Small Multiply Charged Anions as Building Blocks in Chemistry

Dynamics of Anions: From Bound to Unbound States and Everything In Between

Gas-phase anions present an ideal playground for the exploration of excited-state dynamics. They offer control in terms of the mass, extent of solvation, internal temperature, and conformation. The application of a range of ion sources has opened the field to a vast array of anionic systems whose dynamics are important in areas ranging from biology to star formation. Here, we review recent experimental developments in the field of anion photodynamics, demonstrating the detailed insight into photodynamical and electron-capture processes that can be uncovered. We consider the electronic and nuclear ultrafast dynamics of electronically bound excited states along entire reaction coordinates; electronically unbound states showing that photochemical concepts, such as chromophores and Kasha's rule, are transferable to electron-driven chemistry; and nonvalence states that straddle the interface between bound and unbound states. Finally, we consider likely developments that are sure to keep the field of anion dynamics buoyant and impactful.

Hydrogen Bonds Are Never of an "Anti-electrostatic" Nature: A Brief Tour of a Misleading Nomenclature

A large amount of scientific works have contributed through the years to rigorously reflect the different forces leading to the formation of hydrogen bonds, the electrostatic and polarization ones being the most important among them. However, we have witnessed lately with the emergence of a new terminology, anti-electrostatic hydrogen bonds (AEHBs), that seems to contradict this reality. This nomenclature is used in the literature to describe hydrogen bonds between equally charged systems to justify the existence of these species, despite numerous proofs showing that AEHBs are, as any other hydrogen bond between neutral species, mostly due to electrostatic forces. In this Viewpoint, we summarize the state of the art regarding this issue, try to explain why this terminology is very misleading, and strongly recommend avoiding its use based on the hydrogen bond physical grounds.

Interaction of low-energy electrons with radiosensitizers

We provide an experimentalist's perspective on the present state-of-the-art in the studies of low-energy electron interactions with common radiosensitizers, including compounds used in combined chemo-radiation therapy and their model systems. Low-energy electrons are important secondary species formed during the interaction of ionizing radiation with matter. Their role in the radiation chemistry of living organisms has become an important topic for more than 20 years. With the increasing number of works and reviews in the field, we would like to focus here on a very narrow area of compounds that have been shown to have radio-sensitizing properties on the one hand, and high reactivity towards low-energy electrons on the other hand. Gas phase experiments studying electron attachment to isolated molecules and environmental effects on reaction dynamics are reviewed for modified DNA components, nitroimidazoles, and organometallics. In the end, we provide a perspective on the future directions that may be important for transferring the fundamental knowledge about the processes induced by low-energy electrons into practice in the field of rational design of agents for concomitant chemo-radiation therapy.

An environmental impact statement for molecular anions

A molecular anion's (MA's) chemical reactivity and physical behavior can be quite different when it is surrounded by other molecules than when it exists in isolation. This sensitivity to the surrounding environment is especially high for anions because their outermost valence electrons are typically loosely bound and exist in rather spatially diffuse orbitals, allowing even weak intermolecular interactions arising from the environment to have strong effects. This Perspective offers illustrations of such sensitivity for a variety of cases including (i) the effect of solvation on electron binding energies, (ii) how some "well known" anions need to have solvent molecules around to even exist as stable species, (iii) how internal Coulomb repulsions within a multiply charged MA can provide temporary stability toward electron loss, (iv) how MAs arrange themselves spatially near liquid/vapor interfaces in manners that can produce unusual reactivity, (v) how nearby cationic sites can facilitate electron attachment to form a MA site elsewhere, (vi) how internal vibrational or rotational energy can make a MA detach an electron.

Beyond Duality: Rationalizing Repulsive Coulomb Barriers in Host-Guest Cyclodextrin-Dodecaborate Complexes

The repulsive Coulomb barrier (RCB), an intrinsic potential energy barrier along electron detachment or charge-separation coordinates in multiply charged anions (MCAs), provides dynamic stability to MCAs whose electronic and thermodynamic stabilities are largely dictated by strong internal Coulomb repulsions. Spectroscopic and theoretical characterizations of the RCB have been focused on isolated MCAs. In this work, we extend the RCB investigation beyond the previous scope by including noncovalent host-guest cyclodextrin-closo-dodecaborate dianionic complexes χCD·B₁₂X₁₂^2- (χ = α, β, γ; X = H, F-I). Photodechment photoelectron spectroscopy reveals the existence of two distinctly different RCBs, derived from detaching electrons from the guest dianions (RCB₁) or ionizing the host neutrals (RCB₂), respectively, with the latter being substantially smaller than the former. Theoretical calculations support the duality of RCBs in these complexes and further exhibit highly anisotropic nature of the RCBs.

On the existence of CO32- microsolvated clusters: a theoretical study

Microsolvated clusters of multiply charged anions play a crucial role in atmospheric chemistry and some of them were previously registered experimentally. At the same time, there are no experimental observations of [CO₃·(H₂O)_n]^2-. The reasons for this may be related to the thermodynamical or kinetical instability of microsolvated CO₃^2- toward autoionization or autoprotonation processes. In this study we theoretically investigate the potential stability of the [CO₃·(H₂O)_n]^2- microsolvated clusters from both perspectives - thermodynamic and kinetic - and we claim they are stable toward autoionization and kinetically semi-stable toward autoprotonation. In addition, the behaviour of CO₃^2- anions in bulk water solvent was analysed to highlight important precautions for synthetic purposes.

Kasha's Rule and Koopmans' Correlations for Electron Tunnelling through Repulsive Coulomb Barriers in a Polyanion

The long-range electronic structure of polyanions is defined by the repulsive Coulomb barrier (RCB). Excited states can decay by resonant electron tunnelling through RCBs, but such decay has not been observed for electronically excited states other than the first excited state, suggesting a Kasha-type rule for resonant electron tunnelling. Using action spectroscopy, photoelectron imaging, and computational chemistry, we show that the fluorescein dianion, Fl^2-, partially decays through electron tunnelling from the S₂ excited state, thus demonstrating anti-Kasha behavior, and that resonant electron tunnelling adheres to Koopmans' correlations, thus disentangling different channels.

High-Spin Superatom Stabilized by Dual Subshell Filling

Quantum confinement in small symmetric clusters leads to the bunching of electronic states into closely packed shells, enabling the classification of clusters with well-defined valences as superatoms. Like atoms, superatomic clusters with filled shells exhibit enhanced electronic stability. Here, we show that octahedral transition-metal chalcogenide clusters can achieve filled shell electronic configurations when they have 100 valence electrons in 50 orbitals or 114 valence electrons in 57 orbitals. While these stable clusters are intrinsically diamagnetic, we use our understanding of their electronic structures to theoretically predict that a cluster with 107 valence electrons would uniquely combine high stability and high-spin magnetic moment, attained by filling a majority subshell of 57 electrons and a minority subshell of 50 electrons. We experimentally demonstrate this predicted stability, high-spin magnetic moment (S = 7/2), and fully delocalized electronic structure in a new cluster, [NEt₄]₅[Fe₆S₈(CN)₆]. This work presents the first computational and experimental demonstration of the importance of dual subshell filling in transition-metal chalcogenide clusters.

Isolated [B2(CN)6]2-: Small Yet Exceptionally Stable Nonmetal Dianion

We report the observation of a small, yet remarkably stable, metal-free hexacyanodiborate dianion [B₂(CN)₆]^2- in the gas phase. Negative ion photoelectron spectroscopy (NIPES) was employed to measure its spectra at multiple laser wavelengths, yielding a 1.9 eV electron binding energy (EBE) ─a remarkably high value of electronic stability and a ∼2.60 eV repulsive Coulomb barrier (RCB) for electron detachment. This rationalizes the observation of this dianion, although homolytic charge-separation dissociation into two [B(CN)₃]^•- is energetically favorable. Quantum chemical calculations demonstrate a D_3d staggered conformation for both the dianion and radical monoanion, and the calculated EBE and RCB match the experimental values well. The simulated density of states spectrum reproduces all measured electronic transitions, while the simulated vibrational progressions for the ground state transition cover a much narrower EBE range compared to the experimental band, indicating appreciable auto-photodetachment via electronically excited dianion resonances.

Photoelectron spectroscopy of the protoporphyrin IX dianion

Two-dimensional photoelectron spectroscopy using nanosecond and femtosecond lasers has been used to study the protopophyrin IX dianion at photon energies between 1.8-4.1 eV. The photoelectron spectra indicated the presence of two direct detachment channels, tunnelling through the repulsive Coulomb barrier (RCB) and thermionic emission from monoanions. A direct detachment feature suggested a near 0 eV electron affinity, which may be attributable to the repulsive through space interaction of the unshielded carboxylate groups. The minimum height of the repulsive Coulomb barrier (RCB) was found to be between 1.4-1.9 eV. Adiabatic tunnelling through the RCB was seen to occur on a timescale faster than rotational dephasing of the molecule. The observation of thermionic emission below the RCB in the nanosecond spectra originated from monoanions, which were produced via photon-cycling of the dianion.

Measuring Electronic Structure of Multiply Charged Anions to Understand Their Chemistry: A Case Study on Gaseous Polyhedral closo-Borate Dianions

Research on multiply charged anions (MCAs) in the gas phase has been intensively performed during the past decades, mainly to understand fundamental molecular physics phenomena, for example, intramolecular Coulomb repulsion and existence of the repulsive Coulomb barrier. However, the relevance of these investigations with respect to understanding MCAs' chemistry appears often vague. Here, we discuss how insights into the electronic structure obtained from negative ion photoelectron spectroscopy (NIPES) combined with theoretical calculations and collision-induced dissociation can provide a fundamental understanding of the intrinsic chemical reactivity of MCAs and their fragments. This is exemplified in our studies on polyhedral closo-borate dianions [B_nX_n]^2- (n = 6, 10, 11, 12; X = H, F-I, CN) and their fragment ions. For example, the rational design of closo-borate dianions with specific electronic properties is described, which leads to generating highly reactive fragments. Depending on the dianionic precursor, these fragments are tuned to either bind noble gases effectively or activate small molecules like CO and N₂. The intrinsic electronic properties of closo-borate dianions are further compared to their electrochemistry in solutions, revealing solvent effects on the redox potentials. Neutral host molecules such as cyclodextrins are found to bind strongly to [B_nX_n]^2-, and gas phase NIPES provides insights into the intrinsic host-guest interactions. Finally, outlooks including the direct NIPES of molecular fragment ions that cannot be generated in the condensed phase and their utilization in preparative mass spectrometry are discussed.

Small lithium-chloride clusters: Superalkalis, superhalogens, supersalts and nanocrystals

In the present paper, the results of the theoretical investigation of the small lithium-chloride clusters are reported. The geometrical structures, electronic and thermodynamic stability of superalkalis, superhalogens, and their single and double charged ions are obtained using efficient and accurate quantum chemistry methods. Further, low-lying isomers of the Li_n Cl_n ( n = 2 - 5 ) clusters and their stability parameters are calculated. Two ways of formation of the Li_n Cl_n clusters, polymerization of LiCl fragments and combination of superalkalis and superhalogen clusters, are compared. By examination the lattice energy and the average Li-Cl bond length in rectangular Li_n Cl_n ( n ≤ 60 ) clusters, it was concluded that already 50 LiCl are enough to mostly resembles the structure and stability of the bulk LiCl.

Rational design of an argon-binding superelectrophilic anion

Chemically binding to argon (Ar) at room temperature has remained the privilege of the most reactive electrophiles, all of which are cationic (or even dicationic) in nature. Herein, we report a concept for the rational design of anionic superelectrophiles that are composed of a strong electrophilic center firmly embedded in a negatively charged framework of exceptional stability. To validate our concept, we synthesized the percyano-dodecoborate [B₁₂(CN)₁₂]^2-, the electronically most stable dianion ever investigated experimentally. It serves as a precursor for the generation of the monoanion [B₁₂(CN)₁₁]^-, which indeed spontaneously binds Ar at 298 K. Our mass spectrometric and spectroscopic studies are accompanied by high-level computational investigations including a bonding analysis of the exceptional B-Ar bond. The detection and characterization of this highly reactive, structurally stable anionic superelectrophile starts another chapter in the metal-free activation of particularly inert compounds and elements.

Polyion Covalency: Exotic Species from the Unexplored World of Electrostatically Shielded Molecular Ion Chemistry

Standard quantum chemical methods have been employed to describe a variety of kinetically stable polyionic molecular species that are trapped in appreciable potential wells by chemical bonding forces, despite powerful electrostatic opposition that challenges conventional chemical detection and characterization. The studied species are covalent or dative analogs of "anti-electrostatic" hydrogen-bonded (AEHB) species, all illustrating how short-range quantum covalency can overcome the powerful "shielding" opposition of long-range electrostatic forces to form highly charged molecular species, analogous to known neutral or singly ionic counterparts. Computational predictions of representative structural, spectroscopic, and NBO-based electronic signatures of multiply charged analogs of common neutral species (CH₃ CH₃ , CO₂ , FeCO) are provided to suggest the unique material properties characteristic of this shielded domain of polyionic chemical phenomena.

Nanometer patterning of water by tetraanionic ferrocyanide stabilized in aqueous nanodrops

Formation of the small, highly charged tetraanion ferrocyanide, Fe(CN)₆^4–, stabilized in aqueous nanodrops and its influence to the surrounding hydrogen-bonding network of water is reported.

Formation of the small, highly charged tetraanion ferrocyanide, Fe(CN)₆^4–, stabilized in aqueous nanodrops is reported. Ion–water interactions inside these nanodrops are probed using blackbody infrared radiative dissociation, infrared photodissociation (IRPD) spectroscopy, and molecular modeling in order to determine how water molecules stabilize this highly charged anion and the extent to which the tetraanion patterns the hydrogen-bonding network of water at long distance. Fe(CN)₆^4–(H₂O)₃₈ is the smallest cluster formed directly by nanoelectrospray ionization. Ejection of an electron from this ion to form Fe(CN)₆^3–(H₂O)₃₈ occurs with low-energy activation, but loss of a water molecule is favored at higher energy indicating that water molecule loss is entropically favored over loss of an electron. The second solvation shell is almost complete at this cluster size indicating that nearly two solvent shells are required to stabilize this highly charged anion. The extent of solvation necessary to stabilize these clusters with respect to electron loss is substantially lower through ion pairing with either H⁺ or K⁺ (n = 17 and 18, respectively). IRPD spectra of Fe(CN)₆^4–(H₂O)_n show the emergence of a free O–H water molecule stretch between n = 142 and 162 indicating that this ion patterns the structure of water molecules within these nanodrops to a distance of at least ∼1.05 nm from the ion. These results provide new insights into how water stabilizes highly charged ions and demonstrate that highly charged anions can have a significant effect on the hydrogen-bonding network of water molecules well beyond the second and even third solvation shells.

Formation of dianions in helium nanodroplets

The formation of dianions in helium nanodroplets is reported for the first time. The fullerene cluster dianions (C60)n(2-) and (C70)n(2-) were observed by mass spectrometry for n≥5 when helium droplets containing the appropriate fullerene were subjected to electron impact at approximately 22 eV. A new mechanism for dianion formation is described, which involves a two-electron transfer from the metastable He(-) ion. As well as the prospect of studying other dianions at low temperature using helium nanodroplets, this work opens up the possibility of a wider investigation of the chemistry of He(-), a new electron-donating reagent.

Excited states of multiply-charged anions probed by photoelectron imaging: riding the repulsive Coulomb barrier

Recent progress towards understanding the repulsive Coulomb barrier in multiply-charged anion using photoelectron spectroscopy is discussed.

Lead-enhanced gas-phase stability of multiply charged EDTA anions: a combined experimental and theoretical study

Besides their fundamental importance, multiply charged anions (MCAs) are considered as promising molecular capacitors for which their intrinsic stabilities are of great significance. Herein, the gas-phase stabilities of ethylenediaminetetraacetic acid (EDTA) anions (i.e. [EDTA-nH](n-), n = 1-4) and their Pb(II) complexes (i.e. [EDTA + Pb-nH]((2-n)-), n = 3, 4) have been investigated using an approach that combines extractive electrospray ionization mass spectrometry (EESI-MS) measurements, Car-Parrinello molecular dynamics simulations and density functional theory/Tao-Perdew-Staroverov-Scuseria calculations. The EESI-MS data showed that the doubly charged EDTA anions in the form of [EDTA-2H](2-) and [EDTA + Pb-4H](2-) were much more abundantly observed than the singly charged species such as [EDTA-H](-) and [EDTA + Pb-3H](-), respectively. The calculation results indicated that [EDTA-2H](2-) and [EDTA + Pb-4H](2-) anions were thermodynamically more stable than the [EDTA-H](-) and [EDTA + Pb-3H](-) species in the gas phase, respectively. The [EDTA + Pb-3H](-) anions preferred five-coordinated structure, whereas [EDTA + Pb-4H](2-) anions formed either five-coordinated or six-coordinated structures. The calculations further revealed that significant electron clouds drifting from the ligand EDTA to the metal Pb(II) ions and the large distances between the carboxylic groups reduced the Coulomb repulsion among the excess electrons of these MCAs. Our data demonstrated that EESI-MS combined with theoretic calculations were able to provide a deep insight into the fundamental behavior of stability of MCAs in the gas phase and, thus, might be useful tools for studying MCAs for potential molecular capacitors.

Femtosecond Photoelectron Imaging of Aligned Polyanions: Probing Molecular Dynamics through the Electron-Anion Coulomb Repulsion

The first time-resolved photoelectron imaging study of a polyanion is presented. Using the alignment induced through resonance excitation, the photoelectron angular distributions can be qualitatively understood in terms of the position of localized excess charges on the molecular skeleton, which influence the photoemission dynamics. Pump-probe experiments are used to demonstrate that the photoelectron angular distribution is also sensitive to molecular dynamics. This is shown here for the rotational dynamics of a polyanion, in which the photoelectron anisotropy tracks the rotational coherence as it dephases. The methodology can in principle be applied to general molecular dynamics in large polyanions, providing a new route to studying ultrafast structural dynamics in complex gas-phase systems.

Effect of internal energy on the repulsive Coulomb barrier of polyanions

The nature of the repulsive Coulomb barrier in isolated molecular polyanions is studied by means of the photodetachment dynamics of the S(1) excited state of the fluorescein dianion which is bound solely by the repulsive Coulomb barrier. Photoelectron spectra reveal a feature at a constant electron kinetic energy, regardless of the excitation energy. This is explained by using an adiabatic tunneling picture for electron loss through successive repulsive Coulomb barriers correlating to vibrationally excited states. This physical picture is supported by time-resolved photoelectron spectra, showing that the tunneling lifetime is also invariant with excitation energy.

Guiding electron emissions by excess negative charges in multiply charged molecular anions

Using photoelectron imaging, we show the effects of excess negative charges on the directions of outgoing electrons in multiply charged anions. Photoemissions are observed to occur either in a perpendicular or parallel direction, depending on the molecular configurations and origins of the detached electrons. Detachment of the π electrons from biphenyl-disulfonate dianions is shown to occur in a perpendicular direction due to the Coulomb repulsion from the two terminal charges, whereas detachment from the sulfonate groups in linear aliphatic disulfonates occurs in parallel directions.