Halide electron affinities

Miller T M, Leopold D G, Murray K K and Lineberger W C 1986 Electron affinities of the alkali halides and the structure of their negative ions J. Chem. Phys. 85 2368-75... 

Halogens, the elements in Group 17 of the periodic table, have the largest electron affinities of all the elements, so halogen atoms (a n readily accept electrons to produce halide anions (a a. This allows halogens to react with many metals to form binary compounds, called halides, which contain metal cations and halide anions. Examples include NaCl (chloride anion), Cap2 (fluoride anion), AgBr (bromide anion), and KI (iodide anion). 

In principle, the equilibrium approach can be used to measure any of the thermochemical properties listed above. However, in practice, it is most commonly used for the determination of gas-phase acidities, proton affinities, and electron affinities. In addition, equilibrium measurements are used for measuring ion affinities, including halide (F, Cl ) and metal ion (alkali and transition metal) affinities. 

As we have seen, several atomic properties are important when considering the energies associated with crystal formation. Ionization potentials and heats of sublimation for the metals, electron affinities, and dissociation energies for the nonmetals, and heats of formation of alkali halides are shown in Tables 7.1 and 7.2. 

Electron affinity or reduction of halide radical to halide anion (A )... 

Enthalpies of formation for the singlet and triplet states of methylene were obtained from the photodissociation of ketene.131 The data for CH2 (3Bi) were recently confirmed by methods which do not rely on ketene.132,133 In a widely applicable procedure, threshold collision energies for the loss of halide ion from RR C-X- were combined with gas phase acidities of RR CH-Cl to give AHf (RR C ) (Eq. 11).134 Similarly, gas phase acidities of the radicals RR CH were combined with ionization energies of the radical anions RR C -, or electron affinities of the carbenes RR C (Eq. 12).135136... 

The relative position of the electronic level eo to the Fermi level depends on the electrode potential. We perform estimates for the case where there is no drop in the outer potential between the adsorbate and the metal - usually this situation is not far from the pzc. In this case we obtain for an alkali ion eo — Ep — where is the work function of the metal, and I the ionization energy of the alkali atom. For a halide ion eo — Ep = electron affinity of the atom. 

Table 18.2 Occupation probability of the valence orbital of a few alkali and halide ions adsorbed on mercury ( = 4.5 eV). For alkali atoms eo denotes the ionization energy for halide atoms, the electron affinity.

As noted earlier, the superoxide ion reacts with effective electron acceptors. It is a one-electron reductant of moderate strength. However, the superoxide ion can act as a nucleophile if a substrate has a decreased electron affinity. For instance, alkyl halides react with the ion O2 RCH2OO -h HaE. This reaction initiates the next ones RCH2OO ... 

If P is a substance of electron affinity lower than that of the solvent (e.g., AlkHal in SFg), the process leads to the formation of the cation-radical. If P is a substance of electron affinity higher than that of a solvent (e.g., AlkHal in CH3OH), the process leads to the formation of the anion-radical. These possibilities are depicted in the following equations, based on alkyl halides ... 

Cycles such as this can be constructed for other compounds such as oxides (MO), sulfides (MS), higher valent metal halides (MX,j), etc. The difficulty in these cycles sometimes comes in the determination of values for the electron affinity, E. In the case of... 

It is not yet possible to measure lattice energy directly, which is why the best experimental values for the alkali halides, as listed in Table 1.16, are derived from a thermochemical cycle. This in itself is not always easy for compounds other than the alkali halides because, as we noted before, not all of the data is necessarily available. Electron affinity values are known from experimental measurements for... 

Br- (g). The electron affinity of Br (g) is calculable by the method of lattice energies. Selecting the crystal RbBr, because Rb+ and Br have exactly the same nuclear structure, and taking the exponent of the repulsive term to be 10, we have computed, for the reaction, RbBr (c) = Rb+ (g)+Br g), Dz= —151.2 whence the electron affinity of Br (g) becomes 87.9. Using the lattice energies of the alkali bromides as calculated by Sherman,1 we have computed the values 89.6, 85.6, 84.6, 83.6, and 89.6, respectively. Butkow,1 from the spectra of gaseous TIBr, deduced the value 86.5. From data on the absorption spectra of the alkali halides, Lederle1 obtained the value 82. See also Lennard-Jones.2... 

Calculate the values for the proton affinities of the halide anions shown in Table 93 from a Bom-Habtr ihermochcmical cycle and values Gar ionization energies, electron affinities, and bond energies. 

In the meantime, the reactivity of milled aluminum correlated well with the intensity of exoelectron emission. Such an emission decayed with time after termination of milling, along with the suppression of the chemical reaction. The aluminum, which had entirely lost electron emission activity, did not react with butyl bromide at all. Alkyl halides capture free electrons. The emission intensity of the free (unused) electrons under butyl bromide atmosphere was less than 20% of that under benzene atmosphere. In other words, exoelectrons are captured with butyl bromide more easily than with benzene. Butyl bromide has much stronger electron affinity than benzene. 

Other cases of approximately monatomic chromophores occur in 4f-+5d transitions now known in Sm11, Eu11, Tm11,28 Ybn, Cera, Prm, and Tbra.16 (The half-filled shell effect expressed by Eq. (3) is very conspicuous in this distribution of known species.) 5transitions are known in UIU, Np111, Puin, Paiy, U, Np, and Pu17. 5s-+5p transitions are known in complexes of Snn and Sbm and 6s — 6p in Tl1, Pbn, and Bira. The halide ions in solvents of not too high electron affinity and in crystals of alkali metal halides show absorption bands which to a certain approximation can be described as 3p - 4s(Cl), 4p — -5s(Br), and 5p - -6s(I). 

Alkyl- and Aryl-Halides. Because the halo-groups of organic molecules have large electronegativities and electron affinities, all halo-carbon molecules are electrophilic. Their electrochemical reduction potential is a measure of their electrophilicity (and electron affinity), which is illustrated in Figure 12.1 for hexacWorobenzene (C6C16), 1,2,3,4-tetrachlorobenzene (1,2,3,4-C6H2C14), and n-butyl iodide (n-BuI).8,9 Table 12.1 summarizes the reduction potentials for several alkyl-halides and ary 1-chlorides.810... 

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