A Perfluorinated Carboxylic Sulfuric Anhydride: Microwave and Computational Studies of CF3COOSO2OH

Direct Measurements of Covalently Bonded Sulfuric Anhydrides from Gas-Phase Reactions of SO3 with Acids under Ambient Conditions

Sulfur trioxide (SO3) is an important oxide of sulfur and a key intermediate in the formation of sulfuric acid (H2SO4, SA) in the Earth's atmosphere. This conversion to SA occurs rapidly due to the reaction of SO3 with a water dimer. However, gas-phase SO3 has been measured directly at concentrations that are comparable to that of SA under polluted mega-city conditions, indicating gaps in our current understanding of the sources and fates of SO3. Its reaction with atmospheric acids could be one such fate that can have significant implications for atmospheric chemistry. In the present investigation, laboratory experiments were conducted in a flow reactor to generate a range of previously uncharacterized condensable sulfur-containing reaction products by reacting SO3 with a set of atmospherically relevant inorganic and organic acids at room temperature and atmospheric pressure. Specifically, key inorganic acids known to be responsible for most ambient new particle formation events, iodic acid (HIO3, IA) and SA, are observed to react promptly with SO3 to form iodic sulfuric anhydride (IO3SO3H, ISA) and disulfuric acid (H2S2O7, DSA). Carboxylic sulfuric anhydrides (CSAs) were observed to form by the reaction of SO3 with C2 and C3 monocarboxylic (acetic and propanoic acid) and dicarboxylic (oxalic and malonic acid)-carboxylic acids. The formed products were detected by a nitrate-ion-based chemical ionization atmospheric pressure interface time-of-flight mass spectrometer (NO3--CI-APi-TOF; NO3--CIMS). Quantum chemical methods were used to compute the relevant SO3 reaction rate coefficients, probe the reaction mechanisms, and model the ionization chemistry inherent in the detection of the products by NO3--CIMS. Additionally, we use NO3--CIMS ambient data to report that significant concentrations of SO3 and its acid anhydride reaction products are present under polluted, marine and polar, and volcanic plume conditions. Considering that these regions are rich in the acid precursors studied here, the reported reactions need to be accounted for in the modeling of atmospheric new particle formation.

Kinetics of Sulfur Trioxide Reaction with Water Vapor to Form Atmospheric Sulfuric Acid

Although experimental methods can be used to obtain the quantitative kinetics of atmospheric reactions, experimental data are often limited to a narrow temperature range. The reaction of SO3 with water vapor is important for elucidating the formation of sulfuric acid in the atmosphere; however, the kinetics is uncertain at low temperatures. Here, we calculate rate constants for reactions of sulfur trioxide with two water molecules. We consider two mechanisms: the SO3···H2O + H2O reaction and the SO3 + (H2O)2 reaction. We find that beyond-CCSD(T) contributions to the barrier heights are very large, and multidimensional tunneling, unusually large anharmonicity of high-frequency modes, and torsional anharmonicity are important for obtaining quantitative kinetics. We find that at lower temperatures, the formation of the termolecular precursor complexes, which is often neglected, is rate-limiting compared to passage through the tight transition states. Our calculations show that the SO3···H2O + H2O mechanism is more important than the SO3 + (H2O)2 mechanism at 5-50 km altitudes. We find that the rate ratio between SO3···H2O + H2O and SO3 + (H2O)2 is greater than 20 at altitudes between 10 and 35 km, where the concentration of SO3 is very high.

Microwave and Computational Study of Pivalic Sulfuric Anhydride and the Pivalic Acid Monomer: Mechanistic Insights into the RCOOH + SO3 Reaction

The microwave spectrum of pivalic sulfuric anhydride, (CH₃)₃CCOOSO₂OH (PivSA), has been observed by rotational spectroscopy. The compound was formed by the reaction of SO₃ with (CH₃)₃CCOOH (pivalic acid) in a supersonic jet in a manner analogous to that previously observed with other carboxylic acids. Computational analysis indicates that the reaction is best described as a pericyclic process coupled with a 60° rotation of the t-butyl group. Product formation can occur through either a sequential (two-step) or a concerted (one-step) pathway. The former involves an internal rotation of the t-butyl group through a 0.11 kcal/mol barrier followed by the pericyclic reaction that joins the moieties. The latter passes through a second-order saddle point in which the internal rotation and pericyclic reaction occur simultaneously. This path is the most energetically favorable, as the zero-point corrected energy at the saddle point structure is 0.16 kcal/mol below that of a putative (CH₃)₃CCOOH-SO₃ precursor complex. Additional computational work involving a series of carboxylic acids is reported, which explores the effects of gas-phase acidity and basicity of the RCOOH reactant on reaction energetics. These calculations, together with prior experimental and theoretical studies of the acetic and trifluoroacetic derivatives, demonstrate that the basicity of the carbonyl oxygen, not the acidity of the COOH proton, is the important driving factor for the reaction. As a precursor to the experimental work on the title molecule, microwave spectra of the parent and OD forms of the pivalic acid monomer were recorded and are reported here as well. A convenient synthesis of SO₃ is also described.

Reaction of SO3 with HONO2 and Implications for Sulfur Partitioning in the Atmosphere

Sulfur trioxide is a critical intermediate for the sulfur cycle and the formation of sulfuric acid in the atmosphere. The traditional view is that sulfur trioxide is removed by water vapor in the troposphere. However, the concentration of water vapor decreases significantly with increasing altitude, leading to longer atmospheric lifetimes of sulfur trioxide. Here, we utilize a dual-level strategy that combines transition state theory calculated at the W2X//DF-CCSD(T)-F12b/jun'-cc-pVDZ level, with variational transition state theory with small-curvature tunneling from direct dynamics calculations at the M08-HX/MG3S level. We also report the pressure-dependent rate constants calculated using the system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory. The present findings show that falloff effects in the SO₃ + HONO₂ reaction are pronounced below 1 bar. The SO₃ + HONO₂ reaction can be a potential removal reaction for SO₃ in the stratosphere and for HONO₂ in the troposphere, because the reaction can potentially compete well with the SO₃ + 2H₂O reaction between 25 and 35 km, as well as the OH + HONO₂ reaction. The present findings also suggest an unexpected new product from the SO₃ + HONO₂ reaction, which, although very short-lived, would have broad implications for understanding the partitioning of sulfur in the stratosphere and the potential for the SO₃ reaction with organic acids to generate organosulfates without the need for heterogeneous chemistry.

Molecular-Scale Mechanism of Sequential Reaction of Oxalic Acid with SO3: Potential Participator in Atmospheric Aerosol Nucleation

Recent research has shown the almost barrierless cycloaddition reaction of the carboxylic acid with one SO₃ to form products with group of -OSO₃H, which can form stable clusters with the nucleation precursors through hydrogen bonds (Mackenzie et al., Science 2015, 349, 58). Oxalic acid (OA), the simplest and prevalent dicarboxylic acid, was selected as an example to clarify the possibility to react with two SO₃ sequentially and the nucleation potential of products. The results indicate that OA can sequentially react with two SO₃ through low reaction barriers to form the primary product (oxalic sulfuric anhydride (OSA)) and the secondary product (oxalic disulfuric anhydride (ODSA)). Interactions between atmospheric nucleation precursors and OSA, ODSA, or OA are in the order of ODSA > OSA > OA through evaluating the stability of generated clusters by the topological, thermodynamics, and kinetic analysis, which implies generated products could be nucleation stabilizers with nucleation potential positively correlating with the number of -OSO₃H. This reaction mechanism contributes to a comprehensive understanding of the reactivity of dicarboxylic acid in the polluted environment as well as the role of products in organosulfur chemistry and, to some extent, help to explain the missing sources of new particle formation.