QUANTUM CHEMICAL STUDIES ON THE SULFURIC ACID ADDITION TO THE SMALL
CLUSTERS OF SULFUR CONTAINING COMPAUNDS
M. TOIVOLA1, T. KURTÉN1 , I. K. ORTEGA1,
V. LOUKONEN1 , H. VEHKAMÄKI1 and M. KULMALA1
1
Department of Physics, P.O. Box 64, FI-00014, University of Helsinki, Finland
Keywords: Sulfuric acid , Peroxodisulfuric acid, cluster formation.
INTRODUCTION
Atmospheric sulfur dioxide is mainly emitted by volcanoes and industrial process such as fossil fuel
burning, and it is oxidized in the atmosphere. The main product of oxidation of SO2 is sulfuric acid.
Sulfuric acid is a key component in atmospheric aerosol formation. Earlier publications of sulfuric acid
nucleation experiments gave us reason to expect that some external SO2 oxidation products could take part
of the sulfuric acid nucleation (see e.g. Berndt et al., 2008). The threshold concentration of H2SO4 was
lower if H2SO4 was produced in situ via SO2 oxidation compared to the case where H2 SO4 was taken from
the liquid sample. However, new experiments by Sipilä et al. (2010) showed that disagreement does not
exist. Even the experiments no longer support idea of nucleation of sulfur containing compounds other
than H2 SO4, small amounts of several kind of sulfur containing compounds are measured in the
atmosphere and their participation to the nucleation cannot be completely excluded.
Possible biproducts of H2SO4 oxidation chain are formed from HSO3. Salonen et al (2009) studied the
stability of the dimers of one sulfuric acid and one possible biproduct of the oxidation chain of SO2.
According to the calculations, only the sulfuric acid-peroxodisulfuric acid (H2S2 O8) dimer is more stable
than the sulfuric acid dimer with respect to the electronic energy. This is necessary but not sufficient
condition for some molecule to enhance H2SO4 nucleation. In this work we study sulfuric acid addition to
the (H2S2 O8)l*( H2SO4)m*(H2 O)n clusters (l=0-2, m=1-3, n=0-2) using quantum chemistry calculations.
METHODS
Calculations on clusters have been performed using a systematic multi-step method recently developed by
our group (Ortega et al. 2008). The initial molecule and cluster geometries were taken from earlier
computational studies (Ortega et al. 2008; Salonen et al. 2009) when possible, or generated with the
DL_POLY_2 (Smith et al., 2002) molecular dynamics (MD) program, using both sulfuric acid molecules
and bisulfate- hydronium ion pair in simple MD annealing. The more stable isomers were optimized
using the SIESTA program (Soler et al., 2002) The gradient-corrected BLYP functional (Miehlich et al.,
1989) and the double-ζ polarized (DZP) functions were used. Finally, we calculated single point energies
using the TURBOMOLE program suite (Ahlrichs et al., 1989). Energies were computed at the RIMP2/aug-cc-pV(T+d)Z level (Dunning Jr. et al.., 2001).
CONCLUSIONS
We have studied following clusters: (H2SO4)3*(H2 O)n, (H2SO4)4*(H2 O)n, H2S2O8*(H2SO4)2*(H2 O)n,
H2S2 O8*( H2SO4)3*(H2 O)n, (H2S2 O8)2* H2SO4*(H2 O)n, (H2S2 O8)2*( H2SO4)2*(H2 O)n, where n=0-2. The
minimum energy geometries of the clusters are characterized by several hydrogen bonds and proton
transfer occurs in the several clusters. We calculated the binding energies and sulfuric acid addition
energies.
We studied the addition of the H2SO4 molecule to the cluster containing 3 H2SO4 and 0-2 H2 O molecules.
We wanted to know if the presence of H2S2O8 molecule affect to the H2SO4 addition and we replaced 0-2
H2SO4 molecules (in the initial cluster) with H2S2O8. Figure 1 shows the addition energies (ΔE) for all the
described cases. Only if no water is present the presence of H2 S2O8 makes H2SO4 addition more
favourable compared to pure H2SO4 cluster.
Figure 1. Sulfuric acid addition energies. One sulfuric acid molecule is added to a cluster of three sulfur
containing molecules and 0-2 water molecules.
ACKNOWLEDGEMENTS
The financial support by the Academy of Finland Centre of Excellence program (project no
1118615) is gratefully acknowledged.
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