Oxygen cluster anions revisited: solvent-mediated dissociation of the core O4(-) anion

Temporary Anions of Benzoxazole in Charge-Transfer Cluster Photodetachment

The photoelectron spectra of cluster anions of superoxide (O2-) solvated by one molecule of benzoxazole (BzOx) reveal two competing photodetachment mechanisms: a direct photoemission from the solvated cluster core and an indirect pathway involving temporary anion states of benzoxazole accessed via the O2-·BzOx → O2·BzOx- charge-transfer transitions. Benzoxazole is a bicyclic unsaturated organic molecule that does not form permanent anions. However, its low-lying vacant π* orbitals permit a resonant capture of the electron emitted from the O2- cluster core. The non-Hermitian theory using a complex absorbing potential predicts the existence of two BzOx- π* resonances within the experimental energy range: resonance A (π1*) at 0.891 eV and resonance B (π2*) at 1.76 eV, relative to the onset of the BzOx + e- continuum at the ground-state geometry of neutral BzOx. Within the clusters, the O2·BzOx- charge-transfer states are partially stabilized relative to the free-electron limit by interactions with the O2 molecule. These interactions depend on the electronic states of both species. The theory predicts that at the O2-·BzOx cluster geometry, the O2(X3Σg-)·BzOx-(A) and O2(a1Δg)·BzOx-(A) states lie at 0.56 and 0.47 eV (vertically) above the respective neutral states. The O2(3Σg-)·BzOx-(B) resonance is found 1.43 eV (vertically) above O2(X3Σg-)·BzOx. Intense signatures of both BzOx- resonances and the three above-mentioned charge-transfer cluster states, O2(X3Σg-)·BzOx-(A), O2(a1Δg)·BzOx-(A), and O2(3Σg-)·BzOx-(B) are observed in the 355 nm (3.495 eV) and 532 nm (2.330 eV) photoelectron spectra of the O2-·BzOx cluster anion.

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.

Photoelectron Spectroscopy of OH−-Anion–Water Clusters Generated by Ultrasonic Nebulizer

Investigating molecules in the gas phase is the only way to discover their intrinsic molecular properties; however, it is challenging to produce the gaseous phase of large-molecule chemicals. Thermal evaporation is typically used to convert molecules into gases, but it is still challenging to study ionic molecules in solutions in the gas phase. Electrospray ionization is one of the best methods to generate molecules in the gas phase, and it is uniquely capable of studying large biomolecules, including proteins. However, the molecular temperature required to study the spectroscopic properties of the molecules is very high. In this study, we developed a new, simple evaporation method using an ultrasonic nebulizer to obtain gas-phase molecules. Using this new equipment, we observed OH⁻ anions and their water clusters in the gas phase and obtained their photoelectron spectra. We observed that the vertical electron-detachment energy (VDE) of OH⁻ was 1.90 ± 0.05 eV and the VDEs of its water clusters and OH⁻ (H₂O)_n (n = 1–2) decreased to 1.50 ± 0.05 eV (n = 1) and 1.30 ± 0.05 eV (n = 2), respectively.

Weak covalent interactions and anionic charge-sharing polymerisation in cluster environments

We discuss the formation of weak covalent bonds leading to anionic charge-sharing dimerisation or polymerisation in microscopic cluster environments. The covalent bonding between cluster building blocks is described in terms of coherent charge sharing, conceptualised using a coupled-monomers molecular-orbital model. The model assumes first-order separability of the inter- and intra-monomer bonding structures. Combined with a Hückel-style formalism adapted to weak covalent and solvation interactions, it offers insight into the competition between the two types of forces and illuminates the properties of the inter-monomer orbitals responsible for charge-sharing dimerisation and polymerisation. Under typical conditions, the cumulative effect of solvation obstructs the polymerisation, limiting the size of covalently bound core anions.

Photoelectron Spectroscopy of Biacetyl and Its Cluster Anions

Photoelectron spectroscopy of the biacetyl (dimethylglyoxal) anion reveals the properties of the ground singlet and lowest triplet electronic states of the neutral biacetyl (BA) molecule. Due to the broad and congested nature of the singlet transition, which peaks at a vertical detachment energy VDE = 1.12(5) eV, only an upper bound of the adiabatic electron affinity of BA could be determined: EA(BA) < 0.7 eV. A narrower and more structured triplet band peaking at VDE = 3.17(2) eV reveals the adiabatic electron binding energy of the triplet to be 3.05(2) eV. These results are in good agreement with ab initio (coupled-cluster) calculations. The lowest-energy structures of the anion, singlet, and triplet states of biacetyl are characterized by different orientations of the methyl groups within the molecular frame. In the ground singlet state of neutral BA, the methyl torsion is offset by ∼60° compared to that of the anion, while in the triplet the methyl orientation is similar to that of the anion. Photoelectron spectra of the cluster anions reveal that the intermolecular interactions in the homogeneously solvated (BA) _n^- clusters are significantly stronger than the interactions of BA^- with N₂O or even of BA^- with H₂O. To account for these observations, π-π bonded structures of the dimer and trimer anions of biacetyl are proposed based on density-functional theory calculations. The analysis of the proposed structures indicates that the negative charge in the (BA) _n^- cluster anions, at least in the dimer and the trimer, is significantly delocalized between all BA moieties present and there is a significant degree of covalent bonding within the cluster.

Photoelectron Imaging Spectra of O2-·VOC and O4-·VOC Complexes

The anion photoelectron imaging spectra of O₂^-·VOC and O₄^-·VOC (VOC = hexane, isoprene, benzene, and benzene-d₆) complexes measured using 3.49 eV photon energy, along with the results of ab initio and density functional theory results are reported and analyzed. Photodetachment of these anionic complexes accesses neutrals that model collision complexes, offering a probe of the effects of symmetry-breaking collision events on the electronic structure of normally transparent neutral molecules. The energies of O₂^-·VOC spectral features compared to the bare O₂^- indicate that photodetachment of the anion accesses a modestly repulsive region of the O₂-VOC potential energy surface, with subtle VOC dependence on the relative energies of the O₂ (X ³Σ_g^-)·VOC ground state and O₂ (a ¹Δ_g)·VOC excited state. In contrast, a significantly higher intensity of the transition to the O₂ (a ¹Δ_g)·VOC excited state relative to the O₂ (X ³Σ_g^-)·VOC ground state is observed for VOC = benzene, with a less pronounced effect observed for VOC = isoprene. Similar spectral effects are observed in the O₄^-·benzene and O₄^-·isoprene PE spectra. Several explanations are considered, with involvement of a temporary anion state emerging as the most plausible.

Photoelectron imaging and photodissociation of ozonide in O3(-)⋅(O2)n (n = 1-4) clusters

The photoelectron images of O3 (-) and O3 (-) ⋅ (O2)n (n = 1-4) have been measured using 3.49 eV photon energy. The spectra exhibit several processes, including direct photodetachment and photodissociation with photodetachment of O(-) photofragments. Several spectra also exhibit autodetachment of vibrationally excited O2 (-) photofragments. Comparison of the bare O3 (-) photoelectron spectra to that of the complexes shows that the O3 (-) core is preserved upon clustering with several O2 molecules, though subtle changes in the Franck-Condon profile of the ground state photodetachment transition suggest some charge transfer from O3 (-) to the O2 molecules. The electron affinities of the complexes increase by less than 0.1 eV with each additional O2 molecule, which is comparable to the corresponding binding energy [K. Hiraoka, Chem. Phys. 125, 439-444 (1988)]. The relative intensity of the photofragment O(-) detachment signal to the O3 (-) ⋅ (O2)n direct detachment signal increases with cluster size. O2 (-) autodetachment signal is only observed in the O3 (-), O3 (-) ⋅ (O2)3, and O3 (-) ⋅ (O2)4 spectra, suggesting that the energy of the dissociative state also varies with the number of O2 molecules present in the cluster.

Photochemistry of fumaronitrile radical anion and its clusters

The photodetachment and photochemistry of the radical anion of fumaronitrile (trans-1,2-dicyanoethylene) and its clusters are investigated using photoelectron imaging and photofragment spectroscopy. We report the first direct spectroscopic determination of the adiabatic electron affinity (EA) of fumaronitrile (fn) in the gas phase, EA = 1.21 ± 0.02 eV. This is significantly smaller than one-half the EA of tetracyanoethylene (TCNE). The singlet-triplet splitting in fumaronitrile is determined to be ΔES-T ≤ 2.6 eV, consistent with the known properties. An autodetachment transition is observed at 392 and 355 nm and assigned to the (2)Bu anionic resonance in the vicinity of 3.3 eV. The results are in good agreement with the predictions of the CCSD(T) and EOM-XX-CCSD(dT) (XX = IP, EE) calculations. The H2O and Ar solvation energies of fn(-) are found to be similar to the corresponding values for the anion of TCNE. In contrast, a very large (0.94 eV) photodetachment band shift, relative to fn(-), is observed for (fn)2(-). In addition, while the photofragmentation of fn(-), fn(-)·Ar, and fn(-)(H2O)1,2 yielded only the CN(-) fragment ions, the dominant anionic photofragment of (fn)2(-) is the fn(-) monomer anion. The band shift, exceeding the combined effect of two water molecules, and the fragmentation pattern, inconsistent with an intact fn(-) chromophore, rule out an electrostatically solvated fn(-)·fn structure of (fn)2(-) and favor a covalently bound dimer anion. A C2 symmetry (fn)2(-) structure, involving a covalent bond between the two fn moieties, is proposed.