Dipole-bound negative ions

This is the title of the paper [107], where for the first time such properties, as well as electric dipole transition probabilities, were reported for the bound excited states of the negative ion of Be, namely the Be ls 2s2p P and ls 2p S°, whose existence had previously been predicted theoretically. The paper is cited here in order to provide evidence of the flexibility and efficiency of methods in the framework of the SPSA (Breit-Pauli Hamiltonian) in producing correlated wavefunctions of excited states that are usable toward the calculation of other properties, such as fine and hyperfine structures and oscillator strengths [107]. Note that the computational facilities available to us in the early 1980s were far from optimal. 

Positive and negative ions in water solutions attract water molecules because of their dipole character. The number of water molecules that can be bound to a cation depends on the size of the ion, and the strength of the bond depends on the electrical charge 2 in relation to the ion radius r. The hydration of an ion thus depends on Z/r. 

Even more so than for the case of radiative association, very little experimental information exists for radiative attachment, and the reliability of the simple phase-space treatment has not been tested. One possible problem is that the formation of the negative ion complex may require so-called doorway states known as dipole-bound states, in a similar manner to the Rydberg mechanism for dissociative recombination of positive molecular ions. Such states have been seen in the spectra of negative ions, although a detailed calculation employing them for radiative attachment has not been attempted. 

Hendricks, J. H.> Lyapustina, S. A.> de Clercq, H. L., Snodgrass, J. T., 8c Bowen, K. H. (1996). Dipole bound, nucleic acid base anions studied via negative ion photoelectron spectroscopy. The Journal of Chemical Physics, 104, 7788. 

Fig. 8. An example of the lack of strong interaction between Ca ions bound to proteins and a-helix dipoles. Shown is the double-Ca -binding site of thermolysin (3TLN), with two associated helices (residues Gly-136 to Asn-181). Side chains are drawn only for Asp-138 and Glu-177 (thick lines), two Ca -ligand residues from the helical regions. Only main-chain atoms are shown for other residues. The Ca ions are circles. The positive amino terminus of the dipole from the first helix passes to one side of the Ca positions. The negative carboxy terminus of the dipole from the second helix bypasses the Ca positions at some distance. The only interaction between the ions and the helices is with the side chains of Asp-138 and Glu-177 that protrude from their respective helix axes.

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