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| Titel |
Ab initio modeling O2-(H2O)n and O3-(H2O)n clusters, n ⤠12 |
| VerfasserIn |
Nicolai Bork, Martin B. Enghoff, Jens Olaf P. Pedersen, Kurt V. Mikkelsen, Theo Kurtén, Henrik Svensmark |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250055384
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| Zusammenfassung |
| For almost two decades, empirical evidence has linked the influx of cosmic rays to cloud
formation.[1] Cosmic rays are the primary source of atmospheric ionization and it has
therefore been speculated that ions may provide the link between cosmic rays and clouds.
The exact mechanism is largely unknown, but one plausible mechanism is through catalytic
oxidation of SO2 to H2SO4.[2]
Due to electron affinities and concentrations, O2- and O3- anions are likely
primary products of cosmic ray ionization. Such ions will quickly attract a number
of water and the attached water molecules will be important for any subsequent
reactions.[3] It is well known that solvent molecules alter both thermodynamic
and kinetic properties of most reactions, but further, the solvent water may also
disperse the charge and /or screen the O2- and O3- species from further reactions.
Despite several previous studies, the exact sizes and structures of the most important
O2-(H2O)n and O3-(H2O)n clusters remain uncertain.[4] This information is crucial for
studying all subsequent reactions in the proposed catalytic oxidation of SO2 to
H2SO4.[1,2]
We present an ab initio study of gaseous clusters of O2- and O3- with water. We have
included up to 12 water, constituting the first and second solvation shells. We have
determined the thermodynamics of cluster growth in excellent agreement with existing
experimental data.[3] We find that anionic O2-(H2O)n and O3-(H2O)n clusters are
thermally stabilized at typical atmospheric conditions for at least n = 8. This is considerably
larger than previous assumptions of n = 4 - 5.[4]
The first 4 water molecules are strongly bound to the anion due to delocalization of the
excess charge while stabilization of more than 4 H2O is due to normal hydrogen bonding.
Although clustering up to 12 H2O, we find that the O2 and O3 anions retain at least ca. 80%
of the charge. Further, the O2- and O3- species are located near at the surface of the
cluster and are thus accessible for further reactions in the catalytic H2SO4 oxidation
cycle.
[1] Enghoff et al., Atmos. Chem. Phys., 8, 4911–4923, 2008.
[2] Svensmark et al., Proc. R. Soc. A, 463, 385-396, 2007.
[3] Fehsenfeld and Ferguson, J. Chem. Phys., 61, 8, 1974.
[4] Seta et al., J. Phys. Chem. A, 107, 7, 2003. Lee and Kim, Mol. Phys., 100, 6, 875,
2002. |
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