Inorganic reactions of iodine(III) in acidic solutions and free energy of iodous acid formation

Iodine Clusters in the Atmosphere I: Computational Benchmark and Dimer Formation of Oxyacids and Oxides

The contribution of iodine-containing compounds to atmospheric new particle formation is still not fully understood, but iodic acid and iodous acid are thought to be significant contributors. While several quantum chemical studies have been carried out on clusters containing iodine, there is no comprehensive benchmark study quantifying the accuracy of the applied methods. Here, we present the first study in a series that investigate the role of iodine species in atmospheric cluster formation. In this work, we have studied the iodic acid, iodous acid, iodine tetroxide, and iodine pentoxide monomers and their dimers formed with common atmospheric precursors. We have tested the accuracy of commonly applied methods for calculating the geometry of the monomers, thermal corrections of monomers and dimers, the contribution of spin-orbit coupling to monomers and dimers, and finally, the accuracy of the electronic energy correction calculated at different levels of theory. We find that optimizing the structures either at the ωB97X-D3BJ/aug-cc-pVTZ-PP or the M06-2X/aug-cc-pVTZ-PP level achieves the best thermal contribution to the binding free energy. The electronic energy correction can then be calculated at the ZORA-DLPNO-CCSD(T0) level with the SARC-ZORA-TZVPP basis for iodine and ma-ZORA-def2-TZVPP for non-iodine atoms. We applied this methodology to calculate the binding free energies of iodine-containing dimer clusters, where we confirm the qualitative trends observed in previous studies. However, we identify that previous studies overestimate the stability of the clusters by several kcal/mol due to the neglect of relativistic effects. This means that their contributions to the currently studied nucleation pathways of new particle formation are likely overestimated.

Selective modification of tryptophan in polypeptides via C-N coupling with azoles using in situ-generated iodine-based oxidants in aqueous media

This study demonstrates the C-N coupling of tryptophan with azoles, promoted by an in situ-generated iodine-based oxidant. The protocol was successfully applied to the selective modification of tryptophan in nonprotected polypeptide bearing oxidatively sensitive residues in acidic aqueous media. The present method allows the attachment of reactive handles to polypeptides and the peptide stapling.

Unexpectedly significant stabilizing mechanism of iodous acid on iodic acid nucleation under different atmospheric conditions

Iodous acid (HIO₂) has been shown to play a stabilizing role in the nucleation of iodic acid (HIO₃) (He et al., 2021). However, the stabilization effect and specific stabilizing mechanism of HIO₂ on HIO₃ nucleation under different atmospheric conditions remain unclear. Therefore, we studied these two issues under different temperatures and nucleation precursor concentrations using density functional theory combined with the Atmospheric Cluster Dynamics Code. We found that HIO₂ can form clusters with HIO₃ via strong hydrogen bonds, halogen bonds, and proton-transfer, substantially enhancing the stability of HIO₃ clusters and decreasing the energy barrier of HIO₃-based cluster formation at different temperatures and nucleation precursor concentrations. The particle formation rate and cluster concentrations of HIO₃-HIO₂ nucleation were negatively correlated with temperature and positively correlated with HIO₂ concentration. The enhancements by HIO₂ on the particle formation rate and cluster concentration of HIO₃ nucleation were positively correlated with temperature and HIO₂ concentration. Interestingly, even at a low HIO₂ concentration (1.0 × 10⁵ molecules cm^-3), the enhancement on the particle formation rate and cluster concentration of HIO₃ nucleation by HIO₂ were both unexpectedly up to 4.1 × 10⁴-fold at 283 K. Therefore, HIO₃-HIO₂ nucleation can be extremely rapid in cold regions, and the enhancement by HIO₂ can be significant, especially in warm regions even at relatively high HIO₂ concentrations.

Critical Role of Iodous Acid in Neutral Iodine Oxoacid Nucleation

Nucleation of neutral iodine particles has recently been found to involve both iodic acid (HIO₃) and iodous acid (HIO₂). However, the precise role of HIO₂ in iodine oxoacid nucleation remains unclear. Herein, we probe such a role by investigating the cluster formation mechanisms and kinetics of (HIO₃)_m(HIO₂)_n (m = 0-4, n = 0-4) clusters with quantum chemical calculations and atmospheric cluster dynamics modeling. When compared with HIO₃, we find that HIO₂ binds more strongly with HIO₃ and also more strongly with HIO₂. After accounting for ambient vapor concentrations, the fastest nucleation rate is predicted for mixed HIO₃-HIO₂ clusters rather than for pure HIO₃ or HIO₂ ones. Our calculations reveal that the strong binding results from HIO₂ exhibiting a base behavior (accepting a proton from HIO₃) and forming stronger halogen bonds. Moreover, the binding energies of (HIO₃)_m(HIO₂)_n clusters show a far more tolerant choice of growth paths when compared with the strict stoichiometry required for sulfuric acid-base nucleation. Our predicted cluster formation rates and dimer concentrations are acceptably consistent with those measured by the Cosmic Leaving Outdoor Droplets (CLOUD) experiment. This study suggests that HIO₂ could facilitate the nucleation of other acids beyond HIO₃ in regions where base vapors such as ammonia or amines are scarce.

Systematic Application of the Principle of Detailed Balancing to Complex Homogeneous Chemical Reaction Mechanisms

It is not uncommon for proposed complex reaction mechanisms to violate the principle of detailed balancing. Here, we draw attention to three ways in which such violations can occur: reversible reaction loops where the rate constants do not attain closure, illegal loops, and reversible steps having rate equations in the forward and reverse directions that are inconsistent with the equilibrium expressions. We present two simple methods to test whether a proposed mechanism is consistent with the first two aspects of the principle of detailed balancing. Both methods are restricted to closed homogeneous isothermal reactions having mechanisms that consist of stoichiometrically balanced reaction steps. The first method is restricted to mechanisms in which all reaction steps are reversible; values of Δ_f G° are assigned to all reaction species, equilibrium constants are then computed for all steps, and all rate constants for elementary steps are constrained by the relationship K_eq = k_f/ k_r. The second method is applicable to mechanisms that can consist of a series of reversible and/or irreversible reaction steps. One first examines the subset of reversible steps to determine whether any of these steps are stoichiometrically equivalent to a combination of any of the other steps. If so, the forward and reverse rate expressions must yield equilibrium constants that are in agreement with the stoichiometric relationships. Next, the complete set of steps is examined to look for "illegal reaction loops". Both of these procedures are performed by constructing matrices that represent the stoichiometries of the various reaction steps and then performing row reductions to identify basis sets of loops. A method based on linear programming is described that determines whether a mechanism contains any illegal loops. These methods are applied in the analysis of several published reaction mechanisms.

Chloride Ion Inhibition, Stirring, and Temperature Effects in an Ethylacetoacetate Briggs-Rauscher Oscillator in Phosphoric and Hydrochloric Acids in a Batch Reactor

Briggs-Rauscher oscillating reaction has been investigated in hydrochloric acid media and in the presence of added chloride ions in phosphoric acid media in a batch reactor using ethylacetoacetate as the organic substrate. The changes in key oscillation parameters, namely, induction period, oscillation period, amplitude, and so forth, have been explained in terms of the steps of the skeleton mechanism proposed by Furrow and Noyes and by de Kepper and Epstein. In spite of several shortcomings as outlined in section 2 in the text, the same mechanistic steps are good enough to explain qualitatively most of the changes in oscillation parameters under different stirring rates and temperatures in phosphoric acid and hydrochloric acid media, particularly stirring effects explained well by the theory of imperfect mixing. Potential data indicate that 0.018 M HCl medium has the ability to exhibit near-zero/zero amplitudes before cessation of oscillations. Inhibition reactions from a low Cl^- concentration in the 0.018 M HCl media appear to be responsible for this interesting dynamical transformation, as toward the end of oscillations, the difference between [I^-] and [I^-]_crit [see eq J in the text] becomes very small because of depleted component chemicals due to chemical reactions.

Reexamination of gas production in the Bray-Liebhafsky reaction: what happened to O2 pulses?

Results of high-precision measurements of gas production in the BL reaction are presented, and an efficient kinetic model for their analysis is proposed. Based on this model, the data have been examined pulse by pulse, and for the first time, the entire records of gas production could be successfully reduced to series of just a few key parameters. It has been confirmed that the kinetics of O2(g) production is of the first order with respect to its precursor. Overall, only two steps have been found necessary to fit the observed pulses in gas production. The first step produces the precursor of the recorded O2(g), and its rate has two components. One component provides the peaks, and its approximation in the form of Gaussian functions has been found as satisfactory. The other component provides the constant baseline of gas production between the pulses. Finally, the precursor gives rise to O2(g) in the second step, and the simple first-order kinetics suggests that the precursor is otherwise relatively unreactive, making O2(aq) a logical candidate. However, the rate constant of this process showed almost perfect linearity with the actual concentrations of H2O2, and it was affected only little by variations in the rate of stirring. It thus seems possible that this final step in gas production, responsible for the majority of O2 produced in pulses, might not be the interphase transport O2(aq) → O2(g), as expected. Instead, it might be a truly chemical process, giving rise to O2(g) in a reaction of H2O2 with another precursor, which is not involved significantly in any other process, but it is not O2(aq). If this is true, the second-order rate constant of this process in the system with initial composition of 0.360 M KIO3, 0.345 M H2O2, and 0.055 M HClO4 at 60 °C would be 0.25-0.30 M(-1)·s(-1), depending on the rate of stirring.