Bound Anions

This section defines and explains some basic chemical and quantum-mechanical concepts concerning anions. Bound anions (where M is lower in energy than M, at the minimum-energy geometry of the former) are considered first, and subsequently we discuss metastable anions, also known as temporary anion resonances. For easy reference, a list of acronyms is provided at the end of this chapter. 

Insofar as one is able to map out potential energy surfaces for both M and M , one could compute such a vibrational progression by evaluating the appropriate Franck-Condon factors, Wunioni = O)lv neutral( ))- he other hand, taking a classical-mechanical view of the nuclear motion, it is often convenient to define the VDE as a continuously varying function of molecular geometry  

Note that Eq. [2] defines the VDE, which excludes relaxation of the neutral species following electron detachment. Including that relaxation energy defines the adiabatic electron detachment energy, which is more often called the adiabatic electron affinity (AEA), 

One note of caution about terminology the phrase electron affinity by itself (as opposed to AEA) is used somewhat ambiguously in the literature, in the sense that the EA in question might be the AEA, or it could be the EA for a vertical process. We caution against this ambiguous usage. In contrast, the vertical EA and the VDE are two different names for precisely the same energetic quantity, and the use of one term over the other is simply a matter of taste. 

In view of this discussion, it is tempting to conceptualize the weakly-bound anions of polar molecules as dipole-bound anions. Solution of the Schrodinger equation for an electron interacting with a point dipole reveals that bound states are obtained for dipole moments g 1.625 debye, ° ° with no further molecular structure required. (In practice this threshold should be modified to something like g 2.4 debye, owing to the possibility of rotational-to-electronic energy transfer, but the point remains that 

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Yourshaw I, Zhao Y and Neumark D M 1996 Many-body effects in weakly bound anion and neutral clusters zero... 

Bailey C G, Dessent C E FI, Johnson M A and Bowen K FI 1996 Vibronic effects in the photon energy-dependent photoelectron spectra of the CFIjCN dipole-bound anion J. Chem. Phys. 104 6976-83... 

Fig. 12. Schematic representation of the a-helix-inducing effect of specific bound anions like CIO4 or SCN" in the case of N-co-trialkylaled BPAA the extended helical conformation contracts by the insertion of the anions. The water molecules of the hydration shells are omitted 1091...

The acceleration by anions under both conditions was attributed to displacement of one of the water molecules presumed to be tetrahedrally coordinated with the mercuric ion, the subsequent reaction being then envisaged as displacement of the anion or water molecule by the aromatic the anions which cause reaction to take place more slowly were presumed to be more tightly bound to the mercuric ion than water. It has, however, been pointed out that less tightly bound anions would be unlikely to displace the more tightly bound water molecules in the first place438. 

Dipole-bound anions (5a, 4f) in which the extra electron is attracted primarily by the dipole force field of the polar molecule and for which rotation-to-electronic coupling is most important in inducing electron ejection. 

Threshold CID can be used to measure the energy required for decarboxylation, in order to determine the enthalpy of formation of R, which can be used to calculate the gas-phase acidity. While nominally straight-forward, the decarboxylation approach is limited to systems that have a bound anion, R, and requires an instrument with the capability of carrying out energy-resolved CID. However, it does have an advantage of being a regiospecific approach. 

The unusual rate enhancement of nucleophiles in micelles is a function of two interdependent effects, the enhanced nucleophilicity of the bound anion and the concentration of the reactants. In bimolecular reactions, it is not always easy to estimate the true reactivity of the bound anion separately. Unimolecular reactions would be better probes of the environmental effect on the anionic reactivity than bimolecular reactions, since one need not take the proximity term into account. The decarboxylation of carboxylic acids would meet this requirement, for it is unimolecular, almost free from acid and base catalysis, and the rate constants are extremely solvent dependent (Straub and Bender, 1972). 

H2PO4 to A-16) with a concomitant large increase in fluorescence quantum yield. Such an increase may be due to the rigidifkation of the receptor by the bound anion, which decreases the efficiency of non-radiative de-excitation. 

The cathodic shifts of the redox potentials of compounds [68], [69] and [71] on addition of halide anions are due to the stabilization of the cobaltocenium cation by the bound anion which causes the redox couple to shift to a more... 

Kadish et al. (1989) have described the effect of axially bound anions on the electroreduction of tris(IV) porphyrins in THF. Cyclic voltammetric investigations of zinc tetraphenylporphyrin in dichloromethane in the presence of background electrolyte anions reveal significant perturbations of the met-alloporphyrin s first one-electron oxidation, ranging from 0.86 V for TBAPF6 to 0.50 V for TBAC1 (Seely et al., 1994). 

Fig. 3 Relative dipole-bound anion formation rates in RET collisions between Rydberg Xe(nf) atoms with (a) adenine (circles) or imidazole (squares) molecules and (b) adenine-imidazole complex produced in a supersonic beam. Experimental data are fitted to curvecrossing model calculations which lead to the experimental determination of EAdS values, equal to 11 meV for adenine, 23 meV for imidazole and 54 meV for adenine-imidazole complex (reproduced by permission of the American Chemical Society).

Fig. 14 Relative dipole bound anion formation rates in RET collisions between Rydberg Xe(nf) atoms and a supersonic beam of (R)-l-phenylethanol (E/ ) with 2-pyrrolidinmethanols (PRand P5).

Davis, J. T. Lipophilic G-quadruplexes are self-assembled ion pair receptors, and the bound anion modulates the kinetic stability of these complexes. J Am Chem Soc 2003, 125, 10830-10841. 

As can be seen, generally all electron affinities predicted by ASCF are negative, indicating a more stable neutral system with respect to the anion. The inclusion of correlation via CCSD(T) and NOF approximates them to the available adiabatic experimental EAs, accordingly with the expected trend. The EAs tend to increase in moving from ACCSD(T) to ANOF and then from ANOF to the experiment. It should be noted that the NH anion is predicted to be unbound by CCSD(T), whereas the positive vertical EA value via NOE corresponds to the bound anionic state. 

Skurski, P., Gutowski, M., Simons, J. How to choose a one-electron basis set to reliably describe a dipole-bound anion. Int. J. Quantum Chem. 2000, 80, 1024-38. 

This process can be represented by the chemical interaction showing the displacement of the bound anion (A") by the eluent anion (E"). 

Figure 3.6. Model to interpret CS planes in figure 3.4, (001) projection (a) weakly bound anion vacancies (b) elimination of pairs of the vacancies by shearing by, e.g., a jb along an octahedral edge.

Fig. 3. H-ENDOR spectra of a protein-bound anionic flavin semiquinone (oxynitrilase) and a protein-bound neutral flavin semiquinone (Azotobacter flavodoxin). The ENDOR spectra were recorded at the magnetic field settings indicated. Taken from Ref. with permission...

The properties of the semiquinone from of the ETF isolated from the methylotrophic bacterium resemble those of the bacterial flavodoxins with the exception that flavodoxins form neutral semiquinones whereas this ETF forms an anionic semiquinone. Nearly quantitative anionic semiquinone formation is observed either in the presence of excess dithionite or when excess trimethylamine and a catalytic amount of trimethylamine dehydrogenase are added. Of interest is the apparent stability of the anionic semiquinone towards oxidation by O2 but not to oxidizing agents such as ferricyanide. This appears to be the first example of an air-stable protein-bound anionic flavin semiquinone. Future studies on the factors involved in imparting this resistance to O2 oxidation by the apoprotein are looked forward to with great interest. 

The ligand X is an anion that should have intermediate binding strength to Pt(II). Complexes with labile anions such as NO,a or C10.4-are usually highly toxic (14-20). Complexes with strongly bound anions are inert, although in some cases the dissociation of these anions appears to be activated in vivo (15,21). Examples of effective anions are... 

At pH 5.5 EH2, EH, CMP, and CMPH species are present with fractional protonations of 12, 119, and CMP being approximately 0.8, 0.6, and 0.6 (pX values 6.1, 5.8, and 5.8). Since the pX values are sensitive to ionic strength, bound anions may be present in some free protein species but less than at pH 4.5. The complex of EHi,4 with CMPHo., would produce EH2-CMP with no release of protons as observed by Hummel and Witzel. There are several important protein species involved at this pH and there is some controversy as to what they are and how significant. 

The spectra obtained with less strongly bound anions (log K 4, cf. Table 4), as well as the acidic spectral form of the free enzyme, have similar band structures (51) but are usually even broader and have maximal absorbancies in the range 150 to 250 M-1 cm-1. There is no simple correlation between the position of the anion in the spectrochemi-cal series and the absorption spectrum, but the lowest-energy peak of the Cl--, Br--, and I--complexes is shifted towards the red in that order as illustrated in Fig. 9. 

The equation can also be illustrated in Figure 9.1. When a semiconductor such as Ti02 absorbs photons, the valence band electrons are excited to the conduction band. For this to occur, the energy of a photon must match or exceed the band-gap energy of the semiconductor. This excitation results in the formation of an electronic vacancy or positive hole at the valence band edge. A positive hole is a highly localized electron vacancy in the lattice of the irradiated Ti02 particle. This hole can initiate further interfacial electron transfer with the surface bound anions. 

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