ARTICLE pubs.acs.org/JPCA
Structure and Energetics of Nanometer Size Clusters of Sulfuric Acid with Ammonia and Dimethylamine Joseph W. DePalma, Bryan R. Bzdek, Douglas J. Doren, and Murray V. Johnston* Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
bS Supporting Information ABSTRACT: The structures of positively and negatively charged clusters of sulfuric acid with ammonia and/or dimethylamine ((CH3)2NH or DMA) are investigated using a combination of Monte Carlo configuration sampling, semiempirical calculations, and density functional theory (DFT) calculations. Positively charged clusters of the formula [(NH4+)x(HSO4)y]+, where x = y + 1, are studied for 1 e y e 10. These clusters exhibit strong cationanion interactions, with no contribution to the hydrogenbonding network from the bisulfate ion protons. A similar hydrogen-bonding network is found for the [(DMAH+)5(HSO4)4] cluster. Negatively charged clusters derived from the reaction of DMA with [(H2SO4)3(NH4+)(HSO4)2] are also studied, up to the fully reacted cluster [(DMAH+)4(HSO4)5]. These clusters exhibit anionanion and ionmolecule interactions in addition to cationanion interactions. While the hydrogen-bonding network is extensive for both positively and negatively charged clusters, the binding energies of ions and molecules in these clusters are determined mostly by electrostatic interactions. The thermodynamics of amine substitution is explored and compared to experimental thermodynamic and kinetic data. Ammonia binds more strongly than DMA to sulfuric acid due to its greater participation in hydrogen bonding and its ability to form a more compact structure that increases electrostatic attraction between oppositely charged ions. However, the greater gas-phase basicity of DMA is sufficient to overcome the stronger binding of ammonia, making substitution of DMA for ammonia thermodynamically favorable. For small clusters of both polarities, substitutions of surface ammonium ions are facile. As the cluster size increases, an ammonium ion becomes encapsulated in the center of the cluster, making it inaccessible to substitution.
’ INTRODUCTION The role of atmospheric particles in global climate change has been established, but is not well understood. New particle formation (NPF) is a key process governing atmospheric particle concentrations.1,2 Particles arising from NPF can grow large enough (∼100 nm) to act as cloud condensation nuclei (CCN)17 and may ultimately affect global climate by scattering solar radiation (albedo effect) as well as by influencing precipitation patterns.8 Uncertainties in modeling aerosol effects on climate can be reduced through an improved molecular-level understanding of the mechanisms underlying NPF. Two of the most important species implicated in NPF are sulfuric acid and ammonia.914 Ternary homogeneous nucleation (THN) studies of sulfuric acidwaterammonia systems have shown that ammonia decreases the thermodynamic barrier to nucleation, thereby increasing the nucleation rate.1517 However, recent work has shown that organic species can contribute significantly to nanoparticle growth during NPF.1821 In fact, amines and organic aminium salts appear to be particularly promising candidates for enhancing NPF rates, either by reducing the barrier to nucleation22,23 or by contributing to particle growth.19,20,2426 Nonetheless, the exact nature and role of organic species in NPF is poorly understood, as direct observation of the chemical composition of ambient particles and clusters
