Quantum Mechanical Study of Sulfuric Acid Hydration: Atmospheric Implications

Dean's Office, College of Arts and Sciences, and Department of Chemistry, Bucknell University, Lewisburg, Pennsylvania 17837, United States.
The Journal of Physical Chemistry A (Impact Factor: 2.69). 03/2012; 116(9):2209-24. DOI: 10.1021/jp2119026
Source: PubMed


The role of the binary nucleation of sulfuric acid in aerosol formation and its implications for global warming is one of the fundamental unsettled questions in atmospheric chemistry. We have investigated the thermodynamics of sulfuric acid hydration using ab initio quantum mechanical methods. For H(2)SO(4)(H(2)O)(n) where n = 1-6, we used a scheme combining molecular dynamics configurational sampling with high-level ab initio calculations to locate the global and many low lying local minima for each cluster size. For each isomer, we extrapolated the Møller-Plesset perturbation theory (MP2) energies to their complete basis set (CBS) limit and added finite temperature corrections within the rigid-rotor-harmonic-oscillator (RRHO) model using scaled harmonic vibrational frequencies. We found that ionic pair (HSO(4)(-)·H(3)O(+))(H(2)O)(n-1) clusters are competitive with the neutral (H(2)SO(4))(H(2)O)(n) clusters for n ≥ 3 and are more stable than neutral clusters for n ≥ 4 depending on the temperature. The Boltzmann averaged Gibbs free energies for the formation of H(2)SO(4)(H(2)O)(n) clusters are favorable in colder regions of the troposphere (T = 216.65-273.15 K) for n = 1-6, but the formation of clusters with n ≥ 5 is not favorable at higher (T > 273.15 K) temperatures. Our results suggest the critical cluster of a binary H(2)SO(4)-H(2)O system must contain more than one H(2)SO(4) and are in concert with recent findings (1) that the role of binary nucleation is small at ambient conditions, but significant at colder regions of the troposphere. Overall, the results support the idea that binary nucleation of sulfuric acid and water cannot account for nucleation of sulfuric acid in the lower troposphere.

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    • "Improving our understanding of the configuration and thermodynamics of H 3 O + (H 2 O) 8 , or H + (H 2 O) 9 , clusters is an important step in elucidating how water molecules forms larger clusters and, subsequently, aerosols in the atmosphere. This work is a continuation of long-standing efforts to explore ion-molecule interactions[4] [5] [6] [7] [8], hydrogen-bonded interactions[9-11][16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] water cluster formation[3] [4] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20], and atmospheric processes[5] [6] [7] [8] [12] [21] [22] [23] [24] [25] [26] [27] [28]. "
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    ABSTRACT: Using molecular dynamics configurational sampling combined with ab initio energy calculations, we determined the low energy isomers of the bisulfate hydrates. We calculated the CCSD(T) complete basis set (CBS) binding electronic and Gibbs free energies for 53 low energy isomers of HSO(4)(-)(H(2)O)(n=1-6) and derived the thermodynamics of adding waters sequentially to the bisulfate ion and its hydrates. Comparing the HSO(4)(-)/H(2)O system to the neutral H(2)SO(4)/H(2)O cluster, water binds more strongly to the anion than it does to the neutral molecules. The difference in the binding thermodynamics of HSO(4)(-)/H(2)O and H(2)SO(4)/H(2)O systems decreases with increasing number of waters. The thermodynamics for the formation of HSO(4)(-)(H(2)O)(n=1-5) is favorable at 298.15 K, and that of HSO(4)(-)(H(2)O)(n=1-6) is favorable for T < 273.15 K. The HSO(4)(-) ion is almost always hydrated at temperatures and relative humidity values encountered in the troposphere. Because the bisulfate ion binds more strongly to sulfuric acid than it does to water, it is expected to play a role in ion-induced nucleation by forming a strong complex with sulfuric acid and water, thus facilitating the formation of a critical nucleus.
    Full-text · Article · Apr 2012 · The Journal of Physical Chemistry A
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