Formation of the chelate bonds in the cluster O−2(CO2)n, CO−3(CO2)n, and NO−2(CO2)n

Probing Anion-Molecule Complexes of Atmospheric Relevance Using Anion Photoelectron Detachment Spectroscopy

Bimolecular reaction and collision complexes that drive atmospheric chemistry and contribute to the absorption of solar radiation are fleeting and therefore inherently challenging to study experimentally. Furthermore, primary anions in the troposphere are short lived because of a complicated web of reactions and complex formation they undergo, making details of their early fate elusive. In this perspective, the experimental approach of photodetaching mass-selected anion-molecule complexes or complex anions, which prepares neutrals in various vibronic states, is surveyed. Specifically, the application of anion photoelectron spectroscopy along with photoelectron-photofragment coincidence spectroscopy toward the study of collision complexes, complex anions in which a partial covalent bond is formed, and radical bimolecular reaction complexes, with relevance in tropospheric chemistry, will be highlighted.

Solvent Cage Concept for the Homolytic Fragmentation of the Peroxynitrite-CO2 Adduct, ONOOCO2. Chem Res Toxicol 2022;35:1135-1145. [PMID: 35763359 DOI: 10.1021/acs.chemrestox.1c00355] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The toxicity of peroxynitrite, ONOO^-, is directed by carbon dioxide via the formation of the corresponding adduct, ONOOCO₂^-. Entity ONOOCO₂^- is believed to be a highly unstable compound that primarily decomposes to nitrate and carbon dioxide, but it also undergoes fractional homolysis to generate carbonate radical anion, CO₃^•-, and nitrogen dioxide, NO₂^•, in a so-called solvent (radical) cage reaction. Recently, Koppenol et al. reviewed their proposal that ONOOCO₂^- is a relatively long-lived intermediate, arguing that "the solvent cage as proposed is physically not realistic". To further address whether ONOOCO₂^- could be a long-lived species, bond dissociation enthalpies (BDE) were calculated by the composite reference method (SMD)W1BD. Anion ONOOCO₂^- can exist in two conformers, s-cis-gauche and s-trans-gauche with predicted gas-phase O-O BDEs of about 10.8 and 9.5 kcal mol^-1, respectively. Therefore, both conformers should have very short lifetimes. The (SMD)W1BD method was also used to evaluate the thermodynamic parameters of interest, revealing that the homolytic decomposition of ONOOCO₂^- is the most reasonable pathway. Moreover, previously reported experimental chemically induced dynamic nuclear polarization data also support the intermediacy of the radical cage and the formation of products CO₂ and NO₃^- at a total yield of about 70%. Because the solvent radical cage concept for the decay of ONOO^- in the presence of CO₂ is supported by a variety of spectrometric methods as well as by quantum chemical calculations at high levels of theory, it provides strong evidence against the "out-of-cage" construct. For clarification of the nature of the transient UV/vis absorption(s) between 600 and 700 nm, as observed by Koppenol et al., several experimental approaches are suggested.

Electron Attachment to the (O2···CO2) van der Waals Complex Results in a Monomeric Anion (O2-CO2)-, a Possible Form of CO4. J Phys Chem A 2021;125:5794-5799. [PMID: 34184897 DOI: 10.1021/acs.jpca.1c04114]

We found that electron attachment to the van der Waals complex (O₂···CO₂) turns the weak intermolecular bond into a pseudochemical bond of significant strength. The resulting monomeric molecular anion (O₂-CO₂)^- may be a form of CO₄^-, the gaseous anionic species suspected to be present in Earth's ionosphere whose chemical characteristics have not been comprehensively identified since its existence was first predicted by Conway in 1962. The measured vertical detachment energy of CO₄^- is very large (4.56 ± 0.05 eV), while the known electron affinity of its component species is much smaller (0.448 eV, O₂) or even negative (-0.6 eV, CO₂). These characteristics are correctly borne out by theoretical calculations that show that electron attachment transforms the van der Waals complex to a single contiguous molecular anion, with the formation of a pseudochemical bond between O₂ and CO₂ through an extended π-orbital system.

Negative Reactant Ion Formation in High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS)

Due to the operation at background pressures between 10-40 mbar and high reduced electric field strengths of up to 120 Td, the ion-molecule reactions in High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) differ from those in classical ambient pressure IMS. In the positive ion polarity mode, the reactant ions H⁺(H₂O)_n, O₂⁺(H₂O)_n, and NO⁺(H₂O)_n are observed in the HiKE-IMS. The relative abundances of these reactant ion species significantly depend on the reduced electric field strength in the reaction region, the operating pressure, and the water concentration in the reaction region. In this work, the formation of negative reactant ions in HiKE-IMS is investigated in detail. On the basis of kinetic and thermodynamic data from the literature, the processes resulting in the formation of negative reactant ions are kinetically modeled. To verify the model, we present measurements of the negative reactant ion population in the HiKE-IMS and its dependence on the reduced electric field strength as well as the water and carbon dioxide concentrations in the reaction region. The ion species underlying individual peaks in the ion mobility spectrum are identified by coupling the HiKE-IMS to a time-of-flight mass spectrometer (TOF-MS) using a simple gated interface that enables the transfer of selected peaks of the ion mobility spectrum into the TOF-MS. Both the theoretical model as well as the experimental data suggest the predominant generation of the oxygen-based ions O^-, OH^-, O₂^-, and O₃^- in purified air containing 70 ppm_v of water and 30 ppm_v of carbon dioxide. Additionally, small amounts of NO₂^- and CO₃^- are observed. Their relative abundances highly depend on the reduced electric field strength as well as the water and carbon dioxide concentration. An increase of the water concentration in the reaction region results in the generation of OH^- ions, whereas increasing the carbon dioxide concentration favors the generation of CO₃^- ions, as expected.

The metallo-formate anions, M(CO2)−, M = Ni, Pd, Pt, formed by electron-induced CO2 activation

The metallo-formate anions, M(CO₂)⁻, M = Ni, Pd, and Pt, were formed by electron-induced CO₂ activation.

Reactions between microhydrated superoxide anions and formic acid

Reactions between water clusters containing the superoxide anion, O₂˙⁻(H₂O)_n (n = 0–4), and formic acid, HCO₂H, were studied experimentally in vacuo and modelled using quantum chemical methods.

Photoelectron spectroscopic and computational study of (M-CO2)(-) anions, M = Cu, Ag, Au

In a combined photoelectron spectroscopic and computational study of (M-CO2)(-), M = Au, Ag, Cu, anionic complexes, we show that (Au-CO2)(-) forms both the chemisorbed and physisorbed isomers, AuCO2(-) and Au(-)(CO2), respectively; that (Ag-CO2)(-) forms only the physisorbed isomer, Ag(-)(CO2); and that (Cu-CO2)(-) forms only the chemisorbed isomer, CuCO2(-). The two chemisorbed complexes, AuCO2(-) and CuCO2(-), are covalently bound, formate-like anions, in which their CO2 moieties are significantly reduced. These two species are examples of electron-induced CO2 activation. The two physisorbed complexes, Au(-)(CO2) and Ag(-)(CO2), are electrostatically and thus weakly bound.

Theoretical studies on clusters of carbonate with carbon dioxide, CO31–/2–(CO2)n, forn= 1–5 — Comparison of carbonate clusters with sulfate clusters

Density functional theory (DFT) calculations were performed on the geometries and energies of CO31–/2–(CO2)nclusters with n = 1–5. For small clusters (n = 1 or 2), coupled cluster energies were obtained. Up to three CO2molecules are bound covalently to the dianion. Only weak electrostatic bonds were found in the monoanions. Calculated binding energies for the monoanions are in reasonable agreement with experimental values. The calculated adiabatic electron detachment energy for the dianion is –0.07 eV at n = 5, indicating that at least six CO2molecules will have to be added to CO32–before the dianionic cluster becomes, in the gas phase, more stable than the monoanionic one. In comparison, for sulfate – carbon dioxide clusters, stabilization occurs at n = 2. Carbonate clusters are compared with sulfate clusters for three solvent molecules: CO2, SO2, and H2O. Carbonate clusters have larger binding energies than sulfate clusters. For a given dianion, binding energies are largest for SO2and smallest for H2O. However, in all cases, stabilization of the carbonate dianion by clustering is more difficult to achieve than stabilization of the sulfate dianion.

Infrared spectra of O2- x (CO2)n clusters (n=1-6): asymmetric docking at the pi* orbital

Isolated superoxide ions solvated by CO2 have been studied by infrared photodissociation spectroscopy and density-functional theory, using CO2 evaporation upon infrared excitation of the O2- x (CO2)n (n=1-6) parent ions. We can assign the observed frequencies to the asymmetric stretch vibration and its combination bands with the symmetric stretch and the overtone of the bending vibration of CO2 in various binding situations. We interpret our findings with the help of density-functional theory. Our data suggest that only one CO2 moiety binds strongly to the O2-, whereas the rest of the CO2 molecules are weakly bound, which is consistent with the experimental spectra. The lobes of the pi* orbital of O2- provide a template for the structure of the microsolvation environment.