Theoretical Study on Stable Small Clusters of Oxalic Acid with Ammonia and Water

Thermodynamically stable small clusters of oxalic acid (CO₂H)₂, ammonia (NH₃), and water (H₂O) are studied through quantum chemical calculations. The (CO₂H)₂–NH₃ core system with up to three waters of hydration was examined by B3LYP density functional theory and MP2 molecular orbital theory with the aug-cc-pVDZ basis set. The (CO₂H)₂–NH₃ core complexes are observed to hydrogen bond strongly and should be found in appreciably significant concentrations in the atmosphere. Subsequent hydration of the (CO₂H)₂–NH₃ core, however, is found to be somewhat prohibitive under ambient conditions. Relative populations of the examined clusters are predicted and the binding patterns detailed. Atmospheric implications related to new particle formations are discussed

Theoretical Study on the Structure and Stabilities of Molecular Clusters of Oxalic Acid with Water

The importance of aerosols to humankind is well-known, playing an integral role in determining Earth’s climate and influencing human health. Despite this fact, much remains unknown about the initial events of nucleation. In this work, the molecular properties of common organic atmospheric pollutant oxalic acid and its gas phase interactions with water have been thoroughly examined. Local minima single-point energies for the monomer conformations were calculated at the B3LYP and MP2 level of theory with both 6-311++G­(d,p) and aug-cc-pVDZ basis sets and are compared with previous works. Optimized geometries, relative energies, and free energy changes for the stable clusters of oxalic acid conformers with up to six waters were then obtained from B3LYP calculations with 6-31+G­(d) and 6-311++G­(d,p) basis sets. Initially, cooperative binding is predicted to be the most important factor in nucleation, but as the clusters grow, dipole cancellations are found to play a pivotal role. The clusters of oxalic acid hydrated purely with water tend to produce extremely stable and neutral core systems. Free energies of formation and atmospheric implications are discussed

Theoretical Study of the Hydrogen Abstraction of Substituted Phenols by Nitrogen Dioxide as a Source of HONO

The mild yet promiscuous reactions of nitrogen dioxide (NO₂) and phenolic derivatives to produce nitrous acid (HONO) have been explored with density functional theory calculations. The reaction is found to occur via four distinct pathways with both proton coupled electron transfer (PCET) and hydrogen atom transfer (HAT) mechanisms available. While the parent reaction with phenol may not be significant in the gas phase, electron donating groups in the ortho and para positions facilitate the reduction of nitrogen dioxide by electronically stabilizing the product phenoxy radical. Hydrogen bonding groups in the ortho position may additionally stabilize the nascent resonantly stabilized radical product, thus enhancing the reaction. Catechol (<i>ortho</i>-hydroxy phenol) has a predicted overall free energy change Δ<i>G</i>⁰ = −0.8 kcal mol^–1 and electronic activation energy <i>E</i>_<i>a</i> = 7.0 kcal mol^–1. Free amines at the ortho and para positions have Δ<i>G</i>⁰ = −3.8 and −1.5 kcal mol^–1; <i>E</i>_a = 2.3 and 2.1 kcal mol^–1, respectively. The results indicate that the hydrogen abstraction reactions of these substituted phenols by NO₂ are fast and spontaneous. Hammett constants produce a linear correlation with bond dissociation energy (BDE) demonstrating that the BDE is the main parameter controlling the dark abstraction reaction. The implications for atmospheric chemistry and ground-level nitrous acid production are discussed

Theoretical Study of the Gaseous Hydrolysis of NO₂ in the Presence of Amines

The effects on the hydrolysis of NO₂ in the presence of methylamine and dimethylamine molecules were investigated by theoretical calculations of a series of the molecular clusters 2NO₂-<i>m</i>H₂O–CH₃NH₂ (<i>m</i> = 1–3) and 2NO₂-<i>m</i>H₂O-(CH₃)₂NH (<i>m</i> = 1, 2). With methylamine included in the clusters, the energy barrier is reduced by 3.2 kcal/mol from that with ammonia, and the corresponding products may form without an energy barrier. The results show that amines have larger effects than ammonia in promoting the hydrolysis of NO₂ on thermodynamics. The additional water molecules can stabilize the transition states and the product complexes, and we infer that adding more water molecules in the reactions mainly act as solvent and promoting to form the methylamine nitrate (CH₃NH₃⁺NO₃^–). In addition, the interactions of CH₃NH₂ and (CH₃)₂NH on the hydration of HNO₃ are also more effective than NH₃, and the NH₄NO₃, CH₃NH₃NO₃, and (CH₃)₂NH₂NO₃ complexes tend to form the larger aerosols with the increasing of water molecules. The equilibrium geometries, harmonic vibrational frequencies, and intensities of both HONO–CH₃NH₂ and HONO–NH₃ complexes were investigated. Calculations predict that the binding energies of both HONO–CH₃NH₂ complexes are larger than HONO–NH₃ complexes, and the OH stretching vibrational frequencies and intensities are most affected. The natural bond orbital analysis was performed to describe the donor–acceptor interactions on a series of complexes in the reactions 2NO₂ + H₂O + CH₃NH₂ and 2NO₂ + H₂O + (CH₃)₂NH, as well as the complexes of HONO–NH₃ and HONO–CH₃NH₂. The results show that the interactions with amines are relatively larger, and the higher stabilization energies between CH₃NH₂ and HONO are found

Hydrolysis of Sulfur Dioxide in Small Clusters of Sulfuric Acid: Mechanistic and Kinetic Study

The deposition and hydrolysis reaction of SO₂ + H₂O in small clusters of sulfuric acid and water are studied by theoretical calculations of the molecular clusters SO₂–(H₂SO₄)_<i>n</i>–(H₂O)_<i>m</i> (<i>m</i> = 1,2; <i>n</i> = 1,2). Sulfuric acid exhibits a dramatic catalytic effect on the hydrolysis reaction of SO₂ as it lowers the energy barrier by over 20 kcal/mol. The reaction with monohydrated sulfuric acid (SO₂ + H₂O + H₂SO₄ – H₂O) has the lowest energy barrier of 3.83 kcal/mol, in which the cluster H₂SO₄–(H₂O)₂ forms initially at the entrance channel. The energy barriers for the three hydrolysis reactions are in the order SO₂ + (H₂SO₄)–H₂O > SO₂ + (H₂SO₄)₂–H₂O > SO₂ + H₂SO₄–H₂O. Furthermore, sulfurous acid is more strongly bonded to the hydrated sulfuric acid (or dimer) clusters than the corresponding reactant (monohydrated SO₂). Consequently, sulfuric acid promotes the hydrolysis of SO₂ both kinetically and thermodynamically. Kinetics simulations have been performed to study the importance of these reactions in the reduction of atmospheric SO₂. The results will give a new insight on how the pre-existing aerosols catalyze the hydrolysis of SO₂, leading to the formation and growth of new particles

Mechanism of the Gaseous Hydrolysis Reaction of SO₂: Effects of NH₃ versus H₂O

Effects of ammonia and water molecules on the hydrolysis of sulfur dioxide are investigated by theoretical calculations of two series of the molecular clusters SO₂-(H₂O)_<i>n</i> (<i>n</i> = 1–5) and SO₂-(H₂O)_<i>n</i>-NH₃ (<i>n</i> = 1–3). The reaction in pure water clusters is thermodynamically unfavorable. The additional water in the clusters reduces the energy barrier for the reaction, and the effect of each water decreases with the increasing number of water molecules in the clusters. There is a considerable energy barrier for reaction in SO₂-(H₂O)₅, 5.69 kcal/mol. With ammonia included in the cluster, SO₂-(H₂O)_<i>n</i>-NH₃, the energy barrier is dramatically reduced, to 1.89 kcal/mol with <i>n</i> = 3, and the corresponding product of hydrated ammonium bisulfate NH₄HSO₃-(H₂O)₂ is also stabilized thermodynamically. The present study shows that ammonia has larger kinetic and thermodynamic effects than water in promoting the hydrolysis reaction of SO₂ in small clusters favorable in the atmosphere