Coexistence of Physisorbed and Solvated HCl at Warm Ice Surfaces

Distinctive Heterogeneous Reaction Mechanism of ClNO2 on the Air-Water Surface Containing Cl

The heterogeneous reaction of nitryl chloride (ClNO2) on the air-water surface plays a significant role in the chloride lifecycle. The air-water surface is ubiquitous on ice surfaces under supercooled conditions, affecting the uptake and heterogeneous reaction processes of trace gases. Previous studies suggest that ClNO2 is formed on Cl-doped ice surfaces following the N2O5 uptake. Herein, a distinctive heterogeneous reaction mechanism of ClNO2 is suggested on an air-water surface containing Cl under supercooled conditions using combined classic molecular dynamics (MD) and Born-Oppenheimer MD simulations. It is found that N2O5 dissociates into a NO2+ and NO3- ionic pair on the top air-water surface. In the top layer of the surface containing barely any Cl-, NO2+ proceeds through hydrolysis and produces H3O+ and HNO3. Thus, surface acidification appears because of H3O+ yields. With NO2+ diffusion to the deep layer of the surface, NO2+ reacts with Cl- and forms ClNO2. Note that ClNO2 formation competes with NO2+ hydrolysis, and the rate of ClNO2 formation is 27.7[Cl-] larger than that of NO2+ hydrolysis. Afterward, the reaction of ClNO2 with Cl- becomes barrierless with the catalysis by H3O+, which is not feasible on a neutral surface. Cl2 is thus generated and escapes into the atmosphere (low solubility of Cl2), contributing to the Cl radical. The proposed mechanism bolsters the current understanding of ClNO2's fate and its role in Cl chemistry in extremely cold environments like the Arctic and other high-latitude regions in wintertime.

Uptake of Hydrogen Peroxide from the Gas Phase to Grain Boundaries: A Source in Snow and Ice

Hydrogen peroxide is a primary atmospheric oxidant significant in terminating gas-phase chemistry and sulfate formation in the condensed phase. Laboratory experiments have shown an unexpected oxidation acceleration by hydrogen peroxide in grain boundaries. While grain boundaries are frequent in natural snow and ice and are known to host impurities, it remains unclear how and to which extent hydrogen peroxide enters this reservoir. We present the first experimental evidence for the diffusive uptake of hydrogen peroxide into grain boundaries directly from the gas phase. We have machined a novel flow reactor system featuring a drilled ice flow tube that allows us to discern the effect of the ice grain boundary content on the uptake. Further, adsorption to the ice surface for temperatures from 235 to 258 K was quantified. Disentangling the contribution of these two uptake processes shows that the transfer of hydrogen peroxide from the atmosphere to snow at temperatures relevant to polar environments is considerably more pronounced than previously thought. Further, diffusive uptake to grain boundaries appears to be a novel mechanism for non-acidic trace gases to fill the highly reactive impurity reservoirs in snow's grain boundaries.

Simulating core electron binding energies of halogenated species adsorbed on ice surfaces and in solution via relativistic quantum embedding calculations

In this work, we investigate the effects of the environment on the X-ray photoelectron spectra of hydrogen chloride and chloride ions adsorbed on ice surfaces, as well as of chloride ions in water droplets. In our approach, we combine a density functional theory (DFT) description of the ice surface with that of halogen species using the recently developed relativistic core-valence separation equation of motion coupled cluster (CVS-EOM-IP-CCSD) via the frozen density embedding formalism (FDE), to determine the K and L_1,2,3 edges of chlorine. Our calculations, which incorporate temperature effects through snapshots from classical molecular dynamics simulations, are shown to reproduce experimental trends in the change of the core binding energies of Cl^- upon moving from a liquid (water droplets) to an interfacial (ice quasi-liquid layer) environment. Our simulations yield water valence band binding energies in good agreement with experiment, which vary little between the droplets and the ice surface. For halide core binding energies there is an overall trend for overestimating experimental values, though good agreement between theory and experiment is found for Cl^- in water droplets and on ice. For HCl on the other hand there are significant discrepancies between experimental and calculated core binding energies when we consider structural models that maintain the H-Cl bond more or less intact. An analysis of models that allow for pre-dissociated and dissociated structures suggests that experimentally observed chemical shifts in binding energies between Cl^- and HCl would require that H⁺ (in the form of H₃O⁺) and Cl^- are separated by roughly 4-6 Å.

Environment Effects on X-Ray Absorption Spectra With Quantum Embedded Real-Time Time-Dependent Density Functional Theory Approaches

In this work we implement the real-time time-dependent block-orthogonalized Manby-Miller embedding (rt-BOMME) approach alongside our previously developed real-time frozen density embedding time-dependent density functional theory (rt-TDDFT-in-DFT FDE) code, and investigate these methods' performance in reproducing X-ray absorption spectra (XAS) obtained with standard rt-TDDFT simulations, for model systems comprised of solvated fluoride and chloride ions ([X@( H 2 O ) 8 - , X = F, Cl). We observe that for ground-state quantities such as core orbital energies, the BOMME approach shows significantly better agreement with supermolecular results than FDE for the strongly interacting fluoride system, while for chloride the two embedding approaches show more similar results. For the excited states, we see that while FDE (constrained not to have the environment densities relaxed in the ground state) is in good agreement with the reference calculations for the region around the K and L1 edges, and is capable of reproducing the splitting of the 1s1 (n + 1)p1 final states (n + 1 being the lowest virtual p orbital of the halides), it by and large fails to properly reproduce the 1s1 (n + 2)p1 states and misses the electronic states arising from excitation to orbitals with important contributions from the solvent. The BOMME results, on the other hand, provide a faithful qualitative representation of the spectra in all energy regions considered, though its intrinsic approximation of employing a lower-accuracy exchange-correlation functional for the environment induces non-negligible shifts in peak positions for the excitations from the halide to the environment. Our results thus confirm that QM/QM embedding approaches are viable alternatives to standard real-time simulations of X-ray absorption spectra of species in complex or confined environments.

Generating Excess Protons in Microsolvated Acid Clusters under Ambient Conditions: An Issue of Configurational Entropy versus Internal Energy

Acid dissociation, and thus liberation of excess protons in small water droplets, impacts on diverse fields such as interstellar, atmospheric or environmental chemistry. At cryogenic temperatures below 1 K, it is now well established that as few as four water molecules suffice to dissociate the generic strong acid HCl, yet temperature-driven recombination sets in simply upon heating that cluster. Here, the fundamental question is posed of how many more water molecules are required to stabilize a hydrated excess proton at room temperature. Ab initio path integral simulations disclose that not five, but six water molecules are needed at 300 K to allow for HCl dissociation independently from nuclear quantum effects. In order to provide the molecular underpinnings of these observations, the classical and quantum free energy profiles were decomposed along the dissociation coordinate in terms of the corresponding internal energy and entropy profiles. What decides in the end about acid dissociation, and thus ion pair formation, in a specific microsolvated water cluster at room temperature is found to be a fierce competition between classical configurational entropy and internal energy, where the former stabilizes the undissociated state whereas the latter favors dissociation. It is expected that these are generic findings with broad implications on acid-base chemistry depending on temperature in small water assemblies.

Predictive Simulations of Ionization Energies of Solvated Halide Ions with Relativistic Embedded Equation of Motion Coupled Cluster Theory

A subsystem approach for obtaining electron binding energies in the valence region is presented and applied to the case of halide ions (X^{-},X=F-At) in water. This approach is based on electronic structure calculations combining the relativistic equation-of-motion coupled cluster method for electron detachment and density functional theory via the frozen density embedding approach, using structures from classical molecular dynamics with polarizable force fields for discrete systems (in our study, droplets containing the anion and 50 water molecules). Our results indicate that one can accurately capture both the large solvent effect observed for the halides and the splitting of their ionization signals due to the increasingly large spin-orbit coupling of the p_{3/2}-p_{1/2} manifold across the series, at an affordable computational cost. Furthermore, owing to the quantum mechanical treatment of both solute and solvent electron binding energies of semiquantitative quality are also obtained for (bulk) water as by-products of the calculations for the halogens (in droplets).

Pre-melting and the adsorption of formic acid at the air-ice interface at 253 K as seen by NEXAFS and XPS

Interactions between trace gases and ice are important in environmental chemistry and for Earth's climate. In particular, the adsorption of trace gases to ice surfaces at temperatures approaching the melting point has raised interest in the past, because of the prevailing pre-melting. Here, we present Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy data at ambient partial pressure of water to better define the onset temperature of pre-melting at the interfacial region of ice. Further, this study directly compares the interaction between an organic acid common in the atmosphere, formic acid, and that of an aliphatic carbon with ice at 253 K. It makes use of X-ray Photoelectron Spectroscopy (XPS) with its inherent narrow probing depth covering both the surface and near-surface bulk region when detecting electrons. We use the tender X-ray range for excitation to locate the organic species within the interfacial region with an extended probing depth compared to published XPS work. Electron kinetic energy dependent C1s photoemission data indicate that, at low coverage of a few 10¹⁴ molecules cm^-2, the presence of formic acid is restricted to the upper ice layers of the interfacial region. Increasing the dosage, formic acid penetrates 6-7 nm into the air-ice interface. The presence of the more hydrophobic aliphatic carbon is restricted to the upper ice monolayers. This direct comparison of an organic acid with an aliphatic compound confirms the emerging picture where solutes enter the interfacial region of ice at a depth related to their specific tendency to form solvation shells.