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We investigate the molecular interactions between phosphoric acid (
) and common atmospheric nucleation precursors (
,
and
) using computational methods. The equilibrium geometries and vibrational frequencies are obtained using the three DFT functionals M06-2X, PW91 and ωB97X-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.