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One of the main sources of uncertainty for understanding global warming is understanding the formation of larger secondary aerosols. The beginning stages start with the formation of prenucleation complexes from precursor monomers of acids, bases, and organic molecules. The detailed interactions responsible for prenucleation and subsequent aerosol formation are difficult to decipher experimentally. We present a computational chemistry study of the interactions between three different acid molecules and two different bases. By combining a comprehensive search routine covering many thousands of configurations at the semiempirical level with high level quantum chemical calculations of approximately 1000 clusters for every possible combination of clusters containing a sulfuric acid molecule, a formic acid molecule, a hydrochloric acid molecule, an ammonia molecule, a dimethylamine molecule, and 0–3 water molecules, we have completed an exhaustive search of the DLPNO-CCSD(T)/CBS//ωB97X-D/6-31++G** Gibbs free energy surface for this system. This first detailed study of HCl interacting with two other acids and two bases reveals the subtleties that exist in the formation of prenucleation complexes for this system. We find that nitric acid forms stronger interactions in dry clusters than hydrochloric acid does. Often as the clusters grow larger with hydration, the sequential energies of clusters containing hydrochloric acid become more favorable than those with nitric acid. The detailed geometries of each minimum free energy cluster are often more important than traditional acid or base strength, which makes a priori prediction of which atmospheric species will be most important for driving prenucleation growth quite difficult. The results presented in this paper add to the conclusions that hydrogen bond topology and the detailed structural interactions that are subtle interplays between enthalpy and entropy are as important as conventional ideas such as acid/base strength.


Environmental Science: Atmospheres, Issue 9

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