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The experimental and theoretical study of molecular anions has undergone explosive growth over the past 40 years. Advances in techniques used to generate anions in appreciable numbers as well as new ion-storage, ion-optics, and laser spectroscopic tools have been key on the experimental front. Theoretical developments on the electronic structure and molecular dynamics fronts now allow one to achieve higher accuracy and to study electronically metastable states, thus bringing theory in close collaboration with experiment in this field. In this article, many of the experimental and theoretical challenges specific to studying molecular anions are discussed. Results from many research groups on several classes of molecular anions are overviewed, and both literature citations and active (in online html and pdf versions) links to numerous contributing scientists' Web sites are provided. Specific focus is made on the following families of anions: dipole-bound, zwitterion-bound, double-Rydberg, multiply charged, metastable, cluster-based, and biological anions. In discussing each kind of anion, emphasis is placed on the structural, energetic, spectroscopic, and chemical-reactivity characteristics that make these anions novel, interesting, and important.

We have carried out ab initio electronic structure calculations on a portion of DNA, the results of which provide support for a mechanism that produces single-strand breaks (SSBs) with low-energy electrons. This mechanism involves attaching a low-energy electron (ca. 1 eV) to a π* orbital of a DNA base to form a shape-resonance state. This π* anion then undergoes a sugar-phosphate C-O bond rupture over a small barrier to produce SSBs. In addition to supporting the efficacy of such a mechanism, our results suggest that solvation plays a crucial role in the rate of SSB formation when such very short-lived shape resonances are involved. In particular, they suggest that either the π* anion must be rendered electronically stable by solvation or its detachment lifetime must be several orders of magnitude longer in the solvated species than in the nonsolvated species.

We overview our recent theoretical predictions and the innovative experimental findings that inspired us concerning the mechanisms by which very low-energy (0.1-2 eV) free electrons attach to DNA and cause strong (ca. 4 eV) covalent bonds to break causing so-called single-strand breaks. Our primary conclusions are that (i) attachment of electrons in the above energy range to base π* orbitals is more likely than attachment elsewhere and (ii) attachment to base π* orbitals most likely results in cleavage of sugar-phosphate CO σ bonds. Later experimental findings that confirmed our predictions about the nature of the electron attachment event and about which bonds break when strand breaks form are also discussed. The proposed mechanism of strand break formation by low-energy electrons involves an interesting through-bond electron-transfer process.

The chemical structure and bonding of Al4C and Al4C- have been studied by photoelectron spectroscopy and ab initio calculations. While Al4C is known to be a tetrahedral molecule, the data reported here suggest that Al4C- has D 4 h symmetry (when averaged over zero-point vibrational motions) and thus is the smallest species identified to date that contains a tetracoordinated planar carbon atom. The experimental vertical electron detachment energy of Al4C- (2.65 ± 0.06 eV) compares well to 2.71 eV calculated at the CCSD(T)/6-311+G(2df) level of theory. The excellent agreement between the calculated and experimental electron detachment energies, excitation energies, and other spectral features allows us to elucidate the structure of the tetracoordinated planar-carbon-containing Al4C- anion.

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Photoelectron spectra of the MX 2 Ϫ ͑MϭLi, Na; XϭCl, Br, I͒ superhalogen anions have been obtained for the first time. The first vertical detachment energies ͑VDEs͒ were measured to be 5.92Ϯ0.04 (LiCl 2 Ϫ), 5.86Ϯ0.06 (NaCl 2 Ϫ), 5.42Ϯ0.03 (LiBr 2 Ϫ), 5.36Ϯ0.06 (NaBr 2 Ϫ), 4.88Ϯ0.03 (LiI 2 Ϫ), and 4.84Ϯ0.06 eV (NaI 2 Ϫ), which are all well above the 3.61 eV electron detachment energy of Cl Ϫ , the highest among atomic anions. Experimental photoelectron spectra have been assigned on the basis of ab initio outer valence Green function ͑OVGF͒ calculations. The corresponding theoretical first VDEs were found to be 5.90 (LiCl 2 Ϫ), 5.81 (NaCl 2 Ϫ), 5.48 (LiBr 2 Ϫ), 5.43 (NaBr 2 Ϫ), 4.57 (LiI 2 Ϫ), and 4.50 eV (NaI 2 Ϫ), in excellent agreement with the experimental values. Photodetachment from the top four valence molecular orbitals (2 g 2 2 u 2 1 u 4 1 g 4) of MX 2 Ϫ was observed. Analysis of the polestrength showed that all electron detachment channels in this study can be described as primarily one-electron processes.