The problem of the binding of an excess electron to polar molecules and their clusters has long fascinated researchers. Although excess electrons bound to such species tend to be very extended spatially and to have little spatial overlap with the valence electrons of the neutral molecules, inclusion of electron correlation effects is essential for quantitatively describing the electron binding. The major electron correlation contribution may be viewed as a dispersion interaction between the excess electron and the electrons of the molecule or cluster. Recent work using a one-electron Drude model to describe excess electrons interacting with polar molecules is reviewed.

Theory of dipole-bound anions

While the naturally occurring amino acids are not zwitterions in the vapor phase, they are in aqueous solutions, implying that water plays an important role in inducing zwitterion formation. Together, these observations inspire the question, “How many water molecules are required to induce zwitterion formation in a given amino acid molecule?” In this paper, we address this question in the context of mass spectrometric and size-selected photoelectron spectroscopic studies of hydrated amino acid anions. We utilize the facts that zwitterions possess very large dipole moments, and that excess electrons can bind to strong dipole fields to form dipole bound anions, which in turn display distinctive and recognizible photoelectron spectral signatures. The appearance of dipole-bound photoelectron spectra of hydrated amino acid anions, beginning at a given hydration number, thus signals the onset of greatly enhanced dipole moments there and, by implication, of zwitterion formation. We find that five water molecules are needed to transform glycine into its zwitterion, while four each are required for phenylalanine and tryptophan. Since the excess electron may also make a contribution to zwitterion stabilization, these numbers are lower limits for how many water molecules are needed to induce zwitterion formation in these amino acids when no extra (net) charges are involved.

https://pages.jh.edu/chem/bowen/Publication%20PDF/Zwitterion%20formation%20in%20hydrated%20amino%20acid,%20dipole%20bound%20anions%20How%20many%20water%20molecules%20are%20required.pdf

Zwitterion formation in hydrated amino acid, dipole bound anions: How many water molecules are required?

A theoretical prediction of the nonlinear optical (NLO) properties of the hydrogen fluoride trimer anion (FH)2{e}(HF) reveals that this electron-solvated cluster possesses exceptionally large static first hyperpolarizability. Using the d-aug-cc-pVDZ basis set supplemented with diffusive bond functions (BF), we obtained the mean static first hyperpolarizability (β0) for (FH)2{e}(HF) and its neutral core (HF)2{}(HF) at the CISD level of theory. The β0 is 8.1 × 106 au for (FH)2{e}(HF) and 5.1 au for (HF)2{}(HF). Obviously, the excess electron is the key factor in bringing to (FH)2{e}(HF) the significantly large NLO response. The other properties of (FH)2{e}(HF) are also much larger than those of its neutral core. The values of the static dipole moment, mean polarizability, and anisotropy of the polarizability are, respectively, μ = 1.5764 au, α0 = 654.67 au, and Δα = 224.71 au for the anion, and μ = 0.9351 au, α0 = 17.34 au, Δα = 5.40 au for the neutral core. In addition, the field dependences of the polariz...

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An ab Initio Prediction of the Extraordinary Static First Hyperpolarizability for the Electron-Solvated Cluster (FH)2{e}(HF)

Recently, a novel inorganic electride stable at room temperatures has been obtained by reducing a complex nanoporous oxide 12CaO·7Al2O3 (C12A7) in a Ca atmosphere (Matsuishi, S.; Toda, Y.; Miyakawa, M.; Hayashi, K.; Kamiya, T.; Hirano, M.; Tanaka, I.; Hosono, H. Science 2003, 301, 626). In this system, up to 2.3 × 1021/cm3 electrons can be accommodated in a three-dimensional network of cages formed by a positively charged oxide framework. We demonstrate theoretically that at all concentrations, ne, the electrons are neither associated with specific atoms nor fully delocalized. At low ne, the electrons are isolated from each other and resemble the color centers in insulating materials. They are well localized in some of the lattice cages and yield strong inhomogeneous lattice distortions that provide polaron-type cage-to-cage electron hopping. As ne increases, the electrons form a denser electron gas and become more evenly spread over all available lattice cages. At sufficiently high ne, the system becomes...

From Insulator to Electride: A Theoretical Model of Nanoporous Oxide 12CaO·7Al2O3

Vertical electron detachment energies (VDEs) are calculated for a variety of (H2O)n− and (HF)n− isomers, using different electronic structure methodologies but focusing in particular on a comparison between second-order Moller–Plesset perturbation theory (MP2) and coupled-cluster theory with noniterative triples, CCSD(T). For the surface-bound electrons that characterize small (H2O)n− clusters (n
				≤ 7), the correlation energy associated with the unpaired electron grows linearly as a function of the VDE but is unrelated to the number of monomers, n. In every example considered here, including strongly-bound “cavity” isomers of (H2O)24−, the correlation energy associated with the unpaired electron is significantly smaller than that associated with typical valence electrons. As a result, the error in the MP2 detachment energy, as a fraction of the CCSD(T) value, approaches a limit of about −7% for (H2O)n− clusters with VDEs larger than about 0.4 eV. CCSD(T) detachment energies are bounded from below by MP2 values and from above by VDEs calculated using second-order many-body perturbation theory with molecular orbitals obtained from density functional theory. For a variety of both strongly- and weakly-bound isomers of (H2O)20− and (H2O)24−, including both surface states and cavity states, these bounds afford typical error bars of ±0.1 eV. We have found only one case where the Hartree–Fock and density functional orbitals differ qualitatively; in this case the aforementioned bounds lie 0.4 eV apart, and second-order perturbation theory may not be reliable.

Accuracy and limitations of second-order many-body perturbation theory for predicting vertical detachment energies of solvated-electron clusters

Photoelectron spectra of (HF)(3)(-) reveal coexistence of two anionic isomers with vertical electron detachment energies (VDE) of 0.24 and 0.43 eV. The results of electronic-structure calculations, performed at the coupled cluster level of theory with single, double, and noniterative triple excitations, suggest that the two isomers observed experimentally are an open, zigzag, dipole-bound anion and an asymmetric solvated electron, in which the dipole-bound anion of (HF)(2) is solvated by one HF monomer at the side of the excess electron. The theoretical VDE of 0.21 and 0.44 eV, respectively, are in excellent agreement with the experimental data.

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C. S. Hall

Papers

Solvated electrons in very small clusters of polar molecules: (HF)3-