Probing the Partial Activation of Water by Open-Shell Interactions, Cl(H2O)1-4

The effects of the gas-liquid interface and gas phase on Cl/ClO radical interaction with water molecules

In the marine boundary layer (MBL), chlorine (Cl) and chlorine monoxide (ClO) are powerful oxidants with high concentrations. The gas-liquid interface is also ubiquitous in the MBL as a favorable site for atmospheric reactions. Understanding the role of water in Cl/ClO radical chemistry is essential for predicting their behavior in the atmosphere and developing effective strategies for mitigating their harmful effects. However, the research studies on the system of Cl/ClO radicals on the surface of water droplets are still insufficient. In previous studies, we have found unique results related to the hydroxyl radical at the interface using ab initio molecular dynamics (AIMD). In this work, we have used AIMD to investigate interactions between Cl/ClO radicals and water molecules at the gas-liquid interface. Radical mobility, radial distribution functions, coordination, and population analyses were conducted to investigate the surface preference, bonding pattern, and track Cl/ClO radicals in the water droplets. In addition, density functional theory (DFT) analysis was conducted to compare the results at the gas-liquid interface with those in the gas phase. We found that Cl/ClO radicals tend to remain near the gas-liquid interface in water droplet systems and outside of water clusters in gas phase systems. The ClO radical can form O*-H and Cl-O bonds with water molecules; however, neither the O*-O hemibond nor the Cl-H bond was detected in all systems. Different dominant structures were obtained for ClO in the interface and gas phase. The ClO radical can be bonded to one water molecule from its oxygen side, (H2O)0-Cl-O*-(H2O)1 at the interface, or to two water molecules from the chlorine and oxygen sides, (H2O)1-Cl-O*-(H2O)1 in the gas phase. Meanwhile, the Cl radical can only form a dominant structure like Cl*-(H2O)1 at the gas-liquid interface by making a Cl*-O hemibond. Providing a thorough explanation of the Cl/ClO radical behavior at the gas-liquid interface, this study will improve our understanding of the MBL's oxidizing capacity and pollution causes.

Structural, Thermodynamic, and Spectroscopic Evolution in the Hydration of Copper(II) Ions, Cu2+(H2O)2-8

Gas-phase clusters of the hydrated Cu(II) cation with 2-8 water molecules were investigated using ab initio quantum chemistry. Isomer structures, energies, and vibrational spectra were computed across this size range, yielding a qualitative picture of this ion as an intact Cu2+ hydrate that also partially oxidizes the surrounding water network at equilibrium. At sufficient cluster sizes, these ion hydrates also become thermodynamically preferred over competitive Cu(II) hydroxide hydrates. Competitive coordination environments were found to exist at some cluster sizes, due to both hydrogen-bonding and d-orbital chemical effects, and the dominant coordination number was found in some cases to be temperature-dependent. Clear spectral signatures of the ion's coordination environment were computed to exist at each cluster size, which should make experimental verification of these computational predictions straightforward. Through comparison to recent studies of hydrated CuOH+, the effective charge on the metal center was shown to converge to approximately +1.5 in both cases, despite qualitatively different behavior of their radical spin densities. Therefore, nominally Cu(II) ions exhibit considerable electronic, chemical, and structural flexibility. The electronic origins of this flexibility─including key roles played by the water network itself─are investigated in this work and should provide a conceptual foundation for future studies of copper-based, water-oxidation catalysts.

Persistence of a Delocalized Radical in Larger Clusters of Hydrated Copper(II) Hydroxide, CuOH+(H2O)3-7

The structures, vibrational spectra, and electronic properties of copper hydroxide hydrates CuOH+(H2O)3-7 were investigated with quantum chemistry computations. As a follow-up to a previous analysis of CuOH+(H2O)0-2, this investigation examined the progression as the square-planar metal coordination environment was filled and as solvation shells expanded. Four-, five-, and six-coordinate structures were found to be low-energy isomers. The delocalized radical character, which was discovered in the small clusters, was found to persist upon continued hydration, although the hydrogen-bonded water network in the larger clusters was found to play a more significant role in accommodating this spin. Partial charges indicated that the electronic structure includes more Cu2+···OH- character than was observed in smaller clusters, but this structure remains decidedly mixed with Cu+···OH· configurations and yields roughly half-oxidation of the water network in the absence of any electrochemical potential. Computed vibrational spectra for n = 3 showed congruence with spectra from recent predissociation spectroscopy experiments, provided that the role of the D2 tag was taken into account. Spectra for n = 4-7 were predicted to exhibit features that are reflective of both the mixed electronic character and proton-/hydrogen-shuttling motifs within the hydrogen-bonded water network.

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.

Photochemistry of Monohydrated Chloromethane: Formation of Free and Hydrated Cl- and CH3+ Ions from a Solvent-Shared Semi-Ion-Pair

The effect of water molecule on the excited states of CH₃Cl(H₂O), as compared to those of the isolated chloromethane, has been studied at the multireference configuration interaction with singles and doubles (MR-CISD), including extensivity corrections. Eight new Rydberg states are due to the water molecule but the common states of both systems are not severely altered. Potential energy curves of 23 singlet states along the C-Cl coordinate have also been computed at the MR-CISD level. The dissociation energy of the C-Cl bond decreases from ∼0.4 to 0.5 eV due to the water molecule. As for CH₃Cl (de Medeiros, V. C., J. Am. Chem. Soc. 2016, 138, 272-280), a stable ion-pair has also been characterized. However, for CH₃Cl(H₂O), this ion-pair is better described as a solvent-shared semi-ion-pair, CH₃^+δ(H₂O)Cl^-δ. This species is connected with three ionic dissociation channels, with two being due to the water molecule. The presence of these new ionic channels, particularly the lowest energy one, [H₃C-O]⁺ + Cl^-, raises a very important question of atmospheric relevance: can the interaction of chloroalkanes with water decrease its deleterious effect on the ozone layer? Several potentially new competing dissociation channels are also studied. The latter results can help to set up the most important states to be included in nonadiabatic dynamic calculations to study how the yields of the ionic channels change due to the water molecule.

Electronic Structure and Vibrational Signatures of the Delocalized Radical in Hydrated Clusters of Copper("II") Hydroxide CuOH+(H2O)0-2

The copper hydroxide ion, CuOH⁺, serves as the catalytic core in several recently developed water-splitting catalysts, and an understanding of its chemistry is critical to determining viable catalytic mechanisms. In spite of its importance, the electronic structure of this open-shell ion has remained ambiguous in the literature. In particular, computed values for both the thermodynamics of hydration and the vibrational signatures of the mono- and dihydrates have shown prohibitively large errors compared to values from recent experimental measurements. In this work, the source of this discrepancy is demonstrated to be the propensity of this ion to exist between traditional Cu(I) and Cu(II) oxidation-state limits. The spin density of the radical is accordingly shown to delocalize between the metal center and surrounding ligands, and increasing the hydration serves to exacerbate this behavior. Equation-of-motion coupled-cluster methods demonstrated the requisite accuracy to resolve the thermodynamic discrepancies. Such methods were also needed for spectral simulations, although the latter also required a direct simulation of the role of the deuterium "tag" molecules that are used in modern predissociation spectroscopy experiments. This nominally benign tag molecule underwent direct complexation with the open-valence metal ion, thereby forming a species akin to known metal-H₂ complexes and strongly impacting the resulting spectrum. Thermal populations of this configuration and other more traditional noncovalently bound isomers led to a considerable broadening of the spectral lineshapes. Therefore, at least for the CuOH⁺(H₂O)_0-2 hydrates, these benchmark ions should be considered to be delocalized radical systems with some degree of multireference character at equilibrium. They also serve as a cautionary tale for the spectroscopy community, wherein the role of the D₂ tag is far from benign.