to dimethylamine. Highly basic compounds such as guanidine may explain experimentally observed particle formation events at low precursor vapor concentrations, whereas less basic and more abundant bases such as ammonia and amines are likely to explain measurements at high concentrations.
  • Elm, Jonas; Myllys, Nanna; Kurten, Theo (2017)
    We investigate the molecular interactions between phosphoric acid and common atmospheric nucleation precursors using computational methods. The equilibrium geometries and vibrational frequencies are obtained using the three DFT functionals M06-2X, PW91 and B97X-D. The single-point energy is corrected using a high-level CCSD(T)-F12a/VDZ-F12 calculation. The molecular interaction between phosphoric acid and sulphuric acid is found to be strong with reaction free energy of similar magnitude as the interaction between dimethylamine and sulphuric acid. The strong hydrogen bonding of phosphoric acid to sulphuric acid indicates that concentrations of as low as 10(2)-10(4) molecules/cm(3) will offer equivalent or higher stability as the sulphuric acid dimer for the formation of atmospheric molecular clusters. We assess and utilise the DLPNO-CCSD(T) method for studying larger clusters involving phosphoric acid and sulphuric acid and find that having a phosphoric acid molecule present in the cluster enhances the further addition of sulphuric acid molecules. [GRAPHICS] .