AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
The Role of Hydration in Formation and Reactivity of Sulfuric Acid Clusters Containing Ammonia and Amines
JOSEPH DEPALMA, Douglas Doren, Murray Johnston, University of Delaware
Abstract Number: 108 Working Group: Aerosol Chemistry
Abstract Our previous computational work on charged and uncharged sulfuric acid clusters containing ammonia and amines has elucidated chemical properties and processes which are important to new particle formation and growth, including: cluster free energy of formation, molecular binding within clusters, reactivity (amine substitution for ammonia in clusters), and neutralization (optimum number of ammonia/amine molecules for a given number of sulfuric acid molecules). Yet water is ever present in the atmosphere, and is expected to influence the structure and thermochemistry of these clusters. Ambient clusters containing water are difficult to study experimentally, making computational methods an indispensible tool for their characterization. In the work to be presented here, clusters of the form [(BH$^(+))$_(x)(HSO$_(4)$_(-))$_(x)(H$_(2)O)$_(y)], with B = NH$_3, DMA, x = 4,5,6 and y = 1-10 were investigated using classical numerical sampling, varying levels of quantum chemistry, and Boltzmann averaging over stable populations. Preliminary results suggest that water increases the hydrogen bonding in these clusters, thereby decreasing the binding energy between acid and base molecules. The change in binding energy does not appear to have a cluster size dependence, and is more pronounced with ammonia than amines, as ammonium salts have higher solubility in water than alkyl amines. The increased solubility decreases the anion-anion interactions found in “dry” uncharged clusters containing ammonia. Water does not solvate the amine clusters well, so the effects on binding energy are not as great as ammonia. Amine substitution for ammonia appears to be favorable, with values comparable to “dry” positively charged clusters. Sulfate ions are not observed in clusters with either ammonia or amines, suggesting that bisulfate is the preferred cluster composition in this size range, with or without water. Computational results will be discussed in the context of likely cluster growth pathways in the atmosphere.