Electron affinities of halogens

Work on the direct determination of the electron affinities of halogen atoms was begun by Mayer,7 who, with his students, measured directly the equilibrium constant for dissociation of alkali halogenide gas molecules into ions or of gas halogenide ions into atoms and electrons. Other methods have also been used, especially some involving mass... 

Figure 7.19 Precision and accuracy plot of the corrected values of the electron affinities of halogenated and methylated benzoquinones. These should be compared to the parallel lines in Figure 6.17. The compounds are listed in Table 6.3.

Figure 10.5 The electron affinities of halogenated benzoquinones, cyanoethylene, cyanobenzenes, and nitrobenzenes versus the number of substituents, data from [20],...

The electron affinities of halogenated aromatic and aliphatic compounds and nitro compounds have been evaluated. Additional electron affinities for halogenated benzene, freons, heterocyclic compounds, dibenzofuran, and the chloro- and fluoroben-zenes are reported from ECD data. The first positive Ea for the fluorochloroethanes were obtained from published ECD data. The Ea of halogenated aromatic radicals have been estimated from NIMS data. The AEa of all the halobenzenes have been calculated using CURES-EC. The Ea of chlorinated biphenyls and chlorinated napthalenes obtained from reduction potentials have been revised based on variable solution energy differences. 

Calvin continued his studies at the University of Alinnesota, where he investigated the electron affinities of halogen atoms. He received a Ph.D. degree in 1935. As a Rockefeller Foundation fellow at the University of Manchester in England (1935-1937), Calvin worked with Alichael Polanyi, who introduced him to the interdisciplinary approach, on coordination catalysis, the activation of molecular hydrogen, and metalloporphyrins. In 1937 he joined the faculty of the University of California, Berkeley, as an instmctor, and remained there for the balance of his career. 

There he worked with George Glockler on the electron affinity of halogens (initially iodine and later bromine and chlorine as well) from space-charge effects—Calvin s problem was to measure the amount of energy released when a halogen atom captures an electron and for that he had first to devise the methods for doing so. 

Dissociation of hydrogen halides in plasma with production of hydrogen and halogens can successfully compete with electrolysis and in some cases can be very attractive for practical applications. The experimental data related to these processes, however, are not sufficient today for clear conclusions to be drawn about their detailed mechanism, kinetics, and energy efficiency. The dissociative attachment of electrons to hydrogen halides is usually very fast (see Chapter 2) because of the high electron affinity of halogen atoms ... 

Quantum-chemical estimations show also that the ability to form additional bonds in halides of the Group 11 metals increases from chlorides to iodides. A comparison of the observed ionization potentials and electron affinities of halogens [139] shows that it requires a smaller expense of energy to add the second electron to an 1 ion than to Cr. The ions are not found yet, but if they are ever observed in mass spectra, the lifetime of 1 can be predicted to exceed that of. ... 

Much of tills chapter concerns ET reactions in solution. However, gas phase ET processes are well known too. See figure C3.2.1. The Tiarjioon mechanism by which halogens oxidize alkali metals is fundamentally an electron transfer reaction [2]. One might guess, from tliis simple reaction, some of tlie stmctural parameters tliat control ET rates relative electron affinities of reactants, reactant separation distance, bond lengtli changes upon oxidation/reduction, vibrational frequencies, etc. 

The electronic configuration of each halogen is one electron less than that of a noble gas, and it is not surprising therefore, that all the halogens can accept electrons to form X" ions. Indeed, the reactions X(g) + e - X (g), are all exothermic and the values (see Table 11.1), though small relative to the ionisation energies, are all larger than the electron affinity of any other atom. 

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). 

However, no evidence for even a transitory existence of -(X-Y) has been obtained except in the cases of X = Y = halogen or CNS (ref. 575a). and it is probable that the breakdown is concerted with reduction. The mole of cleavage appears to be governed by the relative electron affinities of X- and Y-, for example, hypobromous acid and hydroxylamine are cleaved by reducing ions as follows... 

The first electron affinity of each halogen in the series chlorine to iodine is shown in the table below. 

A few years ago experimental values were available for Q, S, /, and Z), but not for E the procedure adopted in testing the equation was to use the equation with calculated values of Uq (Equation 13-5) to find E, and as a test of the method to examine the constancy of E for a series of alkali halogenides containing the same halogen. The values obtained in this way were found to be constant to within about 3 kcal/mole. However, later experimental determinations of the values of the electron affinities of the halogen atoms by direct methods have shown that Equation 13-5 for the crystal energy is in general reliable only to about 2 percent. 

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