Kinetic instability of sulfurous acid in the presence of ammonia and formic acid

Gas-Phase Formation of Sulfurous Acid (H2SO3) in the Atmosphere

Sulfurous acid (H2SO3) is known to be thermodynamically instable decomposing into SO2 and H2O. All attempts to detect this elusive acid in solution failed up to now. Reported H2SO3 formation from an experiment carried out in a mass spectrometer as well as results from theoretical calculations, however, indicated a possible kinetic stability in the gas phase. Here, it is shown experimentally that H2SO3 is formed in the OH radical-initiated gas-phase oxidation of methanesulfinic acid (CH3S(O)OH) at 295±0.5 K and 1 bar of air with a molar yield of53 - 17 + 7 ${{53}_{-17}^{+\ 7}}$  %. Further main products are SO2, SO3 and methanesulfonic acid. CH3S(O)OH represents an important intermediate product of dimethyl sulfide oxidation in the atmosphere. Global modeling predicts an annual H2SO3 production of ∼8 million metric tons from the OH+CH3S(O)OH reaction. The investigated H2SO3 depletion in the presence of water vapor results in k(H2O+H2SO3) <3×10-18 cm3 molecule-1 s-1, which indicates a lifetime of at least one second for atmospheric humidity. This work provides experimental evidence that H2SO3, once formed in the gas phase, is kinetically stable enough to allow its characterization and subsequent reactions.

Formic acid stability in different solvents by DFT calculations

CONTEXT

New technologies have been developed toward the use of green energies. The production of formic acid (FA) from carbon dioxide (CO[Formula: see text]) hydrogenation with H[Formula: see text] is a sustainable process for H[Formula: see text] storage. However, the FA adduct stabilization is thermodynamically dependent on the type of solvent and thermodynamic conditions. The results suggest a wide range of dielectric permittivity values between the dimethyl sulfoxide (DMSO) and water solvents to stabilize the FA in the absence of base. The thermodynamics analysis and the infrared and charge density difference results show that the formation of the FA complex with H[Formula: see text]O is temperature dependent and has a major influence on aqueous solvents compared to the FA adduct with amine, in good agreement with the experiment. In these conditions, the stability thermodynamic of the FA molecule may be favorable at non-organic solvents and dielectric permittivity values closer to water. Therefore, a mixture of aqueous solvents with possible ionic composition could be used to increase the thermodynamic stability of H[Formula: see text] storage in CO[Formula: see text] conversion processes.

METHODS

Using the Quantum ESPRESSO package, density functional theory (DFT) calculations were performed with periodic boundary conditions, and the electronic wave functions were expanded in plane waves. For the exchange-correlation functional, we use the vdW-DF functional with the inclusion of van der Waals (vdW) forces. Electron-ion interactions are treated by the projector augmented wave (PAW) method with pseudopotentials available in the PSlibrary repository. The wave functions and the electronic densities were expanded employing accurate cut-off energies of 6.80[Formula: see text]10[Formula: see text] and 5.44[Formula: see text]10[Formula: see text] eV, respectively. The electronic density was computed from the wave functions calculated at the [Formula: see text]-point in the first Brillouin-zone. Each structural optimization was minimized according to the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm, with force and energy convergence criteria of 25 meV[Formula: see text]Å[Formula: see text] and 1.36 meV, respectively. The electrostatic solvation effects were performed by the [Formula: see text] package with the Self-Consistent Continuum Solvation (SCCS) approach.

Effect of (H2O)n (n = 1 and 2) on HOCl + Cl reaction

In the present work, we investigate the effect of water molecules (H₂O and (H₂O)₂) on HOCl + Cl˙ → ClO˙ + HCl (R1), and HOCl + Cl˙ → OH˙ + Cl₂ (R2) reactions using quantum chemical and kinetics calculations. The present investigation suggests that a water molecule decreases the energy barrier of both reactions significantly, compared to uncatalyzed reaction. However, the effective rate constants for the water catalyzed path for both channels (R1 and R2) were found to be lower than the bimolecular rate constant of the uncatalyzed path. Further, it was found that the R2 reaction will dominate over the R1 reaction, with or without catalyst. Interestingly, the uncatalyzed title reaction was found to be two times faster than the HOCl + OH˙ reaction, but in the presence of water, HOCl + OH˙ becomes the dominant reaction compared to the HOCl + Cl˙ reaction in the atmosphere. In addition, the concentration of bimolecular complexes formed in the presence of a catalyst are found to be higher than the precursor molecule of the uncatalyzed reaction, which suggests that in the presence of catalyst, the HOCl + Cl˙ reaction would favor the catalyzed path rather than the uncatalyzed path.

Accurate determination of reaction energetics and kinetics of the HO2˙ + O3 → OH˙ + 2O2 reaction

In the present work, we have studied the HO₂˙ + O₃ → HO˙ + 2O₂ reaction using chemical kinetics and quantum chemical calculations. We have employed the post-CCSD(T) method to estimate the barrier height and reaction energy for the title reaction. In the post-CCSD(T) method, we have included zero point energy corrections, contributions from full triple excitations and partial quadratic excitations at the coupled-cluster level, and core corrections. We have also computed the reaction rate in the temperature range of 197-450 K and found good agreement with all the available experimental results. In addition, we have also fitted the computed rate constants with the Arrhenius expression and obtained an activation energy of 1.0 ± 0.1 kcal mol^-1, almost identical to the value recommended by IUPAC and JPL.

Gas phase acidity of water clusters

In the present work, we have estimated the gas-phase acidity of different water clusters, i.e., (H₂O)_n, n = 1-20, 30, 35, 42, 54, 80, and 100. The present work indicates that the gas-phase acidity of the terminal hydrogen atom increases with the size of water clusters and starts converging at (H₂O)₃₀. Furthermore, the present work also indicates that the gas-phase acidity of a terminal hydrogen atom is higher than that of the corresponding bulk hydrogen atom for the same size of water cluster.

Oxidation of HOSO˙ by O2 (3Σg-): a key reaction deciding the fate of HOSO˙ in the atmosphere

In the present work, we have studied the oxidation of HOSO˙ by O₂ (³Σ_g^-) employing quantum chemical and kinetic calculations. The present work reveals that HOSO˙ + O₂ (³Σ_g^-) is a barrierless reaction which proceeds through a stable hydrogen-bonded complex. The estimated atmospheric lifetime of HOSO˙ in the presence of O₂ (³Σ_g^-) is found to be several orders of magnitude less compared to the other oxidation paths of HOSO˙, suggesting that the oxidation of HOSO˙ by O₂ (³Σ_g^-) might be the most dominant oxidation path of HOSO˙ in the atmosphere.

OH + HCl Reaction on the Surface of Ice: An Ab Initio Molecular Dynamics Study

We have investigated the OH + HCl reaction on the surface of ice using Born-Oppenheimer molecular dynamics (BOMD) simulation. The present work revealed that the OH + HCl reaction becomes ∼1 order of magnitude faster on the ice surface compared to the bare reaction. The BOMD simulation also indicates that the Cl radical formed on the ice surface through the title reaction can form two hydrogen bonds at a time with the water molecules present on the ice surface; hence, the Cl radical cannot escape from the ice surface easily.

Effect of microsolvation on the mode specificity of the OH˙(H2O) + HCl reaction

The present study investigates the mode specificity in the microsolvated OH˙(H₂O) + HCl reaction using on-the-fly direct dynamics simulation. To the best of our knowledge, this is the first study which aims to gain insights into the effect of microsolvation on the mode selectivity. Our investigation reveals that, similar to the gas phase OH˙ + HCl reaction, the microsolvated reaction is also predominantly affected by the vibrational excitation of the HCl mode, whereas the OH vibrational mode behaves as a spectator. Interestingly, in contrast to the behavior of the bare reaction, the integral cross section at the ground state of the microsolvated reaction decreases with an increase in translational energy. However, for the vibrational excited states, the reactivity of the microsolvated reaction is found to be higher than that of the bare reaction within the selected range of translational energies.

Oxidation of HOSO˙ by Cl˙: a new source of SO2 in the atmosphere?

In the present work, we have studied the formation of SO₂ in the atmosphere from the oxidation of HOSO˙ by Cl˙ at the CCSD(T)/aug-cc-pV(+d)TZ//MP2/aug-cc-pV(+d)TZ level of theory. The present work reveals that the title reaction is a barrierless reaction that proceeds through a stable intermediate sulfurochloridous acid having a stabilization energy of ∼-56.5 kcal mol^-1. The rate constant values within the temperature range of 213-400 K indicate that the rate of HOSO˙ + Cl˙ = SO₂ + HCl reaction does not change much with the change in temperature. Besides, the reaction was also found to be insensitive towards pressure change. Interestingly, the relative rate of HOSO˙ + Cl˙ reaction with respect to HOSO˙ + OH˙ reaction indicates that HOSO˙ + Cl˙ is always much slower than HOSO˙ + OH˙ reaction, within the temperature range of 213-400 K.

The effect of ammonia and formic acid on the oxidation of CO via a simple Criegee intermediate

In the present work, we have investigated the effect of catalysts (ammonia, formic acid, ammonia dimer, and ammonia water complex) on the oxidation of CO via a simple Criegee intermediate by means of kinetics and quantum chemical calculations. Our finding suggests that, in the presence of ammonia and ammonia dimer the title reaction becomes a barrierless reaction with respect to the isolated reactants (energy barrier = ∼-0.53 and ∼-0.27 kcal mol-1, respectively), whereas in the presence of formic acid and ammonia-water complex the energy barrier of the CI + CO reaction becomes ∼2.84 and ∼0.82 kcal mol-1, respectively. However, among all the catalysts, due to the very low concentration of the ammonia dimer, its contribution towards the title reaction is insignificant as compared to that of the other catalysts. In addition, the relative rate of the other catalyzed channels against the uncatalyzed reaction suggests that the rate of the catalyzed CI + CO reaction is ∼8-10 orders of magnitude lower than the uncatalyzed reaction. However, the concentration of bimolecular complexes formed in the presence of catalysts (except the ammonia dimer) is ∼1-8 orders of magnitude higher than the concentration of bimolecular complexes formed in the uncatalyzed reaction.