| dc.description |
Molecular clusters of atmospheric relevance containing water and two acidic species, HNO3 and HOCl, are studied using several theoretical techniques, with especial emphasis on their bonding characteristics. Stable structures are found with a minimum in their potential energy surface for aggregates with three and four H2O molecules. In the most stable configurations the H atom of HNO3 is partly donated to the O atom of HOCl. A full proton transfer only takes place for some aggregates when four H2O molecules are present. Proton transfer parameters, electron density at the bond critical point, atomic charges and spectroscopic properties are studied for all these species, revealing direct relationships among several of these properties. The proton transfer parameter gives a straightforward indication of the degree of ionization of the aggregates, with negative or positive values for molecular or ionic (i.e. with fully ionized nitric acid) clusters, respectively. The calculated electron densities yield values typical of hydrogen bonded species. A linear correlation is found between proton transfer parameters and electron density values. Atomic charges are calculated using three different methods, namely Mulliken, Natural Bond Order, and Bader, with sometimes fairly large differences in the estimated values. The predicted spectra present large variations in wavenumber and intensity of the main bands, which could be used to identify specific aggregates among complex spectra. Finally, the effect of the strength of the chlorinated acidic species is evaluated by comparing the HOCl clusters studied here with similar aggregates containing HCl. The weaker acid favors a higher degree of proton sharing in HNO3. (C) 2014 Elsevier B.V. All rights reserved. |
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