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.

Observations on the Electronic Character of Anions and Cations near Water Liquid/Vapor Interfaces.

In recent years, many researchers have become interested in chemical reactions, photon-induced processes, and other events taking place at or near aqueous liquid/vapor or ice/vapor interfaces. Such studies relate to a wide variety of atmospheric, oceanographic, and environmental issues. Near these interfaces, atomic and molecular anions and cations display quite different behaviors than when they are fully solvated in the bulk medium. When they exist near an interface, some cations capture an excess electron to produce new neutral-molecule electronic states. Some such cations can use an attached electron to assist in hydrolyzing one of their first-solvent-shell molecules. Anions residing near an interface are less solvent-stabilized than when in the bulk, causing their electron binding energies to decrease as they approach an interface, as a result of which their ability to act as reducing agents increases. Many multiply charged anions even become electronically metastable with respect to electron loss near an interface. Thus, for both cations and anions, it is important to develop tools for characterizing their varying electronic-state nature as they migrate between bulk solvation and liquid-vapor interface positioning.

Two potential paths for OH radical formation on surfaces of pure water microdroplets.

Experimental findings by others suggest that OH radicals are formed in unexpected abundance on or near surfaces of 1-50 µm microdroplets comprised of pure water, but the mechanism by which these radicals are generated is not yet fully resolved. In this work, we examine two possibilities using ab initio electronic structure methods: (1) electron transfer (ET) from a microdroplet surface-bound OH- anion to a nearby H3O+ cation and (2) proton transfer (PT) from such a H3O+ cation to a nearby OH- anion. Our findings suggest that both processes are possible but only if the droplet's underlying water molecules comprising the microdroplet provide little screening of the Coulomb interaction between the anion and cation once they reach ∼10 Å of one another. In the ET event, an OH radical is formed directly; for PT, the OH formation occurs because the new O-H bond formed by the transferred proton is created at a bond length sufficiently elongated to permit homolytic cleavage. Both the ET and PT pathways predict that H atoms will also be formed. Finally, we discuss the roles played by strong local electric fields in mechanisms that have previously been proposed and that occur in our two mechanisms.

Why Is Quantum Chemistry So Complicated?

The myriad tools of quantum chemistry are now widely used by a diverse community of chemists, biologists, physicists, and material scientists. The large number of methods (e.g., Hartree-Fock, density functional theory, configuration interaction, perturbation theory, coupled-clusters, equations of motion, Green's functions, and more) and the multitude of atomic orbital basis sets often give rise to consternation and confusion. In this Perspective, I explain why quantum chemistry has so many different methods and why researchers should understand their relative strengths and weaknesses. I explain how chemistry's use of orbitals and the need for wave functions to be antisymmetric causes computational-effort scaling proportional to the cube or higher power of the number of orbitals. I also illustrate how the fact that the Schrödinger equation's energies are extensive makes it difficult to extract intensive properties such as bond and excitation energies, ionization potentials, and electron affinities.

Molecular Anions Perspective.

This Perspective attempts to shed light on developments in the theoretical and experimental study of molecular anions highlighting more recent workers in the field. The species I discuss include (i) valence-bound (singly and multiply charged) anions including atmospheric, catalytic, superhalogen, interfacial, and more; (ii) dipole- and correlation-bound anions including their role as doorways to other states and their appearance "in space", and (iii) metastable anions focusing on tools needed for their theoretical treatment. I also briefly discuss angular distributions of photodetached electrons and their growing utilization in experiments and theory. A recurring theme is the dependence of electron binding energies (EBEs) on the surrounding environment. Some anions that are nonexistent as isolated species evolve to be stable but with small EBEs when weakly solvated (e.g., as in a cluster or at an air-solvent interface). Others existing in isolation only as metastable species become stable when the underlying molecular framework contains one or more positively charged group (e.g., protonated side chains in a peptide) that generates a stabilizing Coulomb potential. On the other hand, a destabilizing Coulomb potential between/among negative sites in a multiply charged anion decreases the EBEs of each such site and generates a repulsive Coulomb barrier that can affect stability.

Finding Valence Antibonding Levels while Avoiding Rydberg, Pseudo-continuum, and Dipole-Bound Orbitals.

Electronic structure methods are now widely used to assist in the interpretation of many varieties of experimental data. The energies and physical characteristics (e.g., sizes, shapes, and spatial localization) of valence antibonding π* and σ* orbitals play key roles in a variety of chemical processes including photochemical reactions and electron attachment reductions and are used in Woodward-Hoffmann-type analyses to probe reaction energy barriers and energy surface intersections leading to internal conversion or intersystem crossings. One's ability to properly populate such valence antibonding orbitals within electronic structure calculations is often hindered by the presence of other molecular orbitals having similar energies. These intruding orbitals can be of Rydberg, pseudo-continuum, or dipole-bound characteristic. This article shows how, within the most widely available electronic structure codes, one can avoid the pitfalls presented by these intruding orbitals to properly populate a valence π* or σ* orbital and how to subsequently use that orbital in a calculation that includes electron correlation effects and thereby offers the possibility of chemically useful precision. Special emphasis is given to cases in which the electronic state is metastable.

Do not forget the Rydberg orbitals.

Within any molecule or cluster containing one or more positively charged sites, families of Rydberg orbitals exist. Free electrons can attach directly, and anionic reagents with low electron binding energy can transfer an electron into one of these orbitals to form a neutral Rydberg radical. The possibilities that such a radical could form a covalent bond either to another Rydberg radical or to a radical holding its electron in a conventional valence orbital are considered. This Perspective overviews two roles that Rydberg radicals can play, both of which have important chemical consequences. Attachment of an electron into excited Rydberg orbitals is followed by rapid (∼10-6 s) relaxation into the lowest-energy Rydberg orbital to form the ground state radical. Although the excited Rydberg species are stable with respect to fragmentation, the ground-state species is usually quite fragile and undergoes homolytic bond cleavage (e.g., -R2NH dissociates into -R2N + H or into -RNH + R) by overcoming a very small barrier on its potential energy surface, thus generating reactive radicals (H or R). Here, it is shown that as a result of this fragility, any covalent bonds formed by Rydberg radicals are weak and the molecules they form are susceptible to exothermic fragmentations that involve quite small activation barriers. Another role played by Rydberg species arises when the Coulomb potentials provided by the (one or more) positive site(s) in the molecule stabilize low-energy anti-bonding orbitals (e.g., σ* orbitals of weak σ bonds or low-lying π* orbitals) to the extent that electron attachment into these Coulomb-stabilized orbitals is rendered exothermic. In such cases, the overlap of the Rydberg orbitals on the positive site(s) with the σ* or π* orbitals allows either a free electron or a weakly bound electron to an anionic reagent that is attracted toward the positive site by its Coulomb force to be guided/transferred into the σ* or π* orbital instead. After attaching to such an anti-bonding orbital, bond cleavage occurs again, generating reactive radical species. Because of the large radial extent of Rydberg orbitals, this class of bond cleavage events can occur quite distant from the positively charged group. In this Perspective, several examples of both types of phenomena are given for illustrative purposes.

Analysis of Stabilization and Extrapolation Methods for Determining Energies and Lifetimes of Metastable Electronic States.

Two methods that make use of standard electronic structure tools, the stabilization and extrapolation methods, are discussed with an eye toward pointing out their relative strengths and weaknesses and for improving their applications. In the former, whether to utilize energy data from only one or from both branches of an avoided crossing between the quasi-bound and pseudo-continuum states is one issue that is focused on. Another is the decision of where along the stabilization plot's branches (i.e., far from or close to the avoided crossing) to create data points for optimal performance given a reasonable (10-5-10-7 eV) precision in the electronic energy. A third issue is how many parameters to use in fitting energy data to the (one or two) branches of the stabilization plot. In extrapolation methods, one uses energy data computed when the metastable state's energy has been rendered stable by the application of an external potential, which thus produces a one-branch function. The main issues in implementing this method are the functional form for how the energy E depends on the strength of the external potential especially as the energy evolves from the bound-state region toward the unbound region and how to choose data points so that energy values of a reasonable precision are capable of determining the parameters in the formula that produces the metastable state's energy E and half-width Γ/2 (inversely related to the state's lifetime). In addition to explaining, critiquing, and comparing these two methods, several suggestions are offered for their further testing and improvements.

Caralumane Superacids of Lewis and Brønsted Character.

Carborane Brønsted superacids have proven to be useful reagents in a variety of organic and inorganic synthetic processes. In this work, analogs in which the icosahedral CB11 carborane core is replaced by a CAl11 core are studied using ab initio electronic structure tools. Each so-called caralumane Brønsted acid is formed by adding HF, HCl, or HH to a corresponding caralumane Lewis acid possessing a vacant Al-centered orbital that acts to accept an electron pair from the HF, HCl, or HH. The Lewis acid strengths of the species involved, as measured by their F- ion affinities, are all found to exceed the threshold for labeling them Lewis superacids. Also, the deprotonation Gibbs free energies of the Brønsted acids are found to be small enough for them to be Brønsted superacids. When HF or HCl is bound to a caralumane Lewis acid to form the Brønsted acid, the HF or HCl is bound datively to a single Al atom, and hydrogen bonds can be formed between this molecule's H atom and nearby F or Cl atoms attached to other Al atoms. In contrast, when HH is bound to the Lewis acid to form the Brønsted acid, two novel low-energy structures arise, both of which are Brønsted superacids. One has an essentially intact HH molecule attached to a single Al atom in a η2 fashion. In the other, the HH molecule is heterolytically cleaved to generate a hydride ion that attaches to a single Al atom and a proton that binds in a multicenter manner to other Al atoms. The structures and relative energies of a multitude of such caralumane Lewis and Brønsted superacids are provided and discussed.

Ejecting Electrons from Molecular Anions via Shine, Shake/Rattle, and Roll.

The periodically oscillating electromagnetic potential of a photon can, in an electric-dipole transition, "shine" an electron from an anion's bound-state orbital directly into a continuum-state orbital. This occurs in photoelectron and photodetachment spectroscopy, both of which provide much information about the electronic structure of the anion. Alternatively, a molecular anion containing sufficient vibrational energy to "shake/rattle" an electron out of a bound-state orbital can induce electron detachment via a vibration-to-electronic nonadiabatic transition. In this case, the electron binding energy in the anion must be smaller than the vibrational energy-level spacing, so these processes involve anion states of low binding energy, and they eject electrons having low kinetic energy. If the anion's electron binding energy is even smaller, it is possible for a rotation-to-electronic energy transfer to "roll" an electron from the bound-state orbital into the continuum. For each of these mechanisms by which electron detachment can occur, there are different selection rules governing the angular distribution in which the electrons are ejected, and this manuscript discusses these rules, their origins, and their utility when using spectroscopic tools to probe the anion's electronic structure. Several examples of the shine-, shake/rattle-, and role-ejection of electrons from a range of experimental conditions are discussed as are similarities and differences among the corresponding selection rules. Of special novelty are the effects arising when electron ejection occurs from orbitals having very low electron binding energies and thus large radial extent.

Fate of Dipole-Bound Anion States when Hydrated.

Many strongly polar molecules can form an anion by attaching an electron to either an empty or half-filled valence-bound (VB) orbital or a so-called dipole-bound (DB) orbital. These two families of orbitals can be very different in their radial extent (the former are usually more compact, while the latter are quite diffuse) and in the degree to which they are affected by surrounding solvent molecules. In this study, the effects of hydration (representative of strong solvation) on the DB state of a model polar species are investigated with an eye toward determining whether this state is stabilized or even persists when a few to 100 water molecules surround the polar molecule. It is found that in the presence of up to ca. 10-12 water molecules, the excess electron can remain in a DB orbital. However, once there are enough water molecules to form a complete first hydration shell (or more), the excess electron migrates into an orbital localized on the outer surface of the water solvent cage. These findings have implications on the possible role of DB states as doorways to facilitating electron attachment and subsequent electron transfer to VB states. It is shown that even when the electron is bound to the surface of the surrounding solvent, the dipole potential of the solute molecule can influence where on the surface the electron binds. It is also illustrated that using continuum dielectric methods to describe the hydration of DB states is fraught with danger because much of the outermost electron density in such states penetrates outside the boundary of the cavity used in these methods.

Unusual and Conventional Dative Bond Formation by s2 Lone Pair Donation from Alkaline Earth Metal Atoms to BH3, AlH3, and GaH3.

Using ab initio electronic structure methods with flexible atomic orbital basis sets, we examined the nature of the bonding arising from donation of an ns2 electron pair on an alkaline earth atom (Mg or Ca) into a vacant n'p orbital on the group 13 atom of BH3, AlH3, or GaH3. We also examined what happens when an excess electron is attached to form corresponding molecular anions. Although the geometries of MgBH3, MgAlH3, MgGaH3, and CaBH3 are found to be much as one would expect for datively bound molecules, CaAlH3 and CaGaH3 were found to have very unusual geometries in that their Al-H or Ga-H bonds are directed toward the Ca atom rather than away, as in the other compounds. Internal electrostatic Coulomb attractions between the partially positively charged Ca center and the partially negatively charged H centers were suggested as a source of these unusual geometries. The other novel finding is that the electron affinities (EAs) of all six M'-MH3 species lie in the 0.7-1.0 eV range, which is suggestive of ionic electronic structures for the neutrals even though the partial charges on the alkaline earth centers are as low as 0.3 atomic units. Partial positive charge on the alkaline earth atoms combined with substantial electron affinities of the BH3, AlH3, and GaH3 groups, but only when distorted from planar geometries, were suggested to be the primary contributors to the large EAs.

Concluding remarks for advances in ion spectroscopy Faraday Discussion.

Because the Introductory Lecture of this Faraday Discussion emphasized the recent history and exciting developments in the fields of experimental methods and applications of gaseous ion spectroscopy, these Concluding Remarks are, by design, directed somewhat more toward the roles played by theory. In discussing both the experimental and theoretical studies of gaseous ions, it is important to recognize and appreciate the delicate balance workers in the field are pursuing in terms of methodological/tool development and applications to current-day pressing problems in chemistry, physics, materials science, and biology. Without both components of modern research in this field, progress will not be efficient. Substantial discussion is included about the reductive approach that is commonly used to attempt to connect studies of ions in the gas phase (i.e., as isolated species) with properties of these ions as they exist in nature. Issues of how small a model system can be, to what extent surroundings/solvation can be addressed, and how our experimental or theoretical tools might limit us are all discussed in some detail. The current ability of theory to assist in the interpretation of experimental spectral data on gaseous ions is discussed, as are several of the most pressing limitations of theory on this front. Finally, the author offers his thoughts about what advances/improvements in theory are needed and the outlook for when they might be expected, and urges the experimental community to remain in close contact with theory groups developing new methods so that progress can be optimized.

Dissociative electron attachment to HGaF4 Lewis-Brønsted superacid.

The consequences of an excess electron attachment to HGaF4 (HF/GaF3) superacid are investigated on the basis of theoretical calculations employing ab initio methods. It is found that the dipole potential of HGaF4 plays an important role in the initial formation of a dipole-bound anionic state. Due to the kinetic instability of that initially formed anion, a fragmentation reaction occurs promptly and leads to (GaF4)- and H as the final products. The energy profile of this process, its rate, and mechanism are presented and discussed.

Behavioral Health Screening among Massachusetts Children Receiving Medicaid.

OBJECTIVE: To assess the impact of a Massachusetts Medicaid policy change (the Children's Behavioral Health Initiative; CBHI, which required and reimbursed behavioral health [BH] screening with standardized tools at well child visits and developed intensive home- and community-based BH services) on primary care practice examining the relationship of BH screening to subsequent BH service utilization. STUDY DESIGN: Using a repeated cross-sectional design, our 2010 and 2012 Medicaid study populations each included 2000 children/adolescents under the age of 21 years. For each year, the population was randomly selected and stratified into 4 age groups, with 500 members selected per group. Two data sources were used: medical records and Medicaid claims. RESULTS: The CBHI had a large impact on formal BH screening and treatment utilization among children/adolescents enrolled in Medicaid. Screening increased substantially (73%: 2010; 74%: 2012) since the baseline/premandate period (2007) when only 4% of well child visits included a formal screen. BH utilization increased among those formally screened but decreased among those with informal assessments. CONCLUSIONS: CBHI implementation transformed the relationship between primary care and BH services. Changes in regulation and payment resulted in widespread BH screening in Massachusetts primary care practices caring for children/adolescents on Medicaid.